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During a rejected takeoff at a speed > V1, you obviously try to stop with all available means since a runway overrun is a very undesirable result. In this case, could using the parking brake in addition to the normal brakes and reverse thrust help slow down the aircraft faster?

I know that the parking brake can be used to slow down the aircraft, e.g. in the “no breaks available” memory item on an A320 but does it help when the normal brakes are available?

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    $\begingroup$ I don’t believe it helps. Pulling the parking brake would lock the wheels and thus have the aircraft skid on deflated tires and grinding axles. This provides less friction than proper anti-skid rubber-on-asphalt braking and would make matters worse instead better. My hypothesis only, hence comment instead answer. $\endgroup$ – Cpt Reynolds Apr 11 '18 at 23:20
  • $\begingroup$ Wouldn't it be reasonable to assume the parking brake is not much stronger than it needs to be to stop it rolling away from the gate - which is usually on level ground. So it would seem highly unlikely to be able to lock the brakes during the landing roll. Aircraft are very weight challenged - nothing is heavier or bigger than it really needs to be. $\endgroup$ – Penguin Apr 12 '18 at 9:11
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    $\begingroup$ Assuming the regular brakes are working, the limiting factor is the friction between the tires and the runway. The regular brakes should be able to apply enough force to lock the wheels (even if an ABS system prevents them from actually doing so). So parking brakes would add nothing. $\endgroup$ – jamesqf Apr 12 '18 at 18:59
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This is probably aircraft dependant, but I can give some examples for the airplanes that I have flown and will be generally applicable to transport category aircraft. Understand that it won't cover every airplane as an absolute rule though.

In general, these types of airplanes have an antiskid system which keeps the wheels from locking up during heavy braking. If they were to lock completely up, the tires would likely blow, greatly decreasing deceleration and making it more likely to run off the runway (either at the end or off a side).

Parking brakes often have multiple settings because of this. One setting will be one which can be used when the normal brakes have failed, and have a relatively low brake pressure / stopping capability compared to the normal brakes in order to protect the tires. Using this setting will stop the airplane eventually, but you are better off with the normal brakes which have a higher brake pressure with antiskid, and better deceleration.

Another setting would have a much higher pressure, intended to keep the wheels from turning (while it is parked), and this would be undesirable while attempting an abort.

In the end, I will say what I usually say: Follow the guidance in the AFM. If it says to use the emergency brake when you abort, then do it. Most likely it will not (because of the above reasons), and you should not do it. If you take it upon yourself to go against their guidance, you become a test pilot and are either trying something that hasn't been done before, or worse yet, something that has been tried and has had an undesirable result.

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