Hydraulic Overpressure

[NATOPS1]

Anyone seen an overpressure in the 3850-4000+ range?

If so did your rudder boost (if flaps were up) increase but not to the overpressure valve (due to the de-boost)?

I do not know the bypass capability of the pressure relief valve for the rudder but if it can bypass enough to limit the 3000 input to 1100 to 1400 output then a increase to the input of 1000 should not make a huge difference.

Maybe the increase would be in the 1400 to 1600 caution range….

Any ideas?

[Tim Goken]

Each engine driven pump puts out 8.6 gpm at 2525 psi and the system pressure relief valve has the capability to bypass 16 gpm when it is fully open at 3850 psi. With that being said, you should never experience an overpressure of that much. For that to actually happen, you would have to have a pump compensator and system pressure relief valve fail at the same time. It must have happened because there is a procedure in the dash one for it. A friend told me that he had an overpressure while he was flying slicks at Rhein Main in the '80's. He didn't say exactly what that pressure was and nobody actually noticed the rudder booster pressure when the flaps were up.

My thoughts: The rudder pressure reducer should bypass enough pressure in conjunction with the system pressure relief valve to keep it between 1100-1400 psi. If not, the rudder pressure relief valve would open at 1600 psi and maintain no more than that pressure.

[NATOPS1]

Thanks I have the rated output and the pressure relief valve numbers but Im looking for info on if there were a dual failure of the pump pressure regulator and the system pressure relief valve how much could your rudder by-pass to keep pressure down (at the rudder)

The reducer part number is 3A004 Manufacture (98391) I do not have access to milspecs or the like I do have a few pubs numbers that if someone has access and time would like to look up the info..... 03-30ARA-4 and a TO 9H8-15-35-3..

If we could find out what the bypass gpm maybe we can figure out what the pressure increase would be....

I still think it would be minimal and stay within the 14-1600 caution range....

[bischoffm]

The pressure reducer valve reduces the pressure from 3000 psi to 1275 +-75 with a cracking pressure of 1575.

Where the pressure relief valve will not let the pressure exceed 2300 psi and reseats at 1440psi

So when these two fail the pressure could possibly increase to max pump output pressure

[NATOPS1]

Understood, that is where we get our ranges. 1100-1400,1600 Max. @ flaps less than 15%.

The question is what is the bypass capability of the rudder pressure relief valve? If @ the 1575 PSI full bypass can the valve relieve the increased 1000 PSI from the hyd pump?

A dual failure would have to occur to get to this point... Hyd pump pressure regulator fail to limit output to 3000 and the system pressure relief valve failure to limit pressure to 3850....and the indicated pressure increased to 4000. At that point what would the rudder pressure be? Less than 1600 because the relief valve has the capacity to bypass enough or more than 1600?

[Dan Wilson]

A dual failure, even if both pumps in the system were abex pumps (the higher output of the three brands of pumps) the remaining volume that could cause overpressure would be very small indeed and could be easily overcome with small control corrections.

I used to have all the factory numbers that would have let me give you a very specific answer but all my goodie stuff got turned over to the FE section when I had to quit flying.