ATA 29

Nitrogen Bottles:
There is no requirement for a one year visual internal inspection on the nitrogen bottles currently installed in the Westwinds. That was an old requirement for the Walter-Kidde bottles-- all of which have been replaced because they reached their life-limit.

ATA 29

Hydraulic Fuses
AD 98-06-10 requires the functional checking of the hydraulic fuses every three years or 1200 hours, which ever comes first. It means ALL THREE fuses, not just the indicator fuse. We are finding the sign-off for the AD just referencing the indicator fuse. Also, if you read the brake bleeding instructions in the manual, you will find that the fuse is required to be checked EACH time a brake is replaced and bled.

ATA 29

Hydraulic Line Integrity
We have learned that there have been events where hydraulic lines are cracking. On a recent incident, the left main hydraulic annunciator came on about 30 minutes after takeoff, followed by the right hydraulic annunciator. The crew returned to home base, blew the landing gear down, and landing uneventfully. It was found that the pressure line into the lift dump valve was cracked near a B-nut. The hydraulic pumps ran dry for an indeterminate time. As you go about your routine area inspections, please pay attention for hydraulic fluid staining and fluidleaks. As the fleet continues to mature, we want to ensure we are aware of areas that may become fleet symptoms. Advisory Circular 43-4A identifies problem areas for corrosion, and one of them is around B-nuts and line marking tapes. Corrosion will weaken the line sufficiently to cause failure. If you have recently experienced a similar situation, please let us know via e-mail to westwind@worthingtonav.com

ATA 29

Hydraulic leaks in the lines in the pressurized cabin area have proven to be a recurring problem. There will be a session and training for the perma-swedging of hydraulic lines at our M&O in Las Vegas. This would be a good time to have certified training from the manufacturer so that if needed, you may accomplish your own perma-swedging.

ATA 29

Does your emergency hydraulic pump cycle too often? The problem could be a faulty thermal relief value. If there is no evidence of external leakage the thermal relief valve should be suspect for bypassing. This valve performs two functions, it provides pressure relief at 1250 +/- 100 psi.and has a built-in check valve to prevent pressure from backing up thru the pump. This happens to be the only check valve in the emergency hydraulic system. Emergency pressure should hold for at least 60 seconds and pump running time should not exceed 45 seconds.

The hydraulic restrictors in the thrust reverser system can cause erratic operations of the thrust reverser if they are installed in the wrong port of the T/R actuator. The restrictor and the regular union are similar in appearance and can be interchanged if not properly identified before installation. The restrictor should be installed in the “stow “line port and the regular union should be installed in the “extend” line port.

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 29

Hydraulic Filter bypass
During a routine engine run on the right engine, the pressure filter bypass pin pops out (the filter is contaminated).
There will be no cockpit indication. Since the pressure switch is installed between the pump and filter, the annunciator light will remain out. The pressure gauge will read normal from the opposite engine-driven hydraulic pump.

ATA 29

Brake & Hydraulic Indicating Fuse Checks
Westwind 1124/1124A maintenance technicians and operators are reminded of the recurring requirement to perform a functional test of the hydraulic fuses in their aircraft. Airworthiness Directive (AD) 98-06-10,Amendment 39-10387, states that a repetitive functional test of the hydraulic fuses in the brake system and emergency hydraulic indicating system will be done every 1,200 hours or 3 years after the initial inspection,which ever comes first, in accordance with Westwind Service Bulletin 1124-29-132.
This test is to make sure the fuse will stop the flow of hydraulic fluid after a leak has been detected. Some operators feel the AD no longer applies, if they had the brake fuse checked during brake change or if a fuse had been replaced. This is not so. There is no terminating action for the AD at this time. Both brake fuses and the indicating fuse need to be addressed in signing off the AD. Check your records to be sure you are in compliancewith this requirement.