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AUX Pump/Tank malfunction


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The model is a C-130H. Its LH AUX Fuel Pump is rated 38psi(static-no demand). During flight and as long as crossfeed valves are opened this pressure drops. Finally with the AUX pump ON and four crossfeed valves open the output press is 20psi, causing the AUX tank empty light to illuminate.

The three solenoids of the pump were measured and found ok. But, suspecting an internal mechanical failure of the pump, the pump was replaced. The discrepancy remained after the following flight. More careful inspection revealed a damaged pressure hose(the one connected on the pump). The damage was not so obvious, since it was in the internal part of the hose. Anyway, the hose was replaced and the airplane was dispatched for flight. Unfortunately same discrepancy, even with a finding.

My guess is that an internal leakage of a line inside the aux tank, by-passes fuel and causes this press. drop. I am not really familiar of the AUX tank internal components and bladders internal arrangement.

Any ideas what to look for?

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Seems to me you have nothing to weary about or to look for, since the pump giving you 38psi during no flow condition.

The limit (28 to 40 psi) as stated on flight manual is for preflight reference only.(no flow condition) pressure below this limit is acceptable with engine operating, And for the empty light illumination during 4 engines cross-feeding, might be also normal to see the light, if 4 engines under supply of one aux. pump.

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Ok, jakesnk

Since it is the only aircraft and the only pump unable to supply 4 engines let’s have a look to the following possibilities:

1-Pump Electrical Power supply malfunction (which required a thorough check of continuity, resistance and load of the three phases; one phase weak will weaken the pump.

2-Passage obstruction from the pump to the pressure switch.

3-Ejector plumbing within the aux tank for proper connections.

4-Dump and by-pass valves for fully closed positions (left aux dump valve, left by-pass valve) if any... not in a full closed, will lead to loss some pressure.

5-Aux NRV valve might be giving higher than normal obstruction.

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  • 4 weeks later...


I have seen this issue before on various other C-130's. All models here in the states and while deployed. Here are a couple of quick things you can have done or do to help narrow down the troubleshooting parameters.

1). While the aux pump fuel pump is running open the center separation valve and then depress the crossfeed primer button and see what the pressure drops down to. Kill that pump and turn on the opposite side aux pump that you have no issues with and hit the crossfeed primer button and compare these pressure readings. Let us know what the pressure differences are if any.

2). Pull the aux tank sight gage for the aux tank with the questionable pump. Youll see two black hoses coming from the aux pump thats on the door, one large and one small. The large hose is the aux pump discharge hose, make sure this hose at the top where it arcs over to the wall connection is not crimped or bent in a way that restricts more the approx. 40% of the hose diameter. NOTE: Ive seen these hoses restricted enough to cause demand feed issues.

3). If possible see if the pump is made by a company called "ROMEC". The name should be on the silver and red serial plate. Romec pumps have a design issue that occasionaly causes low readings during high demand situations.

Ill stop for now until I hear anything back from you because there are a number of other things that can cause this problem.

Good luck


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Thank you for the tips, Moon.

While further investigating this one, we found pieces of the damaged pressure hose(the one I mentioned was found worn) inside an orifice check valve installed in the dry bay(don't remember the P/N). We replaced the valve, assuming that this would not allow the pump to have a proper output pressure.

I will also try your tips and let you know.

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