View Full Version : 747-400 brake pressure accumulator

12th Sep 2004, 14:49
Dear friends,

on the 747-400, how long can the brake accumulator supply hydraulic pressure to the parking brake?
UAL AOM does not give details about this topic...
Thank you in advance,


Cornish Jack
12th Sep 2004, 15:21
From the Boeing OM - "If the normal and alternate brake systems are not pressurised, parking brake pressure is maintained by the brake accumulator. ............
Sufficient pressure is stored in the accumulator to set and hold the parking brake, but the accumulator is not designed to stop the aircraft."
Seems to me that any situation which requires you to use the parking brake accumulator would, of necessity, be such as to NOT RELY ON THE PARKING BRAKE. Use it initially, yes, but get the chocks in place soonest!!!:ok:

gas path
12th Sep 2004, 15:50
On the 744 it largely depends on how long the pump is left running, but a good guide is pump on for a minimum of 30 secs. after the park brake is set.
Presuming the brake accumulator is to the correct precharge and the anti skid surge accumulators are also precharged correctly, after 10 mins. brake pressure should not drop by more than 275psi. That would give about 8 hrs of effective park brake pressure.
As CJ says 'put in the chocks'.

13th Sep 2004, 17:44
Dear friends,

thank you very much for the kind and detailed answers.
Now this topic is much more understandable to me!
Have nice time,