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wildweasel_pt

Technical question regarding an engine malfunction

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Hi all

Data:

Model - H2 (elongated H model)

Engine - T-56 A15

Event - Secondary fuel light illumination in flight. Pulled ignition CB and light remained on.

Troubleshoot:

- Speed sensitive checked good

- Fuel filter and fuel line clean of debris

- pressure switch checks good

- On start-up after 16% rpm secondary light illumination kicks on and remains on, no swing for series on TIT indicator

- Suspected problem with fuel pump (possible seizure of fuel pump without braking or shaft to gearbox?)

- Fuel pump checked good and paralleling valve apparently also.

Question: What made the paralleling valve close (paralleling operation) if the speed sensitive checked good? Its a valve that needs electrical condition either to open or close not spring loaded like bleed air. Got into a conversation with maintenance and they are going to further analyze the paralleling valve and check solenoid and such but what do you all think about it? Thanks in advance

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Bad Primary pump blocking flow or paralleling valve stuck closed.

As to your question; The valve should not move on its own; energized closed and "deenergized" open electromagnetic so it should return to the open position...

In flight with no other indications only the SFPP Lt, I would guess the Primary pump.

It is easier to "swap" the Paralleling valve....

The fact the light illuminates at 16% during the restart tells me there is no flow past the pump or parallel valve.

If you disconnect the parallel valve it should never close and you will not get parallel or series indications.

If the light illuminates with it disconnected you have a bad pump.

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I agree with NATOPS completely. Usually, you have Fuel Flow and the Parallel valve at 16%, but you never really build up the 120PSI of pressure until about 35% RPM or so, and no two engines have the light come on at exactly the same time. Since the light is coming on immediately at 16%, I would suspect either total blockage on the outlet of the primary pump (or the pump itself), or an extremely weak pressure switch that is actuating at a very low PSI. Under such a condition, the Tank boost pump, the Centrifugal boost pump, and the Secondary pump all together would put out positive pressure, even if the primary pump is working correctly.

Additionally, if the Parallel valve is not installed and torqued exactly correctly, the valve could stick in the actuated position.

I'd be curious how your maintenance checked the pump without using a dedicated test stand or swapping out for another pump.

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Good point Lkuest on the weak switch. try and start with no fuel pump pressure (it will start) and see if the light comes on at 16%. There is a discussion about weak pressure switches in the books but not under the situation you have described.

Good luck and let us know what you find.

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Good point Lkuest on the weak switch. try and start with no fuel pump pressure (it will start) and see if the light comes on at 16%. There is a discussion about weak pressure switches in the books but not under the situation you have described.

Good luck and let us know what you find.[/QUO

Did the secondary pump light illuminate in flight during Aux or Ext. tank fuel feed? I seem to remember in the -1 that this could happen and it was not really an issue, more of an annoyance. However, I believe you needed to do all the troubleshooting to determine if there are any other issues contributing to this. If all was found to be clear, serviceable and functional, then is was not a malfunction.

Ramrod

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Did the secondary pump light illuminate in flight during Aux or Ext. tank fuel feed? I seem to remember in the -1 that this could happen and it was not really an issue, more of an annoyance. However, I believe you needed to do all the troubleshooting to determine if there are any other issues contributing to this. If all was found to be clear, serviceable and functional, then is was not a malfunction.

Ramrod

Lockheed Service News Bulletin V8N4 has more information on this issue, but you're right, just because it is flickering only during crossfeeding, it doesn't mean there's nothing wrong.

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It was found to be a bad solenoid on the paralleling valve. So the input on it wasn't reflected like it should. It got stuck closed. What makes me confused is why did it go to close position in flight? Something i need to discuss further with maint.

Thanks all for your inputs.

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The valve is deenergized open if the internal part of the valve breaks it can move without electrical input (close).

The electromagnetic side can only move the valve to close (energized) not open so no way to manipulate the system. You could cycle power to the valve a few times (jump the SSC) but other than that not much to do but change valve...

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Let us create this scenario to (somehow) explain a defective paralleling valve:

Due to valve internal malfunction; Paralleling valve at de-energized condition is not retarded to a full open position.

1- During initial rotation (valve supposed to be fully open) but due to not fully open, the fuel pressure before the 16% rpm will build up but not sufficient to trigger the secondary switch.

2- At 16% the valve will be energized to take a full closed position, this will cause farther increase of fuel pressure causing earlier than normal indication of secondary light.

3- At 65% the valve will be de-energized, thus valve will return to the previous position (not fully open) consequently the pressure will slightly drop causing the light to be off. (You may not see a clear indication of TIT cutback).

4- During flight as fuel temperature will decrease thus fuel pressure will increase and it may reach a point that trigger the secondary switch causing the light to be ON specially during cross feeding the engine from high pressure pumps.

Is it a possible scenario ….?

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The fuel heater strainer should increase the temperature of the fuel but anything is possible and in the Herc probable...

Sounds possible to me...

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