How difficult is it to simply disable/disengage the MCAS on Boeing 737 Max 8 & 9 Aircraft?Can computer imposed inputs be overridden on the Boeing 737-MAX?Is the Boeing 737 MAX 8 MCAS exceptional in operating when autopilot is off?Does the Boeing 737 MAX use electro-hydraulic actuators?How long is the horizontal stabilizer on a Boeing 737?Is there any difference between the Boeing 787 and the 737 MAX?What is the maximum take off climb angle of a Boeing 737 MAX?Why is the 737 MAX 7 selling so poorly?How safe is the Boeing 737-800 in high winds?How unusual is the 737 MAX 8's crash history?Is the 737-900 safer than the 737 Max?Where are the grounded 737 Max planes being kept?Why is Germany not able to read data from the Boeing 737 MAX black boxes?
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How difficult is it to simply disable/disengage the MCAS on Boeing 737 Max 8 & 9 Aircraft?
Can computer imposed inputs be overridden on the Boeing 737-MAX?Is the Boeing 737 MAX 8 MCAS exceptional in operating when autopilot is off?Does the Boeing 737 MAX use electro-hydraulic actuators?How long is the horizontal stabilizer on a Boeing 737?Is there any difference between the Boeing 787 and the 737 MAX?What is the maximum take off climb angle of a Boeing 737 MAX?Why is the 737 MAX 7 selling so poorly?How safe is the Boeing 737-800 in high winds?How unusual is the 737 MAX 8's crash history?Is the 737-900 safer than the 737 Max?Where are the grounded 737 Max planes being kept?Why is Germany not able to read data from the Boeing 737 MAX black boxes?
$begingroup$
With regard to the Ethiopian crash of the 737 Max 8 aircraft, where according to preliminary reports, it appears to be related to the MCAS which is a safety system that pushes the nose down in the even of an excessively high angle of attack (pitch attitude), how difficult is it for the pilots to simply disable the MCAS and manually fly the plane?
To clarify my question, what I'm asking is can the pilot disengage the MCAS and if so, how do they do that? Is it just flipping a switch or popping a breaker, or is it a complicated set of procedures?
safety aircraft-systems boeing-737 mcas
asked Mar 15 at 18:08
Devil07Devil07
5,7431656
$endgroup$
|
show 1 more comment
$begingroup$
With regard to the Ethiopian crash of the 737 Max 8 aircraft, where according to preliminary reports, it appears to be related to the MCAS which is a safety system that pushes the nose down in the even of an excessively high angle of attack (pitch attitude), how difficult is it for the pilots to simply disable the MCAS and manually fly the plane?
To clarify my question, what I'm asking is can the pilot disengage the MCAS and if so, how do they do that? Is it just flipping a switch or popping a breaker, or is it a complicated set of procedures?
safety aircraft-systems boeing-737 mcas
asked Mar 15 at 18:08
Devil07Devil07
5,7431656
$endgroup$
15
$begingroup$
This question could be just as answerable without bringing up the two recent 737 Max crashes at all. You might consider just editing that part out; that would go a long way toward alleviating concerns that you want speculation on an accident, rather than operational details on an airplane.
$endgroup$
– a CVn
Mar 15 at 19:08
14
$begingroup$
@aCVn I think context is important. Many people use this site, not all are aviation nerds, by providing context to the question it establishes why the question and answers, are relevant and important. At least, thats my opinion. :)
$endgroup$
– Devil07
2 days ago
2
$begingroup$
Related: Can computer imposed inputs be overridden on the Boeing 737-MAX?
$endgroup$
– ymb1
2 days ago
2
$begingroup$
While this is an interesting question, if we're considering it for practical purposes, we also ought to ask what the purpose of MCAS is and what the consequences of disabling it would be.
$endgroup$
– David K
yesterday
$begingroup$
The purpose of MCAS is to avoid stalls. It does that by shoving the nose down. The consequences of disabling it that it won't try and auto-recover from a stall.
$endgroup$
– ggb667
7 hours ago
|
show 1 more comment
$begingroup$
With regard to the Ethiopian crash of the 737 Max 8 aircraft, where according to preliminary reports, it appears to be related to the MCAS which is a safety system that pushes the nose down in the even of an excessively high angle of attack (pitch attitude), how difficult is it for the pilots to simply disable the MCAS and manually fly the plane?
To clarify my question, what I'm asking is can the pilot disengage the MCAS and if so, how do they do that? Is it just flipping a switch or popping a breaker, or is it a complicated set of procedures?
safety aircraft-systems boeing-737 mcas
asked Mar 15 at 18:08
Devil07Devil07
5,7431656
$endgroup$
With regard to the Ethiopian crash of the 737 Max 8 aircraft, where according to preliminary reports, it appears to be related to the MCAS which is a safety system that pushes the nose down in the even of an excessively high angle of attack (pitch attitude), how difficult is it for the pilots to simply disable the MCAS and manually fly the plane?
To clarify my question, what I'm asking is can the pilot disengage the MCAS and if so, how do they do that? Is it just flipping a switch or popping a breaker, or is it a complicated set of procedures?
safety aircraft-systems boeing-737 mcas
safety aircraft-systems boeing-737 mcas
asked Mar 15 at 18:08
Devil07Devil07
5,7431656
asked Mar 15 at 18:08
Devil07Devil07
5,7431656
edited 2 days ago
Devil07
asked Mar 15 at 18:08
Devil07Devil07
5,7431656
asked Mar 15 at 18:08
Devil07Devil07
5,7431656
asked Mar 15 at 18:08
Devil07Devil07
5,7431656
5,7431656
15
$begingroup$
This question could be just as answerable without bringing up the two recent 737 Max crashes at all. You might consider just editing that part out; that would go a long way toward alleviating concerns that you want speculation on an accident, rather than operational details on an airplane.
$endgroup$
– a CVn
Mar 15 at 19:08
14
$begingroup$
@aCVn I think context is important. Many people use this site, not all are aviation nerds, by providing context to the question it establishes why the question and answers, are relevant and important. At least, thats my opinion. :)
$endgroup$
– Devil07
2 days ago
2
$begingroup$
Related: Can computer imposed inputs be overridden on the Boeing 737-MAX?
$endgroup$
– ymb1
2 days ago
2
$begingroup$
While this is an interesting question, if we're considering it for practical purposes, we also ought to ask what the purpose of MCAS is and what the consequences of disabling it would be.
$endgroup$
– David K
yesterday
$begingroup$
The purpose of MCAS is to avoid stalls. It does that by shoving the nose down. The consequences of disabling it that it won't try and auto-recover from a stall.
$endgroup$
– ggb667
7 hours ago
|
show 1 more comment
15
$begingroup$
This question could be just as answerable without bringing up the two recent 737 Max crashes at all. You might consider just editing that part out; that would go a long way toward alleviating concerns that you want speculation on an accident, rather than operational details on an airplane.
$endgroup$
– a CVn
Mar 15 at 19:08
14
$begingroup$
@aCVn I think context is important. Many people use this site, not all are aviation nerds, by providing context to the question it establishes why the question and answers, are relevant and important. At least, thats my opinion. :)
$endgroup$
– Devil07
2 days ago
2
$begingroup$
Related: Can computer imposed inputs be overridden on the Boeing 737-MAX?
$endgroup$
– ymb1
2 days ago
2
$begingroup$
While this is an interesting question, if we're considering it for practical purposes, we also ought to ask what the purpose of MCAS is and what the consequences of disabling it would be.
$endgroup$
– David K
yesterday
$begingroup$
The purpose of MCAS is to avoid stalls. It does that by shoving the nose down. The consequences of disabling it that it won't try and auto-recover from a stall.
