How Do Pilots Calculate If They Can Land Safely?

[In The News]

DALLAS – After we recently examined some of the factors that affect the performance of an aircraft during take-off, let us now take a look at what calculations are done before landing.

This insight will take a more practical approach rather than delving too deeply into the technicalities. It is worth highlighting that this topic is extremely detailed, but this will provide a more simplistic overview.

Landing performance, like takeoff performance, was previously presented to the crew using paper charts and graphs. These were often stored in bulky manuals, which were cumbersome to use and often required interpolation.

The maximum landing weight would be determined by the crew based on the headwind or tailwind component, with smaller corrections added or subtracted to account for temperature and air pressure differences.

Typically, this is only likely to be an issue when landing on short runways, when stopping distance is more limited. Thankfully, technology has largely replaced paper nowadays, with the speed and accuracy of tablets and laptops eclipsing the paper alternative.

Once the maximum landing weight has been determined, it is as simple as ensuring that the actual landing weight will be less than this limiting value. After all, the heavier an aircraft is, the greater the stopping distance needed. Once you have determined that you will be able to stop safely on the runway, the next question is how much braking is needed.

An example of how pilots can determine the most appropriate Autobrake setting on landing, with the estimated landing distance displayed next to each Autobrake setting at the bottom of the image. Pilot inputs are in green and this screenshot shows calculations for a Boeing 777 at London Heathrow (LHR) using Boeing’s OPT software. Photo: Author

How Autobrake works

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Well enough to stop is perhaps your answer, and whilst that observation is obviously correct, there is, however, a bit more to consider. Many aircraft have a system that applies the wheel brakes automatically after landing, called “Autobrake.” This is traditionally a system that provided various levels of retardation, with the pilot selecting the level of braking that was deemed most appropriate, which varies from landing to landing.

This system would be disconnected by the pilot as the landing speed decreased, which would enable the pilot to make fine manual adjustments to the brake pressure so that the desired taxiway could be reached.

Some modern types, such as the Airbus 350 and 380, employ a concept known as “Brake to Vacate,” in which the aircraft’s wheel braking is modulated to slow it to a suitable taxi speed just in time for turning off the runway at the desired taxiway. In theory, the pilot’s feet should not need to touch the brake pedals until the aircraft reaches walking speed.

The objective of this more modern system is to remove the need for the pilot to disconnect the Autobrake and taper the braking manually in the final stages of the landing roll. With this more advanced system, the desired taxiway is programmed prior to landing, and GPS technology monitors the location of the aircraft on the runway during the landing roll, thereby knowing the runway distance remaining. The same technology can also rapidly apply the brakes if a runway overrun is suspected.

With the more traditional system that is found on most aircraft that are equipped with Autobrake, a desired taxiway exit from the runway will often be considered and discussed with the other pilot during the briefing prior to landing. The airport chart can be used to calculate the landing distance available to reach the desired exit taxiway. Subsequently, this distance can then be compared to the auto brake setting that delivers the projected stopping distance that is most suitable.

As elevation increases, the indicated airspeed on the flight deck under-reads the true speed traveled due to air density changes. This results in longer landing distances at higher airports like Denver (DEN). Photo: Michael Rodeback/Airways

Hot brakes

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Aircraft performance software can calculate the amount of heat generated by the brakes and, more importantly, the time required for the brakes to cool sufficiently before the next flight.

Many airlines encourage their pilots to use only idle reverse thrust on landing to save fuel, and the decision to select or not select reverse can often hinge on whether or not the brakes will have enough time to cool sufficiently before their next flight.

Reverse thrust on jet engines is not as effective as wheel braking but the additional braking force that it delivers can reduce the intensity of wheel braking that is needed, thus keeping the brakes cooler.

Some narrow-body types, on the other hand, may have brake fans housed within the main gear casing, which can significantly shorten brake cooling times by drawing air over the hot brakes on the ground to aid cooling.

Brake fans are less common on long-haul types due to the longer turnaround times that are frequently encountered, which affords more time for brakes to cool.

With the physical characteristics of each runway being unique, factors such as runway slope are carefully considered. Photo: Michael Rodeback/Airways

What if you don’t land?

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So once we have done all our calculations to get us safely stopped on the runway, you may think there is nothing else to consider. Surprisingly, however, there is still more to evaluate. What happens if you are unable to land?

As strange as it may sound, what happens if an approach is aborted is also worth considering. If large obstacles or high terrain lie in the path of the missed approach route, the climb gradient required to safely clear these hazards can be significantly steeper than what the aircraft may be able to achieve if an engine or engines should fail.

