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.

ATA 30

Wing De-ice
We recently had a problem with the wing boots failing to fully inflate on a Westwind II. We looked at the distributor valve, all the aft equipment bay plumbing, and the boots themselves for excessive pinholes. All the pressures were within the system limits. We discovered that the wing lines run under the lav floor area, up each side of the fuselage to the wing root bulkhead fittings, and then to the boots. Because the lines under the floor are the lowest point in the system, moisture had collected in them over the years and turned the bottom of the tubes into Swiss cheese. We made new lines, and the system worked perfectly. Next time you are in the area for maintenance, take a good look at the lines, or even put a little pressure in them and do a leak check. It might disclose a similar problem.

ATA 27

Rudder Trim
Several new Westwind operators have called us asking why their aircraft “wing walks”. We have heard this same question for years, so we thought we would offer a few suggestions to help correct the problem. First, though, the pilot and mechanic need to understand the difference between wing walk and “Dutch roll”. The first is a roll axis problem and the second is a yaw axis problem—two different controls with very similar characteristics. Almost every Westwind we have worked this problem on has turned out to have a very slight Dutch roll. In almost every case, we were able to either eliminate the problem or lessen it to a greater degree by doing the following:

1. Run the rudder trim to the right stop and hold the trim switch in that position for 2-3 seconds. (DO NOT depend on the trim indicator for any of this.)
2. Run the trim to the left stop and again hold the switch.
3. Center the trim and go fly.

The trim actuators are not connected and will run at slightly different speeds. Over a period of time, they will position themselves out of rig and will bend the tab out of shape—causing a Dutch roll. Synchronizing the actuators again will straighten the tab. Don’t worry, it doesn’t hurt the tab at all.