$endgroup$
– ggb667
7 hours ago
15
15
$begingroup$
This question could be just as answerable without bringing up the two recent 737 Max crashes at all. You might consider just editing that part out; that would go a long way toward alleviating concerns that you want speculation on an accident, rather than operational details on an airplane.
$endgroup$
– a CVn
Mar 15 at 19:08
$begingroup$
This question could be just as answerable without bringing up the two recent 737 Max crashes at all. You might consider just editing that part out; that would go a long way toward alleviating concerns that you want speculation on an accident, rather than operational details on an airplane.
$endgroup$
– a CVn
Mar 15 at 19:08
14
14
$begingroup$
@aCVn I think context is important. Many people use this site, not all are aviation nerds, by providing context to the question it establishes why the question and answers, are relevant and important. At least, thats my opinion. :)
$endgroup$
– Devil07
2 days ago
$begingroup$
@aCVn I think context is important. Many people use this site, not all are aviation nerds, by providing context to the question it establishes why the question and answers, are relevant and important. At least, thats my opinion. :)
$endgroup$
– Devil07
2 days ago
2
2
$begingroup$
Related: Can computer imposed inputs be overridden on the Boeing 737-MAX?
$endgroup$
– ymb1
2 days ago
$begingroup$
Related: Can computer imposed inputs be overridden on the Boeing 737-MAX?
$endgroup$
– ymb1
2 days ago
2
2
$begingroup$
While this is an interesting question, if we're considering it for practical purposes, we also ought to ask what the purpose of MCAS is and what the consequences of disabling it would be.
$endgroup$
– David K
yesterday
$begingroup$
While this is an interesting question, if we're considering it for practical purposes, we also ought to ask what the purpose of MCAS is and what the consequences of disabling it would be.
$endgroup$
– David K
yesterday
$begingroup$
The purpose of MCAS is to avoid stalls. It does that by shoving the nose down. The consequences of disabling it that it won't try and auto-recover from a stall.
$endgroup$
– ggb667
7 hours ago
$begingroup$
The purpose of MCAS is to avoid stalls. It does that by shoving the nose down. The consequences of disabling it that it won't try and auto-recover from a stall.
$endgroup$
– ggb667
7 hours ago
|
show 1 more comment
3 Answers
3
active
oldest
votes
$begingroup$
MCAS doesn't have its own on/off switch
It is a fly-by-wire feature designed to account for a particular flight regime that would not (or was not expected to) be encountered very often in normal operations, and is intended to account for some of the aerodynamic effects of the LEAP-1B (CFM International) engine installation for this model. Its activation requires a number of preconditions, but we'll get to that in a moment.
A few things that should disable it (with caveats)
Lower the flaps. It is intended to work only if the flaps are up.
Turn the Stab Trim switches to OFF. This disables the horizontal stabilizer's trim completely, and reverts to manual trim (there are two guarded stabilizer trim switches in the aisle stand, see Windshear's answer). This means that the pilots must move/rotate the trim wheels in order to apply pitch trim during flight.
The manual pitch trim appears to be what a few crews did prior to the LionAir crash in October 2018. It is unclear how many of the crews knew that is was MCAS, versus any other trim or pitch anomaly. The previous LionAir crews on the accident aircraft ended up flying to their destination manually. (Original source i the Preliminary Report from that accident).
... pilots encountered problems involving the AoA as well as the pitot tube used to measure airspeed. In a flight in the same plane the day before, to Jakarta, the pilot experienced many of the same symptoms as the pilots on flight JT610: the stick shaker activated during rotation, an indicated airspeed warning alert appeared, and the aircraft began automatically pushing the aircraft nose down.
The pilot, after determining that his flight display system was malfunctioning, ran a runaway stabilizer non-normal checklist which led to the MCAS being disconnected when the stabilizer trim switches were turned off. The copilot flew the rest of the flight using manual controls and without autopilot.
That Jakarta flight was using an angle of attack sensor that had been replaced after the previous Lion Air flight to Denpasar experienced problems. snip However, it is not clear whether the pilot communicated that he ran a runaway stabilizer non-normal checklist during the flight, which might have alerted the airline’s engineering staff that there was still a problem. (source)
Enable autopilot. It is supposed to only work if the autopilot is off. But...CAVEAT
That last part is a little bit complicated: the autopilot may not stay on if it - the autopilot system - keeps getting spurious signals from the AoA sensors, or if the pitot static system is providing bad data to the FCCs.
How do you know that you need to disable MCAS?
A more apt question might be "when do you know that it's time to turn off Stab Trim switches?"
The problem for a given crew: how do you know what sub-system that is giving you trouble? The AD released in December of 2018 updated the Runaway Stabilizer procedure. Since there are other malfunctions possible in the trim system for pitch, such as a trim runaway, the same symptom may manifest during malfunctions with different causes. (See the example above). If additional symptoms are present, such as a stick shaker activating from an erroneous AoA indication or some other fault, which problem do you solve first?
Which problem are you having?
Are you having both problems, or even something else?
That concern, the relationship between different potential failure modes, gets to the heart of
What is in the pilots manuals?
What training the crews do, or do not, get on this sub-system?
The MCAS is only supposed to activate under certain conditions (in theory): high angle-of-attack, flaps-up, flight with autopilot off. This includes high angle of bank flight in such conditions.
CAVEAT
Take a look at that last bullet in the lower right of the figure. It's only partially correct. When the pilot overrides with yoke trim, MCAS will try again five seconds later (after the pilot has stopped) unless the triggering condition, or signal, goes away.
"In the event of erroneous AOA data, the pitch trim system can trim the stabilizer nose down in increments lasting up to 10 seconds," Boeing explained to operators in a bulletin issued last November, following the JT610 accident. "The nose down stabilizer trim movement can be stopped and reversed with the use of the electric stabilizer trim switches but may restart 5 sec. after the electric stabilizer trim switches are released. Repetitive cycles of uncommanded nose down
stabilizer continue to occur unless the stabilizer trim system is deactivated." (source)
It is this system behavior that is believed to have ultimately overcome the best efforts of the pilots in the LionAir crash. They were airborne for about six minutes before they were no longer able to overcome the horizontal stab position with their trim controls. (Caveat: current as of early 2019. When the final report comes out this may become a lot clearer, and may need revision).
There is an easy-to-understand video of the relationship between the jack screw and the horizontal stabilizer function here. Whenever the jack screw is moving the horizontal stab, a cockpit cue that this is occurring is the rotation of the manual trim wheels (seen at about 3:40 in the video).
Why is this activating during takeoff/departure?
That MCAS appears to activate(sometimes) during routine flight regimes is believed to be catching some crews by surprise - and not just the two accident crews. There is a recent article in Atlantic that excerpts some NASA reports by aircrews in the US about some cases of unexpected/odd pitch behavior in this model aircraft.
answered Mar 15 at 18:39
KorvinStarmastKorvinStarmast
3,0991330
$endgroup$
$begingroup$
is it fair to say that MCAS controls pitch through the automated Stab Trim system? So when MCAS is activated from a high AOA sensor reading, pilots will see Trim wheels spinning forward to reduce pitch?
$endgroup$
– Devil07
2 days ago
$begingroup$
@Devil07 Which line are you getting that from? For that matter, what you just asked is its own question. suggest you ask it. I am addressing the diasabling of that feature. Whatever disables the trim beyond manual control is what disables the MCAS function. Apparently, trim over ride from the yoke does not. I've seen an explanation of the details ... give me a chance to find a link.
$endgroup$
– KorvinStarmast
2 days ago
$begingroup$
@Devil07 FWIW, I have been watching the arguments about the stab trim system and this systems interaction for a few months, since Lion Air, and am leary of jumping into that briar patch.
