Oct 212015

Sometimes, you may get an information that your flight is delayed due to the Air Traffic Control (ATC). Why do Air Traffic Control units delay your flight? These units work in the background and a common passenger hardly knows about them unless there is any specific reason. Air Traffic Controllers work hard all the day and night, throughout the year to keep you safe in the sky. When they delay your flight in interest of safety, they have unavoidable reasons to do so.

Most of time, you don’t know the reasons behind these events and starts guessing by your own logic. You start cursing the airlines, the pilots, the ground staff or the air traffic controllers for these delays. But in reality, the reasons behind delay are beyond the control of any personnel. In this post, I will explain some facts and situations about these delays, so that you can analyse and understand them in a better manner.

Spice Jet Flight Departing From Delhi Airport

Spice Jet Flight Departing From Delhi Airport

1. Why does ATC delay your departure on tarmac? Air Traffic Controllers don’t have any reason to delay your flight intentionally. If your flight gets delayed because of ATC, then the probable reasons are:

i). VVIP movement at the airport: VVIPs may use separate terminals to board or de-board a flight, but they use the same runway, used by other commercial airlines. Every VVIP flight has some associated security procedures and protocols. When a VVIP movement starts, your scheduled flight, sometimes, doesn’t get approval to start engines, taxi out from the stand or to land on that runway within a specified period of time.

This delay is hard to determined, but most of the times, it is not more than 10 minutes. However, at the airports like Delhi, where aircraft take-off or land at every minute during the peak hours, this 10-15 minutes delay can make a huge impact. Delhi, being the capital city, has frequent VVIP movements, sometimes upto 5-6 movements in a day, that leads to the significant delays for the commercial flights on that particular day.

ii). Your flight is far behind in the sequence: At the big airports like Delhi and Mumbai, there are about 60-70 flight movements in the peak hours. So, every flight has a time slot. They have to start and depart within that time window. If they miss their slot, they lose their sequence and get a new slot. It may put them after other flights departing on time.

Sometimes, 5 aircraft ask simultaneously to start their engines within a short span of 1-2 minute. If all 5 aircraft in such cases are parked close to each other on the tarmac, a flight may become number 3, 4 or whatever depending on its sequence. And, if there is 5 minutes delay for one flight, then third flight will be obviously get delayed upto 15 minutes.

iii). Constraints at the destination airport: Sometimes, the destination of your flight also play a significant role in the flight delay. A flight may delayed because of the non-availability of parking stand at the destination, bad weather en-route or at the destination and any other reasons. If you fly to a small airport like Dehradun, where parking space is limited, your departure may get delayed unless a vacant stand is available. If you fly to the airport like Leh or Srinagar, your flight operations can be significantly hampered by bad weather conditions at these airport.

iv). Delay to Westbound flights going to USA and Europe from India, e.g., Delhi: Most of the flights going to Europe/USA from Delhi overfly Pakistan and Afghanistan airspaces. The minimum height that an aircraft can fly on this route is 32000 feet during daytime . A modern jetliner fly upto the height of 40000 feet normally. But, most of the flights prefer to fly at 32000 feet, 34000 feet and 36000 feet (These heights are known as Flight Levels in aviation). This two thousands feet gap is necessary, because Eastbound traffic fly in between at odd heights e.g., 31000 feet, 33000 feet etc.

If there are two aircraft at a single height, say 32000 feet, then they have to maintain 10 minutes time gap between each other. Suppose, there are two flights going from South East Asia (Bangkok, Singapore etc) to Europe (overflying India) at 32000 feet at the interval of 10 minutes, and they don’t want to climb higher. Now, there are a departure from Delhi on the same route that also want to fly at 32000 feet, so it has to be fix behind the second flight after 10 minutes time gap, and then the next flight behind this departure is again 10 minutes behind. There are lot of calculations involved and your departure flight may delayed in this case.

