Thrust reversers, found on almost all jetliners1 and on the larger propliners, redirect engine thrust forwards, helping to slow the aircraft in the event of a landing or rejected takeoff.2 This takes some of the load off the aircraft’s wheelbrakes, reducing brakepad wear and tyre overheating, and considerably increases the total braking force available, allowing the aircraft to stop in a shorter distance than it otherwise would...3

...unless, that is, your aircraft is an A380, in which case, your reversers (all two of them), although they do reduce brake wear, apparently do not decrease the aircraft’s stopping distance (my emphasis):

The two reversers do help slow the A380—but not by much. In fact, unlike the thrust reversers on most airliners, including the Boeing 747 jumbo, they do not stop the aircraft in a shorter distance than brakes and spoilers alone. They do, however, take some of the strain off the brakes and are useful if water or snow makes the runway slippery.

How is this possible? Any increase in available braking force should shorten the aircraft’s stopping distance (even if only by a small amount), so, assuming that the A380’s reversers produce a nonzero amount of braking force, the stopping distance for a given A380 at a given weight on a given runway under given conditions using maximum wheelbraking, maximum aerobraking, and maximum reverse should still be less than the stopping distance for said A380 at said weight on said runway under said conditions using maximum wheelbraking and maximum aerobraking, but no reverse thrust.

Yet, apparently, it isn’t.

Would someone like to explain this?

1: The exceptions being the Fokker F28 and BAe 146.

2: A few jetliners, such as the DC-8, Il-62, and Concorde, can also use reverse thrust in flight, to steepen the aircraft’s descent angle without increasing its speed; in essence, the reversers are used as blown airbrakes.

3: This is especially important when wheelbrake effectiveness is degraded, due to, for instance, landing on a contaminated runway4 (which severely limits the amount of force that can be applied to the wheelbrakes without them locking up), with inoperative spoilers (which severely decreases the amount of weight carried by the wheels, and, thus, the amount by which their brakes can slow the aircraft), with one or more blown tyres (which renders the brakes on those wheel(s) useless for slowing the aircraft), overweight (which forces the aircraft to land at a much higher speed, where the coefficient of friction between the tyres and the runway is much lower than at lower speeds), with inoperative flaps and/or slats (ditto), with accreted ice and/or wing-leading-edge damage (ditto numero dos), with inoperative antiskid (which requires that the pilot(s) brake manually, and severely limit the amount of braking force [especially at high speeds] to prevent the wheels from locking up), with one or more inoperative wheelbrakes (for obvious reasons), numerous other things that I haven’t thought of, or any combination of the above.

4: For instance, one that is flooded (hopefully with water, not Jet A-1) or covered in (usually water) ice.

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    $\begingroup$ I suspect it would be different in case of an emergency braking $\endgroup$ – Antzi May 15 '19 at 2:51
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    $\begingroup$ You're giving too much credence to one isolated statement by one reporter in one article. Typically, stopping performance is a matter of a selected deceleration rate - achieved this some brakes + reverse, of more braking & less reverse. Hence, a "no difference" case. I strongly suspect that author's statement wouldn't prove out with a dive into the A380's performance tables. $\endgroup$ – Ralph J May 15 '19 at 3:20

In all Airbuses auto-braking system aims for specific deceleration. Once the design target has been reached, the system does not apply more brake pressure although there usually is more available. So, if you apply thrust reverser with autobrake selected it will apply less pressure on wheel brakes and as the target deceleration remains the same, the stopping distance remains unaffected.

On slippery runways the autobrakes apply brakes until anti-skid starts to limit the brake pressure so in those conditions reverser would shorten the stopping distance. This would also apply to rejected take-off where maximum braking is commanded.

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    $\begingroup$ And this cannot be overridden by the pilot in case of an emergency, to brake faster, no matter the wear on the brakes? $\endgroup$ – vsz May 15 '19 at 10:29
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    $\begingroup$ @vsz why would locking up the wheels, or overheating the brakes and degrading their efficiency (possibly to zero), make the plane stop quicker? $\endgroup$ – alephzero May 15 '19 at 12:06
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    $\begingroup$ @alephzero I think he means running the breaks at the limits (no locking/overheating), with maximum thrust through the reversers. $\endgroup$ – Baldrickk May 15 '19 at 12:14
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    $\begingroup$ @vsz With autobrakes, no. But of course pilot can step on the brakes anytime and he’ll get full manual braking with anti-skid. Manual operation of brake pedals will disconnect autobrakes. $\endgroup$ – busdriver May 15 '19 at 12:33
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    $\begingroup$ @Joshua Not if the reject is because an outboard engine (on the A380, anyway) failed! Or due to a predictive windshear warning, or a vehicle on the runway, or a cargo fire warning, or etc... Of course, the worst-case planning assumes no reverse during the RTO, but it's entirely possible that it may be available. $\endgroup$ – Ralph J May 15 '19 at 18:23

The A380 has an optional system called Brake-To-Vacate (BTV) which allows the flight crew to calculate required runway length and determine which runway exit they can make before starting descent. Once engaged, the pilot just has to put the aircraft on the runway and BTV takes over, applying spoilers and thrust reversers ahead of brakes to achieve the desired landing performance.

There's a more detailed look at the system here

Stopping performance for certification has to be determined without the use of thrust reversers, so the use of thrust reverser in service doesn't change that particular aspect of the aircraft's certified performance..

  • 4
    $\begingroup$ To clarify, BTV does not automatically deploy spoilers or thrust reversers. For those, a pilot action is always required. Indeed, BTV is a function of autobrake which in turn does not activate until spoilers are deployed. More over, it is true that thrust reversers are not taken into account when certifying performance on dry runways, but they do have effect. $\endgroup$ – busdriver May 15 '19 at 5:08

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