$endgroup$
– KorvinStarmast
2 days ago
$begingroup$
you're answer is great, I'm just trying to understand how it all works together. If MCAS causes aircraft to pitch nose down when it senses excessive AOA, then doesn't it do it through use of trim? If so, when MCAS is pitching nose down, won't the trim wheels physically turn by themselves? Your graphic says "MCAS moves horizontal stabilizer trim".
$endgroup$
– Devil07
2 days ago
1
$begingroup$
To answer your question, yes, as explained in this video
$endgroup$
– KorvinStarmast
2 days ago
|
show 3 more comments
$begingroup$
You can disable the stabaliser trim which will prevent MCAS from making inputs to the aircraft.
answered Mar 15 at 19:10
WindshearWindshear
1313
New contributor
Windshear is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
1
$begingroup$
This is what I was thinking. If MCAS is pitching nose forward through use of stab trim, then the trim wheel will be visibly spinning forward (or will it?), if it is, then disable it and take manual control of trim.
$endgroup$
– Devil07
2 days ago
3
$begingroup$
Yes, the trim wheels will spin in case MCAS sends input. By disabling the stab trim, inputs to it by any system will be disabled.
$endgroup$
– Windshear
2 days ago
1
$begingroup$
that what I thought. Good answer and great picture!
$endgroup$
– Devil07
yesterday
$begingroup$
(stabaliser → stabiliser)
$endgroup$
– Peter Mortensen
yesterday
add a comment |
$begingroup$
From what I have read, the other answers explain the cutout procedure correctly. It is also important to keep in mind Korvin's remark that trim anomalies can occur for a variety of reasons. What I have not seen mentioned often or clearly enough is a crucial difference that distinguishes MCAS from regular trim control, a difference which was apparently not communicated clearly enough to the pilots. As an article by Leeham news points out:
[MCAS is] not stopped by the Pilot pulling the Yoke, which for normal trim from the autopilot or runaway manual trim triggers trim hold sensors.1
This is a difference to previous B737 models which was not mentioned in the manual (as there was no reference to the MCAS at all) and thus was unknown to pilots before the FAA advisory was published.2 From the NASA incident reports published in the Atlantic it becomes clear that the FAA advisory after the LionAir crash was discussed among the pilots. Apparently there were a few incidents where performing the proper cutout procedure saved the airplanes, among them the preceding LionAir flight.
1 The reason for this design decision being that providing the pilot a way to easily override
would negate why MCAS was implemented, the Pilot pulling so hard on the Yoke that the aircraft is flying close to stall.
In other words, MCAS is a mechanism installed to correct an already present pilot error. Logically, this corrective intervention by the flight computer can only be cancelled through a dedicated operating sequence. It's also worth mentioning that even extreme yoke action, working on the elevators only, is not able to fully compensate extreme stabilizer positions, the control surfaces MCAS uses (apart from, I suppose, creating unpleasant flight characteristics).
2 An article in the Seattle Times on March 17, 2019 presents that argument as well.
answered yesterday
Peter A. SchneiderPeter A. Schneider
206210
$endgroup$
$begingroup$
[MCAS is] not stopped by the Pilot pulling the Yoke, -- This is dangerous. Pilots reasonably expect the computer system to go away if they are actively working the controls. After all that's how everything else works. The unexpected can easily cause a pilots brain to lock , especially when they think "I'M ALREADY DOING THE RIGHT ACTION WHY ISN'T IT WORKING!?!?!", and they continue to (ineffectively) fight the controls as the ground gets closer and closer, the stick is shaking and alarms are going nuts.
$endgroup$
– ggb667
6 hours ago
add a comment |
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3 Answers
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3 Answers
3
active
oldest
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active
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active
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$begingroup$
MCAS doesn't have its own on/off switch
It is a fly-by-wire feature designed to account for a particular flight regime that would not (or was not expected to) be encountered very often in normal operations, and is intended to account for some of the aerodynamic effects of the LEAP-1B (CFM International) engine installation for this model. Its activation requires a number of preconditions, but we'll get to that in a moment.
A few things that should disable it (with caveats)
Lower the flaps. It is intended to work only if the flaps are up.
Turn the Stab Trim switches to OFF. This disables the horizontal stabilizer's trim completely, and reverts to manual trim (there are two guarded stabilizer trim switches in the aisle stand, see Windshear's answer). This means that the pilots must move/rotate the trim wheels in order to apply pitch trim during flight.
The manual pitch trim appears to be what a few crews did prior to the LionAir crash in October 2018. It is unclear how many of the crews knew that is was MCAS, versus any other trim or pitch anomaly. The previous LionAir crews on the accident aircraft ended up flying to their destination manually. (Original source i the Preliminary Report from that accident).
... pilots encountered problems involving the AoA as well as the pitot tube used to measure airspeed. In a flight in the same plane the day before, to Jakarta, the pilot experienced many of the same symptoms as the pilots on flight JT610: the stick shaker activated during rotation, an indicated airspeed warning alert appeared, and the aircraft began automatically pushing the aircraft nose down.
The pilot, after determining that his flight display system was malfunctioning, ran a runaway stabilizer non-normal checklist which led to the MCAS being disconnected when the stabilizer trim switches were turned off. The copilot flew the rest of the flight using manual controls and without autopilot.
That Jakarta flight was using an angle of attack sensor that had been replaced after the previous Lion Air flight to Denpasar experienced problems. snip However, it is not clear whether the pilot communicated that he ran a runaway stabilizer non-normal checklist during the flight, which might have alerted the airline’s engineering staff that there was still a problem. (source)
Enable autopilot. It is supposed to only work if the autopilot is off. But...CAVEAT
That last part is a little bit complicated: the autopilot may not stay on if it - the autopilot system - keeps getting spurious signals from the AoA sensors, or if the pitot static system is providing bad data to the FCCs.
How do you know that you need to disable MCAS?
A more apt question might be "when do you know that it's time to turn off Stab Trim switches?"
The problem for a given crew: how do you know what sub-system that is giving you trouble? The AD released in December of 2018 updated the Runaway Stabilizer procedure. Since there are other malfunctions possible in the trim system for pitch, such as a trim runaway, the same symptom may manifest during malfunctions with different causes. (See the example above). If additional symptoms are present, such as a stick shaker activating from an erroneous AoA indication or some other fault, which problem do you solve first?
Which problem are you having?
Are you having both problems, or even something else?
That concern, the relationship between different potential failure modes, gets to the heart of
What is in the pilots manuals?
What training the crews do, or do not, get on this sub-system?
The MCAS is only supposed to activate under certain conditions (in theory): high angle-of-attack, flaps-up, flight with autopilot off. This includes high angle of bank flight in such conditions.
CAVEAT
Take a look at that last bullet in the lower right of the figure. It's only partially correct. When the pilot overrides with yoke trim, MCAS will try again five seconds later (after the pilot has stopped) unless the triggering condition, or signal, goes away.
"In the event of erroneous AOA data, the pitch trim system can trim the stabilizer nose down in increments lasting up to 10 seconds," Boeing explained to operators in a bulletin issued last November, following the JT610 accident. "The nose down stabilizer trim movement can be stopped and reversed with the use of the electric stabilizer trim switches but may restart 5 sec. after the electric stabilizer trim switches are released. Repetitive cycles of uncommanded nose down
stabilizer continue to occur unless the stabilizer trim system is deactivated." (source)
It is this system behavior that is believed to have ultimately overcome the best efforts of the pilots in the LionAir crash. They were airborne for about six minutes before they were no longer able to overcome the horizontal stab position with their trim controls. (Caveat: current as of early 2019. When the final report comes out this may become a lot clearer, and may need revision).
There is an easy-to-understand video of the relationship between the jack screw and the horizontal stabilizer function here. Whenever the jack screw is moving the horizontal stab, a cockpit cue that this is occurring is the rotation of the manual trim wheels (seen at about 3:40 in the video).