2. Why do some flights hold near the runway for significant time before taking-off? Again, there can be many reasons for this. Some probable reasons are:

i). Your flight is again in sequence for take-off: When your flight reach near a runway, it may become number 5 aircraft in the queue, waiting for take-off clearance. A single aircraft normally takes two minutes for the take-off, so number 5 plane will take-off after 10 minutes. And, if there is an arrival on the same runway, the departure has to be delayed further to accommodate that arrival. If there are two-three arrivals before the number 5 flight, this delay will be more .

In Delhi, it’s very rare that more than 2-3 flight reach at the take-off point simultaneously, because Air Traffic Controllers use some advance procedures to deal with these delays. Normally, a flight gets clearance to start only, when ATC think that it can depart without any delay at the take-off point.

ii). Runway is not clear due some unavoidable reasons: Sometimes, previous landing aircraft or previous departing aircraft may encounter a bird strike during it’s roll. In that case, ATC is bound to carry out an inspection of entire runway by operational jeep. This runway inspection is also mandatory during VVIP movement, after an emergency landing and also, after certain intervals in a single day. It may delay your flight by few minutes.

3. Why does your flight circle over your destination sometimes upto 20-30 minutes? This is mainly due to the congestion, again, because of above-stated reasons like, weather, VVIP movements, bird strike on the runway etc. At an airport like Delhi or Mumbai, where 20-30 aircraft approach to land within a short span of time, this delay is sometimes very significant.

Every aircraft normally needs two minutes to land on the runway, and only one aircraft can land at a time. So, 20th aircraft will land only after 30-40 minutes. Also most of the time, there are some departures planned from the same runway between these arrivals, which can further delay your flight. Sometimes, the aircraft absorb these delays during the cruise flight by reducing their speed. But, if they arrive early at their destinations, holding them over the city is only available option.

4. Why does your plane keep moving for 20-25 minutes after the landing? It depends on how big is your destination airport and how far is the runway from the terminal building. For example, if you land on Runway 11 at Delhi Airport and your flight is intended to park near Domestic Terminal, your plane moves on the ground for at least 20-25 minutes to reach its parking bay. Sometimes, your allocated parking stand remains occupied by another aircraft and the controllers don’t have an alternate stand. So, your flight is forced to wait for that stand after arrival.
However, in Delhi, the controllers simultaneously use two runways (in peak hours, all three runways), so that your flight can depart and land on the runway, nearest to your parking stand without any significant delay.

5. Why does your morning flight delayed for hours in the winter? In winter season, foggy weather disrupts flight operations significantly. An Aircraft follows certain visibility criteria to take-off or land on a runway. A good visibility is necessary to fly small aircraft and helicopters, but most of the time, the bigger aircraft can take-off or land during minimal visibility also. However, during the fog, ground visibility can be NIL and it affects bigger aircraft also.

Modern Jet aircraft and big airports (Delhi also) are equipped with the most modern technology which allows safe landing in poor or even zero visibility conditions, but sometimes visibility may drop below the prescribed limit, causing a complete shut-down of operations for few minutes or may be for hours in the morning. This is why sometimes, you wait for hours at the airport, before your flight departs.

When the dense fog lifts, ATC normally has about 50 flights in the queue of departures. Similarly, there are number of flights waiting to land. Even after using two or three runways simultaneously, the accumulated delay can be hours for some flights, who falls in the end of departure list. For a rough idea, it generally takes 3-4 hours to normalize the flight schedules after lifting of the fog at busy airports.

There may be numerous reasons for the flight delay at the airlines end also, like technical snag, non-availability of aircraft, non-availability of crews, operational reasons etc. Sometimes airport operations, bad weather and Air Traffic Control limitations play significant roles in these delay and spoil your plan to enjoy a vacation, to meet a professional commitment or other important works. But, then, An Air Traffic Controller is only a person like you. He can control and direct your flight, but he can not control the weather and avoid certain situations. Trust them and let them do their job. They are perfectly trained to handle all situations to keep you safe from one destination to another.

  One Response to “Why Do Air Traffic Control Units Delay Your Flight?”

  1. very useful

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