ATA 29

Cause & Cure for emergency pump short cycling


Is your aircraft’s emergency hydraulic pump “short cycling”? With power on the aircraft and the HYD EMER PUMP circuit breaker pushed in, the emergency pump electric motor should run until the pump produces 1,000 psi and then shut off. Pressure should slowly drop until it reaches 800 psi, at which point the pump will turn back on until it gets to 1,000 psi again. The time in between the pump cycling off and back on should be no less than two minutes. If the pump cycles in under two minutes, it is considered to be short cycling. All the time, it seems, I see “Emergency hydraulic pump short cycles” written up on the incoming squawk list when planes are in for inspections.


What causes the emergency pump to short cycle? Well, several things may cause the 1,000 psi to drop faster than it should: leaks (internal or external), thermal relief valve malfunctioning, pressure switch malfunctioning, or an improperly-serviced emergency brake system accumulator. The latter is the most common cause that I have seen.


The procedure for “Hydraulic Emergency System Accumulator Charging” is found in section 12-10-03 of the Aircraft Maintenance Manual (AMM). It calls out to charge the accumulator between 170 to 190 psi depending on the outside air temperature.

Outside air temp // Charging Pressure

-20 F to 30 F // 170 +/- 5 psi

31 F to 80 F // 180 +/- 5 psi

81 F to 130 F // 190 +/- 5 psi




This accumulator is very small, and servicing it correctly is extremely critical. Make sure you pump the brakes down all the way (watching the direct reading gauge in the cockpit) while maintaining at least 500 psi of nitrogen in the accumulator. This allows the separator piston to bottom out completely, giving you the calibrated critical area inside the accumulator prior to the final charging pressure.


If the accumulator is under serviced, it will not have enough pressure on the separator piston to push all of the hydraulic fluid out throughout its entire action, causing the hydraulic pump to short cycle as pressure in the system bleeds down to 800 psi.


If a little bit of something is good, then more would obviously be better! Right? Wrong! At least in this case it is wrong. If the accumulator is over serviced, then there will not be enough space for the correct amount of hydraulic fluid to be stored within the accumulator, since more space is taken up on the nitrogen side of the separator piston. Not enough hydraulic fluid being stored in the accumulator correlates to the emergency pump short cycling as well.


NOTE: Please keep in mind that when servicing the hydraulic reservoir that you should not only release the pressure in the T/R accumulator but also pump down the emergency brake system accumulator, as mentioned in Hydraulic Reservoir Servicing 12-10-03, to keep from overfilling the hydraulic reservoir, which results in fluid spewing out of the overpressure relief valve on top of the reservoir lid.