How do pilots handle diversions at one of the world's busiest airports?

It's been recently reported that the wait time at immigration for inbound passengers at London Heathrow is “well in excess of two hours and up to six” — and it could get worse. Reacting to these reports, officials at London Heathrow airport announced this week that they may have to force inbound aircraft to divert to other airports in the U.K. and even Europe as a last-minute resort to ease overcrowding at immigration.

This would cause problems not only for passengers but for the airlines as well. Diversions always result in knock-on delays, something that the industry could really do without right now.

So if the order was given to divert, how would it play out from the pilot's perspective?

Time is Everything

Being able to land an aircraft in a strong crosswind is useful for a pilot, but the most important skill that an airline pilot possesses is good time management. As soon as the fuel truck disconnects at the departure airport, the clock is ticking. We don't have the luxury of just "pulling over" to think things through as you can do on the road. The only way to stop that clock is to land the aircraft.

Related: ‘One of the most demanding maneuvers in aviation’ — How pilots manage crosswind landings

With that in mind, at all stages of a flight, we are always thinking "What if?" What if we were to have an engine failure now? What if a passenger was to get ill now? What if the weather at the destination meant that we were unable to land?

We are always thinking about these situations so should one of them actually happen, we already have a plan up our sleeve ready to throw into action. This even begins way before we board the aircraft, back in the Crew Briefing Center.

How much fuel?

Fuel equals time. The more fuel we have on board, the more time we have at our disposal. However, carrying too much fuel is bad for the environment and bad for an airlines' profit sheet. Why? The more fuel we carry, the heavier the aircraft becomes and the heavier the aircraft is, the more fuel we have to use to fly. As a result, a balance has to be struck between the two.

During the flight planning stage, the airline's operations department will calculate the most fuel and time-efficient route to get us to our destination. The end result is a flight plan that also informs us of the minimum fuel we need to carry in order to depart legally. However, this fuel figure isn't simply a question of just getting from A to B.

In addition to the fuel needed to complete the journey between the departure airport and destination, known as the trip fuel, we need fuel to start the engines and taxi out to the runway. Unsurprisingly, this is known as the taxi fuel. Next, flights don't always go according to plan. Sometimes the winds may not be as forecast or, once airborne, ATC may need us to fly a slightly different route.

For example, the North Atlantic is (or used to be, anyway) a very busy route. Sometimes, we may not get the crossing altitude on which the flight plan and trip fuel were predicated. This will result in more fuel being used to fly the same distance. Crossing the equator, there are often large bands of thunderstorms that we need to avoid. This can result in hundreds of extra miles flown. To cover these eventualities, we always carry a certain amount of contingency fuel, normally 5% of the trip fuel.

However, we can never be 100% sure that we will be able to land at our destination airport. The weather might deteriorate, an incident may close the airport or, as mentioned earlier, we could be directed to divert to another airport due to capacity issues. This all comes down to the "What if?" scenarios pilots are always thinking about.

As result, we always carry fuel to divert to an alternate airport. Most of the time, if the weather is expected to be fine, this will be an airport relatively close to the destination, keeping the diversion fuel to a minimum. However, on days where there is bad weather expected, we may change our choice of alternate airfield to one a bit farther away and carry more fuel.

With the current situation, if we know that there is a chance that we may have to divert to another airport because of capacity issues at Heathrow, we may decide to take more fuel for the flight.

The earlier, the better

As our flight progresses overnight across the Atlantic towards London, we are in constant communication not only with ATC but also with the airline's operations department via a form of text messaging. Datalink enables us to exchange messages with the ground that can give us a heads-up as to what to expect as we get closer to London. In times of mass disruption, for example, during severe snowstorms, the operations department will be thinking hours ahead of our arrival.

If capacity at Heathrow will be greatly reduced, they will take proactive action to divert aircraft to alternate airfields before they even get close to Heathrow. This may seem defeatist, but it's far better to divert early and land safely at another airfield than it is to continue to the destination to hold and then be the last of the pack trying to find somewhere to land.

