738 park brake set but it moves anyways

[JammedStab]

How does that happen?

"...a Boeing 737-800 aircraft operated by xxx Airlines on behalf of Travel Service Poland, was conducting flight TVP7467 from Rhodos Diagoras/Maritsa, Greece (LGRP) to Katowice Intl, Poland (EPKT). In preparation for the departure at parking position 10 with the parking brake set and the parking brake light illuminated at LGRP, the aircraft started to roll forward slowly when the chocks were removed. The flight crew applied brakes, however the aircraft continued to roll and came to a stop with the nose wheel on the grass surface, while the main wheels remained on the apron. Passengers were deplaned, baggages were unloaded and the aircraft was pushed back to its original position.

The operator’s maintenance inspected the aircraft and found no damage. The brakes were also inspected and were determined to be serviceable. The aircraft was released back to service."

[BluSdUp]

Accumulator pressure 0.
Electric Hyd pmps off.
First flt of the day.?
Closing up still doing flt deck prep.?
Jonny-Jonny removes chockes?
VOILA!

" Lets agree the parkbrake was set!?"
"That is what I saw , Sir!"


As far as I recall: No pressure, no parkbreak.

[oceancrosser]

Well if you are use the Parking Brake you better make sure you have brake pressure... I see BluSdUp beat me to it!

[45989]

min 1000psi

Need to look!:ugh:

[Consol]

Never completely trust a park brake anyway.

[RAT 5]

That would suggest the accumulator pre-charge had leaked away. It would be unusual, in every outfit I've worked for, to remove the chocks before completing the Before Start scan = ELEC HYD pumps ON. Prior to that the Boeing scan includes the Brake Accum gauge.
Testing the brake system and finding it serviceable, therefore a/c released back into service, suggests the root cause was not discovered, nor searched for. If the accumulator had leaked the pre-charge there is something awry.

[Centaurus]

Never completely trust a park brake anyway.
Understandable if a flying school light aircraft because rarely a defect like a park brake problem is written up in the maintenance document. The student is told to keep his feet up on the brake pedals during run up.
Never struck the problem on an airliner though.

[framer]

The pre flight procedure has a note beside SET PARK BRAKE that says something to the effect of " don't assume that accumulator pressure is sufficient to prevent aircraft movement". When you set the park brake without pumps on the brake pressure gauge needle moves significantly.

[Old Fella]

Well if you are use the Parking Brake you better make sure you have brake pressure... I see BluSdUp beat me to it!

The park brake being set does not mean it is applied. I am often led to wonder how much "switch flicking" goes on in a robotic manner. It is one thing to position a switch or lever or pedal etc, it is another to check that the system response is what is anticipated. We have all, I have no doubt, been guilty of becoming robotic and sometimes not checking for the indication that our selection has resulted in action as normal.

[Skyjob]

Pressure on the brakes depletes over time naturally.
No reference to how long the aircraft was on stand for prior to incident.
An extended time could've resulted in depleted brake pressures...

[Ian W]

Which is why chocks are used.

[RAT 5]

Pressure on the brakes depletes over time naturally.
No reference to how long the aircraft was on stand for prior to incident.
An extended time could've resulted in depleted brake pressures...


The event mentioned that pax were de-planed. This suggests the crew were well into their preparation. If a new crew then ELEC HYD pumps should have been ON from the pre-flight external inspection and at least during the Before Start Scan. Therefore the accumulator would have been recharged and the whole system pressurised. Therefore it is curious how there could be pax on board and no brake pressure.

Which is why chocks are used.

Indeed; and it is not mentioned if the chocks were removed with ELEC HYD pumps OFF. But see above and ask how that could be the configuration.

If everything appeared normal, and the a/c still moved, it would suggest a malfunction somewhere. A swift 'release to service' seems strange. During the investigation crew actions would need to be included, but I wonder if the techies paid enough attention to the, or just concentrated on the nuts & bolts with the crew saying "no idea, guv, it just rolled away."

[Uplinker]

Funny isn't it, if you compare the millions of cars in the world whose parking brakes just work - for weeks at a time in some cases - without any further attention, to the thousands of aircraft that need special considerations, such as keeping accumulators charged, turning ELEC HYD pumps on and keeping chocks in.....

One would have thought that by now; reliable "set and forget" aircraft parking brakes would have been developed?

[JammedStab]

I thought that the fluid between the park brake valve and the brake itself would be locked in and hold even if the pressure decreases over time upstream of the valve.

[Goldenrivett]

I thought that the fluid between the park brake valve and the brake itself would be locked in and hold even if the pressure decreases over time upstream of the valve.

Any moving parts (brake slide valves etc.) must have a working clearance. This paper explains the leak rate past hydraulic valves.
http://www.sname.org/HigherLogic/System/DownloadDocumentFile.ashx?DocumentFileKey=cc6f4ed6-f71f-474a-873f-32b1c0eef5d8

The American Airline service engineers didn't appreciate the leak rate on the fork lift truck they were using to remove and refit the engines on the DC10. They damaged the rear engine mounting structure, which eventually lead to the failure of the rear engine attachment.

http://libraryonline.erau.edu/online-full-text/ntsb/aircraft-accident-reports/AAR79-17.pdf see page 50.
"The flange could be damaged in an even more insidious manner; the forks could move imperceptibly as a result of either an internal or external pressure leak within the forklift's hydraulic system during pylon removal."

[matkat]

Rat5 the accumulator pressure is not to keep hydraulic power it is simply there to provide a positive pressure on the fluid itself in order that a positive head pressure is delivered to the pump. Agreed no pump pressure and pumps off.

[Uplinker]

@matkat, I think you might be confusing two different things?

An accumulator stores system pressure and is used to smooth out pressure fluctuations and assist the pump in times of short term high demand. On the Airbus, the parking brake accumulator can fully pressurise the parking brake seven times. (This is an emergency brake function as well).

What you seem to be referring to is the air pressure introduced into the hydraulic reservoirs above the fluid which ensures a positive fluid pressure at the pump intake and thereby prevents cavitation.

@Jammedstab, as Golden rivet says, no hydraulic seal is "perfect" - there will always be leakage, (hence why hydraulic pumps have case drains for example). Molecules of fluid will continually escape past the seal at a certain rate, so over time the pressure that seal is holding will gradually reduce - unless the system pressure is being maintained by a pump.