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air lock/fuel starvation with AFP fuel pump

KayS

Well Known Member
Hi All,

i have the bendix style silverhawk throttle body combined with the airflowperformance fuel pump/filter unit without a return line.

the pump/filter assembly is the old spaghetthi style setup from AFP. (the newer assembly looks much cleaner but should have the same or similar components.)

i was thinking to do an tank dry run test during cruise flight with my 7 and in preparation for that i came across a warning from AFP not to do so:

Page 21:
https://airflowperformance.com/wp-content/uploads/2018/12/API-Installation-Service-Manual-9003.pdf

it states that an air lock "will" be formed at the outlet of the boost pump making the lycoming silent. what do you think?

for me that implies that you should never unport the fuel pick ups in the tanks at all, not only running tanks intentionally dry during flight but also unporting situations due to slips at final, ucoordinated turns, aerobatics etc. is this setup especially problematic when it comes to that i kind of situations?

Cheers
Kay
 
i was thinking to do an tank dry run test during cruise flight with my 7 and in preparation for that i came across a warning from AFP not to do so....is this setup especially problematic when it comes to that kind of situations?

It is possible, thus the good folks at AFP quite reasonably insert a CYA warning. It's impossible for the pump supplier to predict or be responsible for the behavior of all possible fuel systems, yet they get blamed if the spit hits fan.

Test your fuel system, on the ground. Drain a tank. Fire up the engine on the other tank. Set runup RPM at least, switch to the empty, let the engine die. Switch back to the tank with fuel, and turn on the boost pump. Do you get fuel pressure? If yes, repeat a few times, and check the other tank the same way next time it's almost empty. Be happy.
 
Dan, thanks for the response! When i do a ground test there will be no windmilling prop and therefore no spark plugs sparking and mech fuel pump pumping. but the ground test will tell me if my setup develops an air lock around the boost pump, right?
 
Dan proposes a good test protocol, that would pull all the fuel out of the lines then switching to the second tank, the boost pump would be the only source of pumping if the engine was not rotating. It sounds like a good test of your specific concern - loss of prime.

Background experience: I have ground tested for fuel flow, leaks etc, and for usable fuel. The RV has quite low unusable at least statically.

When testing with (w/boost pump) it will make a definite change in sound (cavitation) when it begins to suck air. No need to really run until it quits. Even so, it primed and ran just fine after adding more fuel, say a gallon. Only 100cc or less fuel was determined to be unusable on my 7 and a friends 10 with flight/landing attitude. We never tried to check sliding under 1 gallon, but I have flown and wallowed around under 2 gallons and had no loss of pressure (-7).

Our test method included a clear section of tubing to check for suction leaks followed by a ball valve adjusted to generate back pressure to recommended fuel pressure. It would take 10 -15 sec to clear all the air if we let it run after the cavitation sound began and fuel pressure dropped.

As you mentioned though, it had no mechanical pump to suck it dry. Even so, I have not run a tank dry in flight, but would not have been running the boost pump at that time anyway.
 
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Dan, thanks for the response! When i do a ground test there will be no windmilling prop and therefore no spark plugs sparking and mech fuel pump pumping. but the ground test will tell me if my setup develops an air lock around the boost pump, right?

Right. If the boost pump will reliably prime