I broke an MA1a

I broke an MA1a

Postby TillamookTurbine on Thu Mar 19, 2009 8:04 pm

Nothing's foolproof because fool's are so ingenious was my motto today.

I have an MA1A that's been out of service for about 4 years. Last time it was used, it would not light off, I was told. Today, I investigated and found several discrepancies. The boost pump was inop and the power to the igniter box and main fuel valve would not come on at 20%. I diassembled the BP and got it operational. Suspecting a bad oil pressure switch, I merely jumpered around it to get the igniter and main valve open. Light off and acceleration was normal and just as the tach read 100%, a massive fuel leak erupted from the base of the FCU. I immediately shut down; fortunately with no fire. Investigation showed that the fuel screen housing had split (see photo).

Has anybody seen this failure on a GTC-85? Do I have a fuel pressure regulator valve stuck in the FCU? Any help will be appreciated since I'm sending this MA1A with some J85's for a test program in about 2 weeks.

Thanks,
Terry Spath
Attachments
MA1A split screen housing 3_19_09 013.jpg
MA1A split screen housing 3_19_09 013.jpg (174.57 KiB) Viewed 4673 times
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Re: I broke an MA1a

Postby j79guy on Thu Mar 19, 2009 8:39 pm

Terry,

Looks like the FCU main relief valve is stuck. At least the pressure took the path of least resistance. Clean out the relief and you should be good to go. Makes me wonder though, is the fuel system contaminated with water, or perhaps organics growing in it? Might be time to put a little fungicide in the fuel tank.

If you need any parts, let me know.

Robin.
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Re: I broke an MA1a

Postby TillamookTurbine on Thu Mar 19, 2009 8:58 pm

Looks like I'll be taking it apart tomorrow. The tank had a leak before the RASCAL program so we just plumbed it into the pressure fuel system that we fed the J79 and F100 with. However, sitting so long I wouldn't be surprised that the relief poppet is rusted to its seat.

I'll definitely need a housing, if you have one. I could manufacture a home made one but would prefer to buy one from you.

Terry
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Re: I broke an MA1a

Postby j79guy on Thu Mar 19, 2009 9:02 pm

Terry,

I'll pull one off my parts donor tommorow. let me know a ship-to address and it'll get out asap. If you don't want to advertise your address here, send me an e-mail.

Robin.
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Re: I broke an MA1a

Postby TillamookTurbine on Thu Mar 19, 2009 9:39 pm

Thanks Robin:

Spath Engineering
5900 Hangar A Road
Tillamook, Oregon 97141
503-812-0606
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Re: I broke an MA1a

Postby j79guy on Fri Mar 20, 2009 4:43 pm

Terry,

Ok, I have a replacement filter bowl and element for you, it will go out first thing Monday.
I noticed that it is common for these to be grossly over torqued. As they have an o-ring seal, they only need to be snug, and lockwired. If you over torque, the next guy who takes the filter bowl off, can cock the socket on the bowl, and dent or crack the housing. I suspect that your fuel system may be ok, just that the bowl might have been damaged by previous mishandling.

Robin.
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Re: I broke an MA1a

Postby j79guy on Fri Mar 20, 2009 4:50 pm

Looking at your picture, it appears that right in the middle of the split, there is a tell-tale dent where some time in the past a socket cocked over and damaged the filter bowl.
Still wouldn't hurt to flush out the rest of the fuel system and inspect the relief valve.

Robin.
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Re: I broke an MA1a

Postby TillamookTurbine on Sat Mar 21, 2009 8:17 pm

I took the FCU apart and I wish that I could report that I found a big piece of gunk plugging a relief valve. However, everything appeared fairly clean except there was a little grit on the governor spool valve that rides up and down on the governor shaft. I believe this acts as the main metering valve. It's actually appears to meter by holding pressure in the flyweight housing of the FCU. When it goes overspeed, it opens a port in the shaft that vents the flyweight housing thereby reducing metered FP. It can sort of be seen in the second photo. One can also see the flyweights and the housing at the top of the photo which has the P3 (CDP) diaphram. I believe this diaphram controls a bypass valve which is open until P3 reaches a high level; preventing rich flameout.

There is also a spring ball valve which I think holds a constant delta P across the metering valve. I don't think this valve acts as a relief because the ball port is configured 180 degrees from the direction that would relieve pressure.

The fuel pump end has a very light ball valve which I think allows some flow past the fuel pump shaft for lube.

Does anybody have a circuit diagram for this unit? It's kind of hard to trace the passages in the body to figure out where the failure was. If the speeder valve stuck in the underspeed position momentarily because of the grit while the RPM hit 100% I suppose the pressure could climb rapidly thereby rupturing the housing.

Thanks,
Terry
Attachments
Saturday March 21 2009 002.jpg
Fuel Pump end
Saturday March 21 2009 002.jpg (76.5 KiB) Viewed 4612 times
Saturday March 21 2009 005.jpg
Governor and P3 control end
Saturday March 21 2009 005.jpg (72.77 KiB) Viewed 4610 times
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Re: I broke an MA1a

Postby j79guy on Sun Mar 22, 2009 11:02 am

Terry,

Unfortunately, I don't have a governor diagram for this unit. I do suspect though, that it will likely run fine when put back together, if nothing else, just for flushing the little bit of grit out.
Don't stand in the plane of the red lines painted on the enclosure, on first fire up.(!)

Robin.
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Re: I broke an MA1a

Postby f4dj79 on Mon Mar 23, 2009 10:45 am

I am having trouble understanding this failure. There could not have been that much pressure buildup in the fuel control to cause the filter bowl to blow out. My first guess is the wrench damage is the give away. The case was split before operations! The fuel pump pressure to the fuel metering unit can't be much more than 30psi and I just do not believe that even under a hydrualic compression that the system would lead to the bowl blow out. If there is no obvious debris clogging up the works then it must have been something else. The picture actualy shows damage on the bowl that runs across the crack in a crushing pattern.
Oops I have done the same thing here at Boeing once and it is embrassing, we just get new parts and try not to repeat.
WB
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