auto brakes rto question

[JammedStab]

Perhaps the answer is obvious but....

After performing an RTO to a complete stop using autobrakes RTO only and allowing them to stop the aircraft, is it correct to say that you have the equivalent of the parking brake being applied until the Autobrakes are deselected?

[MrBernoulli]

"Equivalent" might be too strong a word to describe this. If you are using the term "parking brake" to help a layman understand what an RTO braking system does, then the analogy may help. But you may also be in danger of confusing someone in to thinking that we try to get airborne with the parking brake on! Obviously, we would never make it in to the air get if we tried that little stunt! ;)

RTO can be considered as a 'switch' that is set, ready to automatically apply maximum braking following the decision to discontinue a takeoff, when certain conditions are met. In the Boeing 777, for example, these conditions occur when the Autobrake selector is set to RTO (armed) and:


the aircraft is on the ground
groundspeed is above 85 knots, and
both thrust levers are retarded to idle.

It is worth bearing in mind that an RTO braking system will also incorporate the antiskid system, to prevent individual wheels locking up during the braking process. A parking brake would likely not incorporate antiskid, because there is no need to when the aircraft is parked and static!


I hope this helps! :ok:

[Goldenrivett]

is it correct to say that you have the equivalent of the parking brake being applied until the Autobrakes are deselected?
If you mean, will the aircraft move? - then they are the same.
But if you mean, your procedure after RTO is to set the parking brake - then no they are not equivalent.
Imagine the fire crews inspecting your aircraft without the parking brake set, and someone disarms the ground spoilers.

[vapilot2004]

There are a few differences, JS, between setting the parking brake and an RTO Autobrake application.

On the 737, the parking brake applies less than 1000psi at the wheels - this figure is not to be confused with hydraulic accumulator pressure. For an RTO, full braking pressure is applied at 3000psi with no modulation for deceleration scheduling - it is the maximum possible. In an RTO event, up to five times the energy is absorbed by the brakes compared to a normal idle-reverse landing.

As Mr. B. indicates above, anti-skid is active for RTO braking but not with the parking brake applied.

[JammedStab]

Thanks for the answers.

Remember that the question is not about the deceleration process in which RTO is used. I am talking about once the aircraft has come to a complete stop without any manual braking applied.

Are the brakes still being continuously applied by the Autobrake RTO system and therefore preventing you from taxiing when thrust is applied?

There is a statement to consider not setting the parking brake after a high speed rejected takeoff. But if we let Autobrake RTO stop the aircraft completely, is the aircraft at that point sitting there with brakes fully applied.

[Chesty Morgan]

If RTO only occurs with both throttles at idle I assume as soon as you move them above idle then the autobrake would release.

[Goldenrivett]

"But if we let Autobrake RTO stop the aircraft completely, is the aircraft at that point sitting there with brakes fully applied."

Affirm.

[MrBernoulli]

Sorry, JammedStab, a re-read proves that I misunderstood your question. Doh! :\

[vapilot2004]

Are the brakes still being continuously applied by the Autobrake RTO system and therefore preventing you from taxiing when thrust is applied?

Yes and no. Brake pressure remains full on until either throttle is moved out of idle (CM got it right) or speed brakes are lowered, or upon manual brake application. This is another difference between the parking brake function and the Autobrakes.

[Cough]

Vapilot.

Just to make the point, CM may be right - it really does depend on what type you are flying!

[Goldenrivett]

When the aircraft has stopped the wheels are seen as not spinning so therefore brakes would be released by the anti-skid system.

Try driving a modern car with anti-skid brakes and let us know what you discover.

[Gryphon]

:= Anti-skid system will deactivate below a given speed.