Why is this activating during takeoff/departure?
That MCAS appears to activate(sometimes) during routine flight regimes is believed to be catching some crews by surprise - and not just the two accident crews. There is a recent article in Atlantic that excerpts some NASA reports by aircrews in the US about some cases of unexpected/odd pitch behavior in this model aircraft.
answered Mar 15 at 18:39
KorvinStarmastKorvinStarmast
3,0991330
$endgroup$
$begingroup$
is it fair to say that MCAS controls pitch through the automated Stab Trim system? So when MCAS is activated from a high AOA sensor reading, pilots will see Trim wheels spinning forward to reduce pitch?
$endgroup$
– Devil07
2 days ago
$begingroup$
@Devil07 Which line are you getting that from? For that matter, what you just asked is its own question. suggest you ask it. I am addressing the diasabling of that feature. Whatever disables the trim beyond manual control is what disables the MCAS function. Apparently, trim over ride from the yoke does not. I've seen an explanation of the details ... give me a chance to find a link.
$endgroup$
– KorvinStarmast
2 days ago
$begingroup$
@Devil07 FWIW, I have been watching the arguments about the stab trim system and this systems interaction for a few months, since Lion Air, and am leary of jumping into that briar patch.
$endgroup$
– KorvinStarmast
2 days ago
$begingroup$
you're answer is great, I'm just trying to understand how it all works together. If MCAS causes aircraft to pitch nose down when it senses excessive AOA, then doesn't it do it through use of trim? If so, when MCAS is pitching nose down, won't the trim wheels physically turn by themselves? Your graphic says "MCAS moves horizontal stabilizer trim".
$endgroup$
– Devil07
2 days ago
1
$begingroup$
To answer your question, yes, as explained in this video
$endgroup$
– KorvinStarmast
2 days ago
|
show 3 more comments
$begingroup$
MCAS doesn't have its own on/off switch
It is a fly-by-wire feature designed to account for a particular flight regime that would not (or was not expected to) be encountered very often in normal operations, and is intended to account for some of the aerodynamic effects of the LEAP-1B (CFM International) engine installation for this model. Its activation requires a number of preconditions, but we'll get to that in a moment.
A few things that should disable it (with caveats)
Lower the flaps. It is intended to work only if the flaps are up.
Turn the Stab Trim switches to OFF. This disables the horizontal stabilizer's trim completely, and reverts to manual trim (there are two guarded stabilizer trim switches in the aisle stand, see Windshear's answer). This means that the pilots must move/rotate the trim wheels in order to apply pitch trim during flight.
The manual pitch trim appears to be what a few crews did prior to the LionAir crash in October 2018. It is unclear how many of the crews knew that is was MCAS, versus any other trim or pitch anomaly. The previous LionAir crews on the accident aircraft ended up flying to their destination manually. (Original source i the Preliminary Report from that accident).
... pilots encountered problems involving the AoA as well as the pitot tube used to measure airspeed. In a flight in the same plane the day before, to Jakarta, the pilot experienced many of the same symptoms as the pilots on flight JT610: the stick shaker activated during rotation, an indicated airspeed warning alert appeared, and the aircraft began automatically pushing the aircraft nose down.
The pilot, after determining that his flight display system was malfunctioning, ran a runaway stabilizer non-normal checklist which led to the MCAS being disconnected when the stabilizer trim switches were turned off. The copilot flew the rest of the flight using manual controls and without autopilot.
That Jakarta flight was using an angle of attack sensor that had been replaced after the previous Lion Air flight to Denpasar experienced problems. snip However, it is not clear whether the pilot communicated that he ran a runaway stabilizer non-normal checklist during the flight, which might have alerted the airline’s engineering staff that there was still a problem. (source)
Enable autopilot. It is supposed to only work if the autopilot is off. But...CAVEAT
That last part is a little bit complicated: the autopilot may not stay on if it - the autopilot system - keeps getting spurious signals from the AoA sensors, or if the pitot static system is providing bad data to the FCCs.
How do you know that you need to disable MCAS?
A more apt question might be "when do you know that it's time to turn off Stab Trim switches?"
The problem for a given crew: how do you know what sub-system that is giving you trouble? The AD released in December of 2018 updated the Runaway Stabilizer procedure. Since there are other malfunctions possible in the trim system for pitch, such as a trim runaway, the same symptom may manifest during malfunctions with different causes. (See the example above). If additional symptoms are present, such as a stick shaker activating from an erroneous AoA indication or some other fault, which problem do you solve first?
Which problem are you having?
Are you having both problems, or even something else?
That concern, the relationship between different potential failure modes, gets to the heart of
What is in the pilots manuals?
What training the crews do, or do not, get on this sub-system?
The MCAS is only supposed to activate under certain conditions (in theory): high angle-of-attack, flaps-up, flight with autopilot off. This includes high angle of bank flight in such conditions.
CAVEAT
Take a look at that last bullet in the lower right of the figure. It's only partially correct. When the pilot overrides with yoke trim, MCAS will try again five seconds later (after the pilot has stopped) unless the triggering condition, or signal, goes away.
"In the event of erroneous AOA data, the pitch trim system can trim the stabilizer nose down in increments lasting up to 10 seconds," Boeing explained to operators in a bulletin issued last November, following the JT610 accident. "The nose down stabilizer trim movement can be stopped and reversed with the use of the electric stabilizer trim switches but may restart 5 sec. after the electric stabilizer trim switches are released. Repetitive cycles of uncommanded nose down
stabilizer continue to occur unless the stabilizer trim system is deactivated." (source)
It is this system behavior that is believed to have ultimately overcome the best efforts of the pilots in the LionAir crash. They were airborne for about six minutes before they were no longer able to overcome the horizontal stab position with their trim controls. (Caveat: current as of early 2019. When the final report comes out this may become a lot clearer, and may need revision).
There is an easy-to-understand video of the relationship between the jack screw and the horizontal stabilizer function here. Whenever the jack screw is moving the horizontal stab, a cockpit cue that this is occurring is the rotation of the manual trim wheels (seen at about 3:40 in the video).
Why is this activating during takeoff/departure?
That MCAS appears to activate(sometimes) during routine flight regimes is believed to be catching some crews by surprise - and not just the two accident crews. There is a recent article in Atlantic that excerpts some NASA reports by aircrews in the US about some cases of unexpected/odd pitch behavior in this model aircraft.
answered Mar 15 at 18:39
KorvinStarmastKorvinStarmast
3,0991330
$endgroup$
$begingroup$
is it fair to say that MCAS controls pitch through the automated Stab Trim system? So when MCAS is activated from a high AOA sensor reading, pilots will see Trim wheels spinning forward to reduce pitch?
$endgroup$
– Devil07
2 days ago
$begingroup$
@Devil07 Which line are you getting that from? For that matter, what you just asked is its own question. suggest you ask it. I am addressing the diasabling of that feature. Whatever disables the trim beyond manual control is what disables the MCAS function. Apparently, trim over ride from the yoke does not. I've seen an explanation of the details ... give me a chance to find a link.
$endgroup$
– KorvinStarmast
2 days ago
$begingroup$
@Devil07 FWIW, I have been watching the arguments about the stab trim system and this systems interaction for a few months, since Lion Air, and am leary of jumping into that briar patch.
$endgroup$
– KorvinStarmast
2 days ago
$begingroup$
you're answer is great, I'm just trying to understand how it all works together. If MCAS causes aircraft to pitch nose down when it senses excessive AOA, then doesn't it do it through use of trim? If so, when MCAS is pitching nose down, won't the trim wheels physically turn by themselves? Your graphic says "MCAS moves horizontal stabilizer trim".
