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  #1  
Old 03-01-2021, 02:10 PM
DanH's Avatar
DanH DanH is online now
 
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Default Fuel Pressure Fitting

I recently viewed some photos taken after a fuel starvation incident, and thought it might be good to pass along a tip for the new guys.

The builders' goal was to access the engine driven fuel pump outlet for a fuel pressure sender. He stacked a 90 degree pump outlet, an automotive aftermarket tee fitting (with female and male ends), and a 90 degree tube end on the hose to the servo.

There are at least two issues here. One, it's a string of fittings with a line leading off at a right angle to the primary thread axis. An offshoot line arranged that way can easily become a lever, providing torque to unscrew one of the connections.

Worse, it appears the servo hose was allowed to contact the engine mount tube, downstream of the tube end, which in addition to the above placed another potential turning moment on the stacked connections.

The combination was all but guaranteed to unscrew one of the fittings eventually, and in fact did so. The subsequent loss of fuel pressure resulted in a forced landing, and yes, it could have caught fire in flight.

There are lots of better ways to skin this cat, but for the new guys, the generally accepted EAB practice is to drill and tap the pump fitting for a straight steel nipple, 1/8" NPT to -3 or -4 flare, which couples to the sender hose. The sender hose cannot unscrew the pump fitting, or the servo hose. The modified fitting is so common that Aircraft Spruce sells them already tapped: https://www.aircraftspruce.com/catal...lickkey=335909

Let's be careful out there.
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Last edited by DanH : 03-01-2021 at 02:13 PM.
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  #2  
Old 03-01-2021, 05:09 PM
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Thanks Dan. I've seen similar kinds of stack-ups out in the wild too. I struggle with how to encourage the builder to look at it a little more, but usually find a way. When it just doesn't look right, it probably isn't.
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  #3  
Old 03-01-2021, 06:01 PM
Kyle Boatright Kyle Boatright is online now
 
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Quote:
Originally Posted by DanH View Post
One, it's a string of fittings with a line leading off at a right angle to the primary thread axis. An offshoot line arranged that way can easily become a lever, providing torque to unscrew one of the connections.
Other than the stackup of fittings, is this different than the main fuel line, which 90's off of the fuel pump fitting and (presumably) could loosen the fuel pump fitting?

For what it is worth, the RV-10 FWF plans call out a 45' fitting coming off of the fuel pump fitting for the pressure line.
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  #4  
Old 03-01-2021, 06:31 PM
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DanH DanH is online now
 
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Quote:
Originally Posted by Kyle Boatright View Post
Other than the stackup of fittings, is this different than the main fuel line, which 90's off of the fuel pump fitting and (presumably) could loosen the fuel pump fitting?
The main line, to the servo, should not be allowed to flop about in space. It should be firmly tied to the engine, so hose, engine, and pump are all moving together. No moment to loosen anything.

Worst case, the pump fitting still can't unscrew from the pump. The hose has it locked to a few degrees of rotation. A poor installation might loosen the locknut, allowing leakage past the o-ring, but the result won't be fuel starvation.
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  #5  
Old 03-01-2021, 08:33 PM
Aluminum Aluminum is offline
 
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Very timely thread.

RV-14A plans call for just such a stack-up, with the little 45deg NPT fitting carrying a vibrating hose attached to a manifold on the firewall.

The other piece of this puzzle that is making me uncomfortable is the big 90deg fitting that goes into the fuel pump, which is a "KB" type not NPT, but it is clocked per plans. I was under the impression that o-ring fuel fittings are supposed to be bottomed out so the o-ring works as designed. Clocking defeats this mechanism in unpredictable ways.
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  #6  
Old 03-01-2021, 08:42 PM
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Thanks Dan! I would have never considered engine-line-pump movement causing fitting to come loose. Funny how there's safety wire on oil filters which are not prone to this but not a b-nut for a fuel line.
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Last edited by avrojockey : 03-01-2021 at 08:45 PM.
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  #7  
Old 03-01-2021, 08:59 PM
TS Flightlines TS Flightlines is offline
 
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This is our fitting---same idea as the 2 piece Vans/Spruce version, except ours is 1 piece. -4 side can be restricted for those with sensitive fuel pressure sensors.

Tom
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  #8  
Old 03-02-2021, 05:33 AM
Bcone1381 Bcone1381 is offline
 
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https://shop.vansaircraft.com/cgi-bi...=pump-fittings
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  #9  
Old 03-02-2021, 06:02 AM
Jetmart Jetmart is offline
 
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Could this failure have anything to do with the possibility of the automotive flare being and different angle then a standard aircraft flare fittings?
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  #10  
Old 03-02-2021, 07:04 AM
TS Flightlines TS Flightlines is offline
 
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The industrial flare angle is 37* like the AN.
Tom
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Teflon Hose Assemblies for Experimentals
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