Duty hours

By making the decision early, not only does it enable us to preserve our fuel, but it also reduces the impact on the crew duty hours. For the safety of everyone onboard the aircraft, pilots and cabin crew are limited to how long they are able to work in one shift, the Flight Duty Period, or FDP.

The length of the FDP depends on what time the duty started and how many sectors the crew is flying. In short, the more unsociable the time the duty commenced and the more sectors the crew will perform, the shorter the FDP. This time can be extended by the addition of an extra pilot and the use of inflight rest.

If a flight was to divert to another airfield, the first thing the crew would do is to calculate when they will be "out of hours." For example, a flight departing Los Angeles at 3 p.m. with three pilots would have a maximum FDP of 16 hours. If the aircraft continued to Heathrow, entered a holding pattern for 30 minutes and then was told to divert, a significant chunk of time would have been taken out of that 16-hour period.

However, if we are told well in advance of our arrival into Heathrow that we will need to divert, we can preserve some of that time and give ourselves the best chance of making it back to Heathrow before our FDP expires.

The Diversion Process

Once the decision has been made to divert, the workload in the flight deck increases dramatically. There are a number of tasks that must be completed before we are able to land at the diversion airport, so dividing the jobs between both pilots (or three pilots on a long-range flight) is key.

The time available is the driving force in prioritizing those tasks.

If you’ve read any of my previous stories, you’ll probably be familiar with the “Fly, Navigate, Communicate” philosophy. If not, the basic premise is that before engaging in any other tasks, make sure one pilot’s sole focus is on the flight path of the aircraft. Next, they need to make sure that the aircraft is traveling exactly where they want it to be. Finally, the other pilot can then communicate with the relevant people to arrange the diversion.

Related: Behind the scenes: What goes on in the flight deck during a diversion?

Fly

The first task is to get the aircraft pointing in the right direction. In most cases, permission will be required from ATC to deviate from our route. However, with technical issues, we may need to get the aircraft moving away from its current flight path before we are able to talk to ATC. In the case of an engine shut down over the Atlantic, this will be done within a few seconds.

In the case of having to divert due to a lack of capacity at Heathrow, it is likely that ATC will ask us in advance what our preferred diversion airport would be. If holding close to Heathrow, we would also let them know by what time we will need to start that diversion to ensure that we have enough fuel.

Navigate

With the diversion approved by ATC, we can start to reprogram the Flight Management Computer (FMC) to navigate the aircraft towards the diversion airport. This requires one pilot to change the destination and then enter the route as prescribed by ATC.

Once we know where we’re going laterally, we also need to think about what we’re doing vertically. The diversion may take us across a mountain range. If so, what’s the lowest altitude we can safely descent to? A new Minimum Enroute Altitude (MEA) must be confirmed by checking the onboard charts.

The next task is to obtain the latest airfield information from the diversion airport. This will provide the runway and approach in use and also the weather conditions. Once these are known, further changes are made to the FMC to ensure that the aircraft is set up to fly the desired approach type. The airfield charts must then be selected on both pilots’ iPads to prepare for an arrival brief at the new airport.

Communicate

The final part of the process is nice to do but not necessarily required. As mentioned above, the time available is what prioritizes the tasks. This is why the "communicate" part comes last.

As we will already have spoken to ATC, this part will involve letting all the relevant stakeholders aware of our diversion. First up is normally the cabin crew.

If it’s a medical diversion, they will probably already be aware that the plan is to divert. However, with a diversion like this, they most probably will not be aware.

With the crew aware of the plan, we are then able to speak to the passengers over the PA system. The crew will then be able to answer any questions they may have.

Finally, if there is time, we will try to let the airline’s operations department know that we are diverting. This will give them the opportunity to contact the ground handling agents and give them time to prepare for our arrival. However, if time is short, this step will be omitted.

Bottom Line

Diverting an aircraft away from its planned destination causes problems for passengers, crew and airlines. Passengers' journeys are delayed, crew workload increases and airlines can find their aircraft out of position for its next flight.

However, the earlier the decision to divert is made, the more we can reduce the knock of effects. Giving crews greater notice allows them to maintain fuel levels and then have the aircraft ready to depart the diversion airfield again just as soon as they are given the green light.