$endgroup$
– Devil07
2 days ago
1
$begingroup$
To answer your question, yes, as explained in this video
$endgroup$
– KorvinStarmast
2 days ago
|
show 3 more comments
$begingroup$
MCAS doesn't have its own on/off switch
It is a fly-by-wire feature designed to account for a particular flight regime that would not (or was not expected to) be encountered very often in normal operations, and is intended to account for some of the aerodynamic effects of the LEAP-1B (CFM International) engine installation for this model. Its activation requires a number of preconditions, but we'll get to that in a moment.
A few things that should disable it (with caveats)
Lower the flaps. It is intended to work only if the flaps are up.
Turn the Stab Trim switches to OFF. This disables the horizontal stabilizer's trim completely, and reverts to manual trim (there are two guarded stabilizer trim switches in the aisle stand, see Windshear's answer). This means that the pilots must move/rotate the trim wheels in order to apply pitch trim during flight.
The manual pitch trim appears to be what a few crews did prior to the LionAir crash in October 2018. It is unclear how many of the crews knew that is was MCAS, versus any other trim or pitch anomaly. The previous LionAir crews on the accident aircraft ended up flying to their destination manually. (Original source i the Preliminary Report from that accident).
... pilots encountered problems involving the AoA as well as the pitot tube used to measure airspeed. In a flight in the same plane the day before, to Jakarta, the pilot experienced many of the same symptoms as the pilots on flight JT610: the stick shaker activated during rotation, an indicated airspeed warning alert appeared, and the aircraft began automatically pushing the aircraft nose down.
The pilot, after determining that his flight display system was malfunctioning, ran a runaway stabilizer non-normal checklist which led to the MCAS being disconnected when the stabilizer trim switches were turned off. The copilot flew the rest of the flight using manual controls and without autopilot.
That Jakarta flight was using an angle of attack sensor that had been replaced after the previous Lion Air flight to Denpasar experienced problems. snip However, it is not clear whether the pilot communicated that he ran a runaway stabilizer non-normal checklist during the flight, which might have alerted the airline’s engineering staff that there was still a problem. (source)
Enable autopilot. It is supposed to only work if the autopilot is off. But...CAVEAT
That last part is a little bit complicated: the autopilot may not stay on if it - the autopilot system - keeps getting spurious signals from the AoA sensors, or if the pitot static system is providing bad data to the FCCs.
How do you know that you need to disable MCAS?
A more apt question might be "when do you know that it's time to turn off Stab Trim switches?"
The problem for a given crew: how do you know what sub-system that is giving you trouble? The AD released in December of 2018 updated the Runaway Stabilizer procedure. Since there are other malfunctions possible in the trim system for pitch, such as a trim runaway, the same symptom may manifest during malfunctions with different causes. (See the example above). If additional symptoms are present, such as a stick shaker activating from an erroneous AoA indication or some other fault, which problem do you solve first?
Which problem are you having?
Are you having both problems, or even something else?
That concern, the relationship between different potential failure modes, gets to the heart of
What is in the pilots manuals?
What training the crews do, or do not, get on this sub-system?
The MCAS is only supposed to activate under certain conditions (in theory): high angle-of-attack, flaps-up, flight with autopilot off. This includes high angle of bank flight in such conditions.
CAVEAT
Take a look at that last bullet in the lower right of the figure. It's only partially correct. When the pilot overrides with yoke trim, MCAS will try again five seconds later (after the pilot has stopped) unless the triggering condition, or signal, goes away.
"In the event of erroneous AOA data, the pitch trim system can trim the stabilizer nose down in increments lasting up to 10 seconds," Boeing explained to operators in a bulletin issued last November, following the JT610 accident. "The nose down stabilizer trim movement can be stopped and reversed with the use of the electric stabilizer trim switches but may restart 5 sec. after the electric stabilizer trim switches are released. Repetitive cycles of uncommanded nose down
stabilizer continue to occur unless the stabilizer trim system is deactivated." (source)
It is this system behavior that is believed to have ultimately overcome the best efforts of the pilots in the LionAir crash. They were airborne for about six minutes before they were no longer able to overcome the horizontal stab position with their trim controls. (Caveat: current as of early 2019. When the final report comes out this may become a lot clearer, and may need revision).
There is an easy-to-understand video of the relationship between the jack screw and the horizontal stabilizer function here. Whenever the jack screw is moving the horizontal stab, a cockpit cue that this is occurring is the rotation of the manual trim wheels (seen at about 3:40 in the video).
Why is this activating during takeoff/departure?
That MCAS appears to activate(sometimes) during routine flight regimes is believed to be catching some crews by surprise - and not just the two accident crews. There is a recent article in Atlantic that excerpts some NASA reports by aircrews in the US about some cases of unexpected/odd pitch behavior in this model aircraft.
answered Mar 15 at 18:39
KorvinStarmastKorvinStarmast
3,0991330
$endgroup$
MCAS doesn't have its own on/off switch
It is a fly-by-wire feature designed to account for a particular flight regime that would not (or was not expected to) be encountered very often in normal operations, and is intended to account for some of the aerodynamic effects of the LEAP-1B (CFM International) engine installation for this model. Its activation requires a number of preconditions, but we'll get to that in a moment.
A few things that should disable it (with caveats)
Lower the flaps. It is intended to work only if the flaps are up.
Turn the Stab Trim switches to OFF. This disables the horizontal stabilizer's trim completely, and reverts to manual trim (there are two guarded stabilizer trim switches in the aisle stand, see Windshear's answer). This means that the pilots must move/rotate the trim wheels in order to apply pitch trim during flight.
The manual pitch trim appears to be what a few crews did prior to the LionAir crash in October 2018. It is unclear how many of the crews knew that is was MCAS, versus any other trim or pitch anomaly. The previous LionAir crews on the accident aircraft ended up flying to their destination manually. (Original source i the Preliminary Report from that accident).
... pilots encountered problems involving the AoA as well as the pitot tube used to measure airspeed. In a flight in the same plane the day before, to Jakarta, the pilot experienced many of the same symptoms as the pilots on flight JT610: the stick shaker activated during rotation, an indicated airspeed warning alert appeared, and the aircraft began automatically pushing the aircraft nose down.
The pilot, after determining that his flight display system was malfunctioning, ran a runaway stabilizer non-normal checklist which led to the MCAS being disconnected when the stabilizer trim switches were turned off. The copilot flew the rest of the flight using manual controls and without autopilot.
That Jakarta flight was using an angle of attack sensor that had been replaced after the previous Lion Air flight to Denpasar experienced problems. snip However, it is not clear whether the pilot communicated that he ran a runaway stabilizer non-normal checklist during the flight, which might have alerted the airline’s engineering staff that there was still a problem. (source)
Enable autopilot. It is supposed to only work if the autopilot is off. But...CAVEAT
That last part is a little bit complicated: the autopilot may not stay on if it - the autopilot system - keeps getting spurious signals from the AoA sensors, or if the pitot static system is providing bad data to the FCCs.
How do you know that you need to disable MCAS?
A more apt question might be "when do you know that it's time to turn off Stab Trim switches?"
The problem for a given crew: how do you know what sub-system that is giving you trouble? The AD released in December of 2018 updated the Runaway Stabilizer procedure. Since there are other malfunctions possible in the trim system for pitch, such as a trim runaway, the same symptom may manifest during malfunctions with different causes. (See the example above). If additional symptoms are present, such as a stick shaker activating from an erroneous AoA indication or some other fault, which problem do you solve first?
Which problem are you having?
Are you having both problems, or even something else?
That concern, the relationship between different potential failure modes, gets to the heart of
What is in the pilots manuals?
What training the crews do, or do not, get on this sub-system?
The MCAS is only supposed to activate under certain conditions (in theory): high angle-of-attack, flaps-up, flight with autopilot off. This includes high angle of bank flight in such conditions.