In fact, for two-engine aircraft, this could be very limiting, because a single-engine failure on a twin will have a more pronounced effect than a single-engine failure on a tri or quad jet. The likelihood of suffering a loss of thrust, and also performing a missed approach at an airport with hemmed-in terrain or obstacles is highly unlikely.

But if pilots always plan for what to do if an engine fails during takeoff, is there any reason why this risk-averse mindset shouldn’t continue to the landing phase as well?

Hong Kong (HKG) is one example, with missed approach climb gradients that can be nearly three times the standard regulatory minimum. It then begs the question, what should be done if one is in Hong Kong and needs to fly a missed approach but cannot comply with the higher missed approach gradient, following an engine failure?

The rugged terrain that surrounds Hong Kong (HKG) can pose a significant challenge when faced with an engine failure on a twin-engined aircraft. By Baycrest – Wikimedia Commons – CC BY-SA 2.5.

Engine Out Procedure

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In this case, pilots would opt to fly the “engine out procedure,” which is a special route that can be flown in the event of an engine failure after the ‘go-no-go’ decision speed has been passed during take-off. This route directs the aircraft away from prominent terrain and man-made obstacles. Since it can provide a safe path for climbing out during takeoff, it can also provide a safe path in the event of an aborted approach.

Each airline tailors these procedures to individual aircraft types, but they are unknown to Air Traffic Control. This can pose a problem if you need to fly them since you must inform the air traffic controller as soon as possible and explain this alternative route.

Missed approach procedures that appear on charts are often designed to direct aircraft away from possible conflicting paths with other aircraft and not necessarily to avoid the higher terrain around an airport, hence having missed approach paths that may not be sympathetic to the local topography.

Air France Cargo Boeing 777-F28 F-GUOC, was the aircraft was that involved in the 2015 incident. Photo: By Maarten Visser – Wikimedia Commons – CC BY-SA 2.0.

Garbage In Garbage Out

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Whilst most aviators will express praise for how technology has elevated safety and efficiency with onboard performance calculations, there is still a glaring weak link in the process. The saying “garbage in, garbage out” is apt here, since the data presented to the pilot is only as good as the data that was input initially.

When errors are made, they are most serious when the weight of the aircraft is entered incorrectly. The worst-case scenario is when performance calculations are generated using a weight that is less than the actual weight of the aircraft. On take-off, this can lead to the disastrous combination of too little engine thrust being used, along with a rotation being attempted before it is aerodynamically possible. At best, this can lead to a potential tail strike!

An Air France Boeing 777 freighter was involved in an incident on takeoff from Paris Charles de Gaulle (CDG) in 2015 as a result of incorrect data being entered into the electronic performance application. The aircraft weight used by the crew in their calculations was 100 tonnes lighter than the actual weight of the aircraft, which was calculated by ground staff and presented to the crew.

This error resulted in the crew using take-off speeds that were significantly slower than required. This in turn led to a premature attempt to rotate, which resulted in the aircraft’s tail strike protection system activating. Thankfully, the aircraft limped into the air without further issue, but the prolonged take-off roll resulted in safety margins being significantly compromised.

Crews are trained in techniques to manage distractions when dealing with safety-critical data. Photo: By Christoph Paulus – Wikimedia Commons – Own work – Public Domain

Landing is no exception

Using the correct weight for landing calculations is also important, so that the aircraft can first safely stop on the runway and, if that is possible, then the correct Autobrake setting is used if it is fitted.

A significant safety event could occur if the crew used an incorrect weight that was lower than the actual weight to determine landing performance, which could underestimate the required airspeed and landing distance. Previously, the focus was primarily on what was done prior to take-off, but the industry is now focusing more heavily on landing preparation as well and ensuring that it receives the same attention to detail.

It should come as no surprise that when pilots take their theoretical exams during their initial training, aircraft performance is assessed separately due to the size of the subject.

This is the time when trainees read textbooks and memorize fine regulatory details that will be tested in a written examination. This fine detail, however, is rarely required on a daily basis, and the preceding paragraphs provide a better overview of what is commonly done on the flight deck.

Therefore, the next time you fly and notice the aircraft smoothly decelerate to a walking pace after landing, you will now realize there is more to it than simply stepping on the brakes after touchdown!

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Featured image: Transaero Boeing 777 flight deck. Photo: By Alex Pereslavtsev – Wikimedia Commons – GFDL 1.2

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