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Suppose Indigo lands at Indira Gandhi international airport and the arrival is schedule at 5:30 a.m. Is the runway to land on fixed by the airline.

If not please explain reason behind not to schedule the runway on which the flights will land.

No one can "book" runways. Traffic on a runway is managed by air traffic control, whose job it is to ensure a safe and expeditious flow of air traffic. Generally speaking, a place on the runway is assigned on a first-come first-served basis.

You may want to explore the air traffic control tag to learn more.

In the USA at the large international busy airports, the airlines have to reserve a landing time slot for arrivals. Indira Gandhi international airport might do the same. "A landing slot, takeoff slot, or airport slot is a permission granted by the owner of an airport designated as Level 3 (Coordinated Airport), which allows the grantee to schedule a landing or departure at that airport during a specific time period."

The choice of runway is determined generally by the wind direction. However,
"Delhi Airport has three near-parallel runways:

• runway 11/29, 4,430 m × 60 m (14,530 ft × 200 ft) with CAT IIIB instrument landing system (ILS) on both sides


• runway 10/28, 3,810 m × 46 m (12,500 ft × 151 ft)


• runway 09/27, 2,813 m × 45 m (9,229 ft × 148 ft)."

So choice of runway might be driven more by how low the clouds (ceiling) are and the equipment on the airplane.

The reason why airports don't schedule exact times and runways for take-off and landings are that there are far too many factors that are unpredictable in advance to know exactly when a plane is going to take off or land or which runway(s) will be available at that time.

If a plane arrives at an airport earlier than scheduled, it makes no sense to have them fly holding patterns wasting fuel instead of just going ahead and sequencing them to land. On the flip side, if a plane arrives later than scheduled, they still have to land. You can't just tell them, "Sorry, you missed your scheduled time, you have to divert or crash now." Similarly, if there are a bunch of planes waiting to use a particular runway (or if a particular runway is unavailable,) there's no reason not to just use another runway, provided there's another one available that meets the wind and performance needs of a given aircraft.

Flight times for airliners are typically scheduled many months in advance. Yet, many factors that can affect their departure, arrival, and total flight times aren't knowable more than a few hours in advance, if that.

Some of the factors that can affect times include:

• Weather shuts down the departure airport during the normal departure time.


• The aircraft is out-of-position due to an earlier delayed flight.


• Part or all of the flight crew and/or cabin crew is out-of-position due to an earlier delayed flight.


• The aircraft has a maintenance issue that causes a delayed departure.


• The planned aircraft has a maintenance issue that prevents it from departing entirely and another aircraft has to be brought in to operate the flight.


• Another aircraft had an emergency and required priority handling.


• Wind. Winds aloft vary by the day and even by the hour. Schedules planned months in advance can only guess how long the actual flight is going to take based on seasonal averages. Actual weather is very often quite different from seasonal averages. Strong tail winds can allow a flight to be completed much faster than expected, while strong head winds can cause a flight to take much longer than expected or, in extreme cases, even cause it to have to divert to add fuel before continuing on to the destination.


• Weather shuts down the arrival airport, causing the flight to circle until the weather clears, divert to another airport, or, if known far enough in advance, hold at its departure airport until the weather clears at the destination.


• Weather en route shuts down the possible routes for the flight for a while, forcing it to wait, even if the weather at the origin and destination airports is clear.


• Passengers board slowly, preventing the aircraft from pushing back on time.


• Cargo isn't loaded quickly enough, preventing the aircraft from pushing back on time.


• The flight is delayed by an ATC, pilot, transit worker, etc. strike (especially if the flight is in or near France.)


• Passengers are connecting to the flight from another flight which arrived late and the airline decides to hold the plane long enough for the connecting passengers to make the connection.


• The power fails at the arrival or departure airport.


• The airline suffers a major IT system failure.


• A passenger (or crew member) has a sudden health emergency.


• The military attempted a coup in the arrival country, the departure country, or a country the flight needed to fly over.


• The surrounding countries decide to close their airspace to the airline's country.


• 
  Someone set an Air Route Traffic Control Center on fire along the plane's route.

This is far from being an exhaustive list, but it should give a feel for why it's completely infeasible to schedule exact times for aircraft to takeoff or land on a given runway.

Similarly, there are several reasons why reserving a particular runway in advance is often not feasible:

• Wind. At the vast majority of airports, different runways will be active depending on current wind direction and magnitude. As much as possible, you want airplanes to be both taking off and landing into the wind in order to allow for slower ground speeds during takeoff and landing (and, thus, less runway distance needed.)


• Several larger aircraft may be arriving or departing that need a longer runway, so other traffic may be moved to a shorter one.


• An emergency aircraft may require one of the runways to be closed to other traffic for a while.


• A runway may be closed for maintenance (e.g. repainting stripes, removing rubber build-up, resurfacing, expansion, etc.)


• A runway may be closed due to maintenance of other equipment in the runway environment (e.g. the lights, ILS, etc.)


• A runway may be closed due to snow and/or ice (or the removal thereof.)


• A runway may be closed for routine inspection.


• A runway may be closed because someone decided to schedule a 5 km run on it.

This list is also far from being exhaustive.

Instead, airlines are assigned a specific block of time to use a gate along with a slot to land and takeoff. This allows the airport to plan the number of arrivals and departures that are likely to occur within a given hour to be less than the maximum capacity the runways are usually capable of handling, while not actually assigning a specific order for those arrivals and departures until they're actually ready for takeoff or sequencing to land. Then ATC can assign the most efficient runway currently available to each aircraft as they actually queue up for arrival or departure.

Even this is far from fool-proof, though. It's much better at handling real-world scenarios than scheduling exact departure and arrival times for each airplane would be, but events that affect a significant portion of the flights at the airport can still quickly lead to delays, especially during busy times of the day. Examples of such events would be storms near the airport, high winds at the airport, snow/ice at the airport, low visibility at the airport requiring more than normal separation between aircraft, a security incident at the airport, or the airport being LaGuardia.