CAVEAT
Take a look at that last bullet in the lower right of the figure. It's only partially correct. When the pilot overrides with yoke trim, MCAS will try again five seconds later (after the pilot has stopped) unless the triggering condition, or signal, goes away.
"In the event of erroneous AOA data, the pitch trim system can trim the stabilizer nose down in increments lasting up to 10 seconds," Boeing explained to operators in a bulletin issued last November, following the JT610 accident. "The nose down stabilizer trim movement can be stopped and reversed with the use of the electric stabilizer trim switches but may restart 5 sec. after the electric stabilizer trim switches are released. Repetitive cycles of uncommanded nose down
stabilizer continue to occur unless the stabilizer trim system is deactivated." (source)
It is this system behavior that is believed to have ultimately overcome the best efforts of the pilots in the LionAir crash. They were airborne for about six minutes before they were no longer able to overcome the horizontal stab position with their trim controls. (Caveat: current as of early 2019. When the final report comes out this may become a lot clearer, and may need revision).
There is an easy-to-understand video of the relationship between the jack screw and the horizontal stabilizer function here. Whenever the jack screw is moving the horizontal stab, a cockpit cue that this is occurring is the rotation of the manual trim wheels (seen at about 3:40 in the video).
Why is this activating during takeoff/departure?
That MCAS appears to activate(sometimes) during routine flight regimes is believed to be catching some crews by surprise - and not just the two accident crews. There is a recent article in Atlantic that excerpts some NASA reports by aircrews in the US about some cases of unexpected/odd pitch behavior in this model aircraft.
answered Mar 15 at 18:39
KorvinStarmastKorvinStarmast
3,0991330
edited yesterday
answered Mar 15 at 18:39
KorvinStarmastKorvinStarmast
3,0991330
answered Mar 15 at 18:39
KorvinStarmastKorvinStarmast
3,0991330
answered Mar 15 at 18:39
KorvinStarmastKorvinStarmast
3,0991330
3,0991330
$begingroup$
is it fair to say that MCAS controls pitch through the automated Stab Trim system? So when MCAS is activated from a high AOA sensor reading, pilots will see Trim wheels spinning forward to reduce pitch?
$endgroup$
– Devil07
2 days ago
$begingroup$
@Devil07 Which line are you getting that from? For that matter, what you just asked is its own question. suggest you ask it. I am addressing the diasabling of that feature. Whatever disables the trim beyond manual control is what disables the MCAS function. Apparently, trim over ride from the yoke does not. I've seen an explanation of the details ... give me a chance to find a link.
$endgroup$
– KorvinStarmast
2 days ago
$begingroup$
@Devil07 FWIW, I have been watching the arguments about the stab trim system and this systems interaction for a few months, since Lion Air, and am leary of jumping into that briar patch.
$endgroup$
– KorvinStarmast
2 days ago
$begingroup$
you're answer is great, I'm just trying to understand how it all works together. If MCAS causes aircraft to pitch nose down when it senses excessive AOA, then doesn't it do it through use of trim? If so, when MCAS is pitching nose down, won't the trim wheels physically turn by themselves? Your graphic says "MCAS moves horizontal stabilizer trim".
$endgroup$
– Devil07
2 days ago
1
$begingroup$
To answer your question, yes, as explained in this video
$endgroup$
– KorvinStarmast
2 days ago
|
show 3 more comments
$begingroup$
is it fair to say that MCAS controls pitch through the automated Stab Trim system? So when MCAS is activated from a high AOA sensor reading, pilots will see Trim wheels spinning forward to reduce pitch?
$endgroup$
– Devil07
2 days ago
$begingroup$
@Devil07 Which line are you getting that from? For that matter, what you just asked is its own question. suggest you ask it. I am addressing the diasabling of that feature. Whatever disables the trim beyond manual control is what disables the MCAS function. Apparently, trim over ride from the yoke does not. I've seen an explanation of the details ... give me a chance to find a link.
$endgroup$
– KorvinStarmast
2 days ago
$begingroup$
@Devil07 FWIW, I have been watching the arguments about the stab trim system and this systems interaction for a few months, since Lion Air, and am leary of jumping into that briar patch.
$endgroup$
– KorvinStarmast
2 days ago
$begingroup$
you're answer is great, I'm just trying to understand how it all works together. If MCAS causes aircraft to pitch nose down when it senses excessive AOA, then doesn't it do it through use of trim? If so, when MCAS is pitching nose down, won't the trim wheels physically turn by themselves? Your graphic says "MCAS moves horizontal stabilizer trim".
$endgroup$
– Devil07
2 days ago
1
$begingroup$
To answer your question, yes, as explained in this video
$endgroup$
– KorvinStarmast
2 days ago
$begingroup$
is it fair to say that MCAS controls pitch through the automated Stab Trim system? So when MCAS is activated from a high AOA sensor reading, pilots will see Trim wheels spinning forward to reduce pitch?
$endgroup$
– Devil07
2 days ago
$begingroup$
is it fair to say that MCAS controls pitch through the automated Stab Trim system? So when MCAS is activated from a high AOA sensor reading, pilots will see Trim wheels spinning forward to reduce pitch?
$endgroup$
– Devil07
2 days ago
$begingroup$
@Devil07 Which line are you getting that from? For that matter, what you just asked is its own question. suggest you ask it. I am addressing the diasabling of that feature. Whatever disables the trim beyond manual control is what disables the MCAS function. Apparently, trim over ride from the yoke does not. I've seen an explanation of the details ... give me a chance to find a link.
$endgroup$
– KorvinStarmast
2 days ago
$begingroup$
@Devil07 Which line are you getting that from? For that matter, what you just asked is its own question. suggest you ask it. I am addressing the diasabling of that feature. Whatever disables the trim beyond manual control is what disables the MCAS function. Apparently, trim over ride from the yoke does not. I've seen an explanation of the details ... give me a chance to find a link.
$endgroup$
– KorvinStarmast
2 days ago
$begingroup$
@Devil07 FWIW, I have been watching the arguments about the stab trim system and this systems interaction for a few months, since Lion Air, and am leary of jumping into that briar patch.
$endgroup$
– KorvinStarmast
2 days ago
$begingroup$
@Devil07 FWIW, I have been watching the arguments about the stab trim system and this systems interaction for a few months, since Lion Air, and am leary of jumping into that briar patch.
$endgroup$
– KorvinStarmast
2 days ago
$begingroup$
you're answer is great, I'm just trying to understand how it all works together. If MCAS causes aircraft to pitch nose down when it senses excessive AOA, then doesn't it do it through use of trim? If so, when MCAS is pitching nose down, won't the trim wheels physically turn by themselves? Your graphic says "MCAS moves horizontal stabilizer trim".
$endgroup$
– Devil07
2 days ago
$begingroup$
you're answer is great, I'm just trying to understand how it all works together. If MCAS causes aircraft to pitch nose down when it senses excessive AOA, then doesn't it do it through use of trim? If so, when MCAS is pitching nose down, won't the trim wheels physically turn by themselves? Your graphic says "MCAS moves horizontal stabilizer trim".
$endgroup$
– Devil07
2 days ago
1
1
$begingroup$
To answer your question, yes, as explained in this video
$endgroup$
– KorvinStarmast
2 days ago
$begingroup$
To answer your question, yes, as explained in this video
$endgroup$
– KorvinStarmast
2 days ago
|
show 3 more comments
$begingroup$
You can disable the stabaliser trim which will prevent MCAS from making inputs to the aircraft.
answered Mar 15 at 19:10
WindshearWindshear
1313
New contributor
Windshear is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
1
$begingroup$
This is what I was thinking. If MCAS is pitching nose forward through use of stab trim, then the trim wheel will be visibly spinning forward (or will it?), if it is, then disable it and take manual control of trim.
