Servicing Hydraulic Accumulators

If you follow the Aircraft Maintenance Manual procedures for servicing the

hydraulic accumulators in your aircraft exactly as written, you MAY NOT

be servicing them correctly!

The AMM procedure does not allow for the fact that you are adding nitrogen

to the accumulators against a hydraulic block (Pressure Maintaining Valve

or Non-Return Valves), which will not allow the piston to move. While the

pressure gage increases and shows the correct pressure level, the end result

is you may not have introduced enough nitrogen VOLUME into the

accumulator to operate the systems in the event of a pressure system failure.

If you service hydraulic fluid in the main reservoir after this, you will have

effectively over-serviced the systems with fluid.

The only correct way to service these accumulators is to have someone in

the cockpit actuating the brake pedals or cycling the brake handle from

“Normal” to “Park” and back during nitrogen servicing, to relieve the

hydraulic block back to return. The same scenario holds true for the Thrust

Reverser accumulator (if installed) as well. Use the hydraulic pressure relief

valve while servicing nitrogen in this accumulator to avoid this under-

serviced condition.

- A publications change request was submitted to the OEM on this issue.