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EDIT: Another way to phrase the question is, given an extra long runway, does it make sense to substitute mechanical braking (which is the primary way to slow down in a typical situation) partially or completely with something else, to reduce brake wear / roll down the runway faster?


Consider an airliner landing on a very long runway (e.g. Boeing 737 landing on 12,000 feet+). The pilots are planning to vacate the runway near the end, so there is a lot of distance to spare.

Ignore noise procedures, and assume there is no traffic behind to suggest an early exit. What would be the preferred way to slow down the aircraft in this scenario?

  1. Light application of reverse thrust only, no brakes?
  2. Light application of brakes only, no reverse thrust?
  3. Moderate braking until below a certain speed (e.g. 80 knots), then "coast" the airplane down the runway?

(This question addresses the primary means to slow down after a typical landing, while this question is about an idealistic case where there is plenty of braking capacity to spare.)

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    $\begingroup$ I feel the answer to question you link to answers this question. It states the breaks are always the primary method. Using air from the engine is always undesirable from an economic view as it's using fuel for something other than thrust for flying. $\endgroup$ – Notts90 Nov 20 '16 at 8:04
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    $\begingroup$ @notts90 I can't see why the linked question is a duplicate. The question is different and the answers are not applicable. $\endgroup$ – kevin Nov 20 '16 at 10:31
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There are several ways to slow down an airliner:

  • aerodynamic drag
  • friction in the wheel bearings
  • reverse thrust
  • wheel brakes

and a few even used a brake chute, but that went out of fashion a long time ago. If the runway is inclined, landing uphill also will slow down the aircraft.

Friction and drag you get for free, so I would rely on those first. Since they are highest when the speed is high, use them first until the aircraft is so far down the runway that it would overshoot without additional application of wheel brakes. Use spoilers and leave the flaps down to maximise the deceleration. Only at the end add wheel brakes so you have slowed down to taxi speed when the exit of the runway is reached.

So the best answer is none of the above but:

  1. Coast along with flaps and spoilers out until you need to brake more to avoid an overshoot. Then apply wheel brakes.

The exact way how brakes should be applied depends on their type.

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  • $\begingroup$ So no reverse thrust ? $\endgroup$ – Antzi Nov 21 '16 at 12:25
  • $\begingroup$ @Antzi: No, that needs fuel and makes noise. Brakes are fine. The key is to take as much energy out of the plane as possible without using either reversers or brakes, and to use brakes for the remainder. $\endgroup$ – Peter Kämpf Nov 21 '16 at 14:00
  • $\begingroup$ @Antzi reversers as needed to slow down, but if you can do so without reversers, do so without reversers. Saves fuel, and might even be required for noise abatement reasons. $\endgroup$ – jwenting Nov 30 '16 at 7:35
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    $\begingroup$ @PeterKämpf Deploying the reversers without actually spooling up the engines also provides a non-negligible amount of force with little or no extra cost, too, right? $\endgroup$ – reirab Dec 20 '16 at 23:31
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    $\begingroup$ @reirab: Yes, particularly at higher speed. Maybe it is worth to deploy them right after touchdown but leave the engines at idle. $\endgroup$ – Peter Kämpf Dec 21 '16 at 7:15
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To make it efficiently its better to start with thrust reversers to reduce speed just after touchdown. As speed reduces the effect of the reversers reduce. To avoid FOD(foreign object damage) risks it is not recommended to use high revers thrust with low ground speed.

This what Airbus says and advises to the flight crews of A330s.

Three systems are involved in braking once the aircraft is on the ground:

• The ground spoilers

• The thrust reversers

• The wheel brakes

THE GROUND SPOILERS

The ground spoilers contribute to aircraft deceleration by increasing aerodynamic drag at high speed. Wheel braking efficiency is improved due to the increased load on the wheels.

REVERSE THRUST EFFICIENCY

Thrust reversers are more efficient at high speeds. Below 70 kt, thrust reversers efficiency rapidly decreases. Below 60 kt with REV MAX selected,engine stall may occur. Therefore, it is recommended to reduce the reverse thrust to REV IDLE at70 kt, and keep REV IDLE until taxi speed.At taxi speed, and not above, stow the thrust reversers before leaving the runway, in order to avoid foreign object ingestion.

WHEEL BRAKES

Wheel brakes contribute the most to aircraft deceleration on the ground. On very long runways, the use of pedal braking may be envisaged if the pilot anticipates that braking will not be needed. To reduce brake wear, the number of brake application should be limited.

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If you have a lot of runway at your disposal, I'd recommend the use of aerodynamic braking (hold a nose high attitude riding on the main gear), speed brakes and thrust reversers, followed by wheel braking starting at around 80 KIAS or so.

The reasons have to do with energy dissipation. For an A380 landing at say 160 KIAS and following this procedure, the pilot has already dissipated 3/4 of his total kinetic energy which he started with at the time he applies the wheel brakes at 80kts, leaving only 1/4 of the original kinetic energy to be dissipated through the brakes in heat. As drag forces are also directly proportional to the square of the velocity, it makes sense that aerodynamic braking and high drag devices will drain off a lot of energy very quickly while at high speed but their effectiveness is greatly diminished at lower speeds.

In addition, the wings are still producing a great deal of lift at touchdown, reducing the static coefficient of friction between the tires and the runway, making braking much less effective at higher speeds.

Thrust reversers are again going to be more effective at high speed as performance is boosted by ram air at the intakes and lower speed operation risks compressor stalls as well as damage from ingestion of FOD kicked up by the fan exhaust.

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  • $\begingroup$ Holding the nose wheel up is a really bad idea for larger aircraft! From the 777 FCTM: "Do not attempt to hold the nose wheels off the runway. Holding the nose up after touchdown for aerodynamic braking is not an effective braking technique and results in high nose gear sink rates upon brake application and reduced braking effectiveness." $\endgroup$ – Bianfable Jan 12 at 9:05
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I experienced such a landing as a passenger and happened to film it:

A319 landing in Stuttgart (EDDS) on runway 25 (10974ft, 3345m), exited on last high speed taxiway. (Sorry for the poor video quality) Touch down at 3min 16 seconds

The pilots touched down at the normal touch down zone, used idle reverse and speedbrakes and low autobrake, then stowed the speedbrake (disables autobrake) and reverse, rolled with idle thrust and no braking down the runway at roughly 60 kts or so, then used light braking at the end of the runway to make the last high speed taxiway.

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