$endgroup$
– Devil07
2 days ago
3
$begingroup$
Yes, the trim wheels will spin in case MCAS sends input. By disabling the stab trim, inputs to it by any system will be disabled.
$endgroup$
– Windshear
2 days ago
1
$begingroup$
that what I thought. Good answer and great picture!
$endgroup$
– Devil07
yesterday
$begingroup$
(stabaliser → stabiliser)
$endgroup$
– Peter Mortensen
yesterday
add a comment |
$begingroup$
You can disable the stabaliser trim which will prevent MCAS from making inputs to the aircraft.
answered Mar 15 at 19:10
WindshearWindshear
1313
New contributor
Windshear is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
1
$begingroup$
This is what I was thinking. If MCAS is pitching nose forward through use of stab trim, then the trim wheel will be visibly spinning forward (or will it?), if it is, then disable it and take manual control of trim.
$endgroup$
– Devil07
2 days ago
3
$begingroup$
Yes, the trim wheels will spin in case MCAS sends input. By disabling the stab trim, inputs to it by any system will be disabled.
$endgroup$
– Windshear
2 days ago
1
$begingroup$
that what I thought. Good answer and great picture!
$endgroup$
– Devil07
yesterday
$begingroup$
(stabaliser → stabiliser)
$endgroup$
– Peter Mortensen
yesterday
add a comment |
$begingroup$
You can disable the stabaliser trim which will prevent MCAS from making inputs to the aircraft.
answered Mar 15 at 19:10
WindshearWindshear
1313
New contributor
Windshear is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
You can disable the stabaliser trim which will prevent MCAS from making inputs to the aircraft.
answered Mar 15 at 19:10
WindshearWindshear
1313
New contributor
Windshear is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
answered Mar 15 at 19:10
WindshearWindshear
1313
New contributor
Windshear is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
answered Mar 15 at 19:10
WindshearWindshear
1313
answered Mar 15 at 19:10
WindshearWindshear
1313
1313
New contributor
Windshear is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
New contributor
Windshear is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
Windshear is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
1
$begingroup$
This is what I was thinking. If MCAS is pitching nose forward through use of stab trim, then the trim wheel will be visibly spinning forward (or will it?), if it is, then disable it and take manual control of trim.
$endgroup$
– Devil07
2 days ago
3
$begingroup$
Yes, the trim wheels will spin in case MCAS sends input. By disabling the stab trim, inputs to it by any system will be disabled.
$endgroup$
– Windshear
2 days ago
1
$begingroup$
that what I thought. Good answer and great picture!
$endgroup$
– Devil07
yesterday
$begingroup$
(stabaliser → stabiliser)
$endgroup$
– Peter Mortensen
yesterday
add a comment |
1
$begingroup$
This is what I was thinking. If MCAS is pitching nose forward through use of stab trim, then the trim wheel will be visibly spinning forward (or will it?), if it is, then disable it and take manual control of trim.
$endgroup$
– Devil07
2 days ago
3
$begingroup$
Yes, the trim wheels will spin in case MCAS sends input. By disabling the stab trim, inputs to it by any system will be disabled.
$endgroup$
– Windshear
2 days ago
1
$begingroup$
that what I thought. Good answer and great picture!
$endgroup$
– Devil07
yesterday
$begingroup$
(stabaliser → stabiliser)
$endgroup$
– Peter Mortensen
yesterday
1
1
$begingroup$
This is what I was thinking. If MCAS is pitching nose forward through use of stab trim, then the trim wheel will be visibly spinning forward (or will it?), if it is, then disable it and take manual control of trim.
$endgroup$
– Devil07
2 days ago
$begingroup$
This is what I was thinking. If MCAS is pitching nose forward through use of stab trim, then the trim wheel will be visibly spinning forward (or will it?), if it is, then disable it and take manual control of trim.
$endgroup$
– Devil07
2 days ago
3
3
$begingroup$
Yes, the trim wheels will spin in case MCAS sends input. By disabling the stab trim, inputs to it by any system will be disabled.
$endgroup$
– Windshear
2 days ago
$begingroup$
Yes, the trim wheels will spin in case MCAS sends input. By disabling the stab trim, inputs to it by any system will be disabled.
$endgroup$
– Windshear
2 days ago
1
1
$begingroup$
that what I thought. Good answer and great picture!
$endgroup$
– Devil07
yesterday
$begingroup$
that what I thought. Good answer and great picture!
$endgroup$
– Devil07
yesterday
$begingroup$
(stabaliser → stabiliser)
$endgroup$
– Peter Mortensen
yesterday
$begingroup$
(stabaliser → stabiliser)
$endgroup$
– Peter Mortensen
yesterday
add a comment |
$begingroup$
From what I have read, the other answers explain the cutout procedure correctly. It is also important to keep in mind Korvin's remark that trim anomalies can occur for a variety of reasons. What I have not seen mentioned often or clearly enough is a crucial difference that distinguishes MCAS from regular trim control, a difference which was apparently not communicated clearly enough to the pilots. As an article by Leeham news points out:
[MCAS is] not stopped by the Pilot pulling the Yoke, which for normal trim from the autopilot or runaway manual trim triggers trim hold sensors.1
This is a difference to previous B737 models which was not mentioned in the manual (as there was no reference to the MCAS at all) and thus was unknown to pilots before the FAA advisory was published.2 From the NASA incident reports published in the Atlantic it becomes clear that the FAA advisory after the LionAir crash was discussed among the pilots. Apparently there were a few incidents where performing the proper cutout procedure saved the airplanes, among them the preceding LionAir flight.
1 The reason for this design decision being that providing the pilot a way to easily override
would negate why MCAS was implemented, the Pilot pulling so hard on the Yoke that the aircraft is flying close to stall.
In other words, MCAS is a mechanism installed to correct an already present pilot error. Logically, this corrective intervention by the flight computer can only be cancelled through a dedicated operating sequence. It's also worth mentioning that even extreme yoke action, working on the elevators only, is not able to fully compensate extreme stabilizer positions, the control surfaces MCAS uses (apart from, I suppose, creating unpleasant flight characteristics).
2 An article in the Seattle Times on March 17, 2019 presents that argument as well.
answered yesterday
Peter A. SchneiderPeter A. Schneider
206210
$endgroup$
$begingroup$
[MCAS is] not stopped by the Pilot pulling the Yoke, -- This is dangerous. Pilots reasonably expect the computer system to go away if they are actively working the controls. After all that's how everything else works. The unexpected can easily cause a pilots brain to lock , especially when they think "I'M ALREADY DOING THE RIGHT ACTION WHY ISN'T IT WORKING!?!?!", and they continue to (ineffectively) fight the controls as the ground gets closer and closer, the stick is shaking and alarms are going nuts.
$endgroup$
– ggb667
6 hours ago
add a comment |
$begingroup$
From what I have read, the other answers explain the cutout procedure correctly. It is also important to keep in mind Korvin's remark that trim anomalies can occur for a variety of reasons. What I have not seen mentioned often or clearly enough is a crucial difference that distinguishes MCAS from regular trim control, a difference which was apparently not communicated clearly enough to the pilots. As an article by Leeham news points out:
[MCAS is] not stopped by the Pilot pulling the Yoke, which for normal trim from the autopilot or runaway manual trim triggers trim hold sensors.1
This is a difference to previous B737 models which was not mentioned in the manual (as there was no reference to the MCAS at all) and thus was unknown to pilots before the FAA advisory was published.2 From the NASA incident reports published in the Atlantic it becomes clear that the FAA advisory after the LionAir crash was discussed among the pilots. Apparently there were a few incidents where performing the proper cutout procedure saved the airplanes, among them the preceding LionAir flight.
1 The reason for this design decision being that providing the pilot a way to easily override
would negate why MCAS was implemented, the Pilot pulling so hard on the Yoke that the aircraft is flying close to stall.
