Why do pilots sometimes deliberately delay flights?
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No one likes a delay. People on business trips miss meetings, those on long journeys miss connections and families miss out on precious time together. Airlines also don’t like delays. They can cost them money in compensation, they cause knock-on delays for subsequent flights and they can throw a spanner in the works when it comes to airport logistics.
They are also an inconvenience for the crew. You may not think about it at the time, but your crew also have a life outside work. They have kids to pick up from school, family events to attend and various appointments to attend.
There are no winners when it comes to delays.
However, every so often, a pilot may decide to delay the flight deliberately.
You may wonder if they are doing this out of spite. A chance to show that they are in charge of that aircraft and what they say goes. However, the reality is quite different.
We all want to get to our destination as quickly (but of course, as safely) as possible. Yet sometimes there are hurdles in our way that might mean that we get airborne too quickly.
Airport Opening Times
Many airports around the world operate 24 hours a day. The big middle Eastern hubs of Dubai, Abu Dhabi and Doha operate throughout the night with flights taking off and landing throughout the small hours of the morning.
It’s for this reason that they have become some of the most successful hub airports in the world. Landing late at night, passengers only have to wait a couple of hours for their connecting flight, vastly reducing their total journey time.
Whilst most other large airports are technically open 24hrs a day, they have strict restrictions on the hours during which aircraft are allowed to take off and land unless it’s an emergency.
These tend to be legacy airports which have grown from humble beginnings on the outskirts of the city but now have become surrounded by the ever-expanding built-up areas. As a result, there tends to be a conflict of interest between the airport and the nearby residents.
The airport would generally prefer to be able to allow aircraft to take off and land throughout the night. However, quite understandably, the residents nearby would quite like to sleep.
What results is a trade-off between the two to the point where there are restrictions placed on the period in which flights can operate.
One of the best examples of this is London Heathrow.
Pre 2020, Heathrow was one of the busiest airports in the world. However, even though there is no formal ban on flights operating throughout the night, defined as between 2300-0600hrs, the UK Government has placed a restriction of a maximum of 5,800 night-time take-offs and landings a year.
According to Heathrow Airport, 80% of these 5,800 flights are between 0430-0600hrs with an average of 16 flights arriving a day (pre-Covid). The airport also has a voluntary ban in place that prevents flights scheduled to land between 0430-0600 from landing before 0430 and they also do not schedule any flight to depart between 2300-0600.
There is also a quota in place that limits the amount of noise the airport can make at night. Each aircraft type is given a score depending on how much noise it makes the more noise it emits, the higher the score. If an aircraft arrives or departs between 2330-and 0600, its noise score counts toward the airport limit.
In addition to the noise quota, there’s also a movement limit that restricts the total number of flights that are allowed to operate within the night quota.
The idea of the two restrictions is that airlines are encouraged to use quieter aircraft such as the B787 and A350 and limit the number of times they are used during the hours of the night.
What results is that a certain number of flights are scheduled to land before 0600hrs, normally those coming from the far-East. A quick look at the arrivals board for Heathrow shows flights from destinations such as Singapore, Kuala Lumpur, Hong Kong and Melbourne via Darwin are all scheduled to arrive before 0600.
Then, from 0615 onwards, the early morning rush begins as flights from the rest of the world, particularly North America are scheduled to land.
However, the keyword here is ‘scheduled’. We all know that the scheduled time of arrival isn’t always the actual time of arrival. Often flights are late, but sometimes, they are early and this is where the problem arises.
For a flight scheduled to land just after 0600hrs, a quick flight time could result in the aircraft arriving at Heathrow too early. With the restriction in place, it could be forced to enter a holding pattern until the airport opens at 6 am. This will use extra fuel, which is not good for either the environment or the airline accountants.
As a result, we have a responsibility to ensure that we don’t arrive too early.
Before departure, the flight paperwork will inform us of the expected flight time. For the most part, these are incredibly accurate, able to predict the duration of the flight with an accuracy of within a few minutes.
So, if we notice that a quick flight time will have us into Heathrow before 6 am, we need to take steps to ensure that this doesn’t happen. The easiest way to do this is by delaying the flight at the gate until such time that we can push back, start our engines taxi out to the runway and then get airborne to arrive in London after 6 am.
When landing at the main hub of the airline you’re flying with, you’d be forgiven for thinking that getting a gate wouldn’t be a problem. Pitch up at whatever time, taxi onto a gate and make your way happily to baggage reclaim.
However, sometimes the bigger the operation, the more complex and intricate it becomes.
Let’s think of this from the flip side. When flying from the hub airport to a small airport with only a handful of gates, if when you land there is still an aircraft at your gate, you have to wait for it. We just accept this as it’s a small airport and that’s to be expected.
However, the same problem also exists at bigger airports — only magnified.
The allocation of gates isn’t always done by the airline. Quite often the airport itself will take responsibility for ‘stand planning’. Whether it’s the airport or the airline allocating gates, it will all be planned out in advance based on the aircraft’s scheduled landing time and then its scheduled departure time.
This creates a ‘block’ of time that the aircraft will occupy the stand.
It is the job of the stand planners to drop these blocks into particular gates so that they fit with other flights around them. There’s no point in allocating a gate to a flight that lands at 2 pm if the flight already at the gate doesn’t depart until 2.30 pm. This job will normally be done overnight so that there is a plan for the day ahead.
The problem comes when an aircraft lands well in advance of its scheduled arrival time and there is still an aircraft at the gate. Of course, in an ideal world, there would be another gate available but at peak times, this isn’t always possible.
The only alternative is for the arriving aircraft to wait until the gate has been vacated, but this can create a problem for ATC.
Many airports are busy enough with aircraft taxiing to and from runways, leaving few areas in which aircraft can just wait for their gate. In these scenarios, ATC will tend to utilise one taxiway to form a queue of aircraft waiting for their gate, keeping them out of their way as much as possible.
The best way to stop this from happening is to delay pushing back from the gate at the departure airport until such time — like with the airport opening times, the flight will arrive at a time that fits in with the schedule.
Arriving late isn’t ideal, but arriving too early can sometimes be even more problematic.
Push and Remote Hold
Now you may be thinking that if an aircraft is waiting at its gate at its departure point (e.g. New York) to avoid waiting for another aircraft at its gate at the destination (e.g. London), then surely have the knock-on effect of causing an aircraft just landed in New York to have to wait for its gate — and you’d be right.
Sitting on the gate blocks it to aircraft just landed and delays their arrival. As a result, some airports enable pilots to ‘push and remote hold’.
This involves pushing back from the gate, starting the engines and then taxiing to a quiet part of the airfield. Here, the crew can shut down the engines to save fuel until such time they are ready to head to the runway for departure.
This frees up their gate for an arrival flight, meaning that there will be fewer passengers missing connections.
If you’ve been on a flight when there is bad weather or during the peak summer months, you will no doubt be familiar with the term ‘slot’ — but what does it actually mean?
A slot is a short window of time, typically 15 minutes, within which ATC are required to get the aircraft airborne. For pilots and ATC, this is represented in the form of a Calculated Take-Off Time (CTOT), or ‘slot’.
Airspace is divided into sectors and controlled largely by the country over which the sector sits. The size of each sector depends on how busy that airspace is in terms of the number of flights. The busier the sector, the smaller its size. These sectors will then be controlled by one or more ATCOs.
Each sector has a limit to the number of aircraft that can pass through it in a given time. This is dependent on airspace geography, the number of intersecting airways and the structure of adjacent airspace.
One of the restrictions that used to be unique to flights heading from the Far East to Europe was the BOBCAT slot
I already mentioned above that the majority of flights scheduled to land in London Heathrow before the 6 am curfew originate from the Far East. The fact that they are able to land before 6 am certainly helps as there is often another restriction they have to comply with along the way — a BOBCAT slot.
One of the most fuel-efficient routes from the Far East to Europe is across India, across the border into Pakistan and then up through Afghanistan. However, Afghan airspace has an extremely limited capacity as there is no radar.
As a result of this, aircraft must have a greater lateral separation than normal, resulting in fewer aircraft being able to transit the airspace than normal.
In order to ensure that too many flights don’t arrive at the Afghan border at the same time, these flights are issued with a particular slot by the agency that deals with this airspace — the Bay Of Bengal Cooperative Air Traffic service (BOBCAT).
A flight leaving Singapore for London that is due to transit through Afghan airspace may be issued with a BOBCAT slot hours before departure. As a result, the pilots know that they have to get airborne within a certain time window to ensure that they make this slot several hours down the route.
Depart too late and they may either have to fly faster to make the BOBCAT slot or be issued with a completely new slot before departure.
Depart too early and they may have to fly slower or enter a holding pattern along the route so that they waste enough time to make their restriction.
ATC may not issue a specific CTOT, so it is up to the pilots to calculate the time at which they need to leave the gate, sometimes resulting in a delay.
Flying an aircraft requires a little more thinking than just blasting off a runway and engaging the autopilot. Careful attention to the flight time and expected arrival time are often needed to ensure that an early arrival doesn’t cause problems.
Strong winds over the Atlantic, particularly in the winter, can cause flights from North America to have significantly quicker flight times than scheduled.
So next time your flight is delayed, pay careful attention to the pilot’s announcement — you may still be arriving early. Just not by too much.
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