Jul 23, 2020

Antiskid operation while braking on accumulator?

Hi, I have a question, and I hope you may be able to help me! I don’t quite understand why the antiskid system keeps working, even in case of full hydraulic loss. (so braking pressure provided by the accumulator). It is said that when braking on accumulator pressure, to apply steady braking pressure as you only have a limited amount of braking applications before the accumulator would be empty. Would the antiskid not be doing the same thing, by modulating pressure to stop the wheels from skidding, hence emptying the accumulator?

This excess pressure gets bled off via the parking brake valve, does this somehow keep the accumulator from depleting?

Jul 23, 2020

That's why its limited. The key word is 'several". That could easily translate to 1,2,5, 7 applications to the main wheel brakes.

Hi @Jordy Gijbels!

Welcome to The 737 Handbook forum and thank you for your question!

The brake accumulator provides braking power when both normal (HYD B) and alternate sources (HYD A) of braking are not available.

Brake Accumulator is pressurised by dry nitrogen. When the HYD B and A is depressurised, stored HYD B in the accumulator is able to provide six full brake applications to the normal brake system (HYD B).

Anti-skid monitors wheel deceleration and controls metered brake pressure from hydraulic brake system or the auto brake pressure from auto brake system to prevent wheel skid.

When anti-skid activates on one of the wheels, it slightly opens the related anti-skid valve to release some of the HYD pressure and prevent the wheel from skidding.

It is true that some of the HYD B will be leaked to the HYD B return during anti-skid operation, but there is enough HYD B power stored in the brake accumulator to stop safely.

Having said that, it is important not to pump the brakes:

  1. Every time the brake pedals are released and reapplied the anti-skid needs few seconds to readjust which increases braking distance

  2. Every time the brake pedals are released, the pressurised hydraulic fluid in the braking system is released to the return. With no HYD A and B, when the brakes are again reapplied, the brake accumulator needs to provide that pressure to the braking system (limited source).

You can find more about this in Chapter 14 Landing Gear in The 737 Handbook

I hope this answers your question:)



Jul 24, 2020

So I guess then, the antiskid is able to control the applied brake pressure with such precision, that not enough stored pressure gets lost to pose much of a problem?

That’s correct, anti-skid only releases just enough pressure to prevent the wheels from locking up