In other words, MCAS is a mechanism installed to correct an already present pilot error. Logically, this corrective intervention by the flight computer can only be cancelled through a dedicated operating sequence. It's also worth mentioning that even extreme yoke action, working on the elevators only, is not able to fully compensate extreme stabilizer positions, the control surfaces MCAS uses (apart from, I suppose, creating unpleasant flight characteristics).
2 An article in the Seattle Times on March 17, 2019 presents that argument as well.
answered yesterday
Peter A. SchneiderPeter A. Schneider
206210
$endgroup$
$begingroup$
[MCAS is] not stopped by the Pilot pulling the Yoke, -- This is dangerous. Pilots reasonably expect the computer system to go away if they are actively working the controls. After all that's how everything else works. The unexpected can easily cause a pilots brain to lock , especially when they think "I'M ALREADY DOING THE RIGHT ACTION WHY ISN'T IT WORKING!?!?!", and they continue to (ineffectively) fight the controls as the ground gets closer and closer, the stick is shaking and alarms are going nuts.
$endgroup$
– ggb667
6 hours ago
add a comment |
$begingroup$
From what I have read, the other answers explain the cutout procedure correctly. It is also important to keep in mind Korvin's remark that trim anomalies can occur for a variety of reasons. What I have not seen mentioned often or clearly enough is a crucial difference that distinguishes MCAS from regular trim control, a difference which was apparently not communicated clearly enough to the pilots. As an article by Leeham news points out:
[MCAS is] not stopped by the Pilot pulling the Yoke, which for normal trim from the autopilot or runaway manual trim triggers trim hold sensors.1
This is a difference to previous B737 models which was not mentioned in the manual (as there was no reference to the MCAS at all) and thus was unknown to pilots before the FAA advisory was published.2 From the NASA incident reports published in the Atlantic it becomes clear that the FAA advisory after the LionAir crash was discussed among the pilots. Apparently there were a few incidents where performing the proper cutout procedure saved the airplanes, among them the preceding LionAir flight.
1 The reason for this design decision being that providing the pilot a way to easily override
would negate why MCAS was implemented, the Pilot pulling so hard on the Yoke that the aircraft is flying close to stall.
In other words, MCAS is a mechanism installed to correct an already present pilot error. Logically, this corrective intervention by the flight computer can only be cancelled through a dedicated operating sequence. It's also worth mentioning that even extreme yoke action, working on the elevators only, is not able to fully compensate extreme stabilizer positions, the control surfaces MCAS uses (apart from, I suppose, creating unpleasant flight characteristics).
2 An article in the Seattle Times on March 17, 2019 presents that argument as well.
answered yesterday
Peter A. SchneiderPeter A. Schneider
206210
$endgroup$
From what I have read, the other answers explain the cutout procedure correctly. It is also important to keep in mind Korvin's remark that trim anomalies can occur for a variety of reasons. What I have not seen mentioned often or clearly enough is a crucial difference that distinguishes MCAS from regular trim control, a difference which was apparently not communicated clearly enough to the pilots. As an article by Leeham news points out:
[MCAS is] not stopped by the Pilot pulling the Yoke, which for normal trim from the autopilot or runaway manual trim triggers trim hold sensors.1
This is a difference to previous B737 models which was not mentioned in the manual (as there was no reference to the MCAS at all) and thus was unknown to pilots before the FAA advisory was published.2 From the NASA incident reports published in the Atlantic it becomes clear that the FAA advisory after the LionAir crash was discussed among the pilots. Apparently there were a few incidents where performing the proper cutout procedure saved the airplanes, among them the preceding LionAir flight.
1 The reason for this design decision being that providing the pilot a way to easily override
would negate why MCAS was implemented, the Pilot pulling so hard on the Yoke that the aircraft is flying close to stall.
In other words, MCAS is a mechanism installed to correct an already present pilot error. Logically, this corrective intervention by the flight computer can only be cancelled through a dedicated operating sequence. It's also worth mentioning that even extreme yoke action, working on the elevators only, is not able to fully compensate extreme stabilizer positions, the control surfaces MCAS uses (apart from, I suppose, creating unpleasant flight characteristics).
2 An article in the Seattle Times on March 17, 2019 presents that argument as well.
answered yesterday
Peter A. SchneiderPeter A. Schneider
206210
edited 6 hours ago
answered yesterday
Peter A. SchneiderPeter A. Schneider
206210
answered yesterday
Peter A. SchneiderPeter A. Schneider
206210
answered yesterday
Peter A. SchneiderPeter A. Schneider
206210
206210
$begingroup$
[MCAS is] not stopped by the Pilot pulling the Yoke, -- This is dangerous. Pilots reasonably expect the computer system to go away if they are actively working the controls. After all that's how everything else works. The unexpected can easily cause a pilots brain to lock , especially when they think "I'M ALREADY DOING THE RIGHT ACTION WHY ISN'T IT WORKING!?!?!", and they continue to (ineffectively) fight the controls as the ground gets closer and closer, the stick is shaking and alarms are going nuts.
$endgroup$
– ggb667
6 hours ago
add a comment |
$begingroup$
[MCAS is] not stopped by the Pilot pulling the Yoke, -- This is dangerous. Pilots reasonably expect the computer system to go away if they are actively working the controls. After all that's how everything else works. The unexpected can easily cause a pilots brain to lock , especially when they think "I'M ALREADY DOING THE RIGHT ACTION WHY ISN'T IT WORKING!?!?!", and they continue to (ineffectively) fight the controls as the ground gets closer and closer, the stick is shaking and alarms are going nuts.
$endgroup$
– ggb667
6 hours ago
$begingroup$
[MCAS is] not stopped by the Pilot pulling the Yoke, -- This is dangerous. Pilots reasonably expect the computer system to go away if they are actively working the controls. After all that's how everything else works. The unexpected can easily cause a pilots brain to lock , especially when they think "I'M ALREADY DOING THE RIGHT ACTION WHY ISN'T IT WORKING!?!?!", and they continue to (ineffectively) fight the controls as the ground gets closer and closer, the stick is shaking and alarms are going nuts.
$endgroup$
– ggb667
6 hours ago
$begingroup$
[MCAS is] not stopped by the Pilot pulling the Yoke, -- This is dangerous. Pilots reasonably expect the computer system to go away if they are actively working the controls. After all that's how everything else works. The unexpected can easily cause a pilots brain to lock , especially when they think "I'M ALREADY DOING THE RIGHT ACTION WHY ISN'T IT WORKING!?!?!", and they continue to (ineffectively) fight the controls as the ground gets closer and closer, the stick is shaking and alarms are going nuts.
$endgroup$
– ggb667
6 hours ago
add a comment |
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15
$begingroup$
This question could be just as answerable without bringing up the two recent 737 Max crashes at all. You might consider just editing that part out; that would go a long way toward alleviating concerns that you want speculation on an accident, rather than operational details on an airplane.
$endgroup$
– a CVn
Mar 15 at 19:08
14
$begingroup$
@aCVn I think context is important. Many people use this site, not all are aviation nerds, by providing context to the question it establishes why the question and answers, are relevant and important. At least, thats my opinion. :)
$endgroup$
– Devil07
2 days ago
2
$begingroup$
Related: Can computer imposed inputs be overridden on the Boeing 737-MAX?
$endgroup$
– ymb1
2 days ago
2
$begingroup$
While this is an interesting question, if we're considering it for practical purposes, we also ought to ask what the purpose of MCAS is and what the consequences of disabling it would be.
$endgroup$
– David K
yesterday
$begingroup$
The purpose of MCAS is to avoid stalls. It does that by shoving the nose down. The consequences of disabling it that it won't try and auto-recover from a stall.
$endgroup$
– ggb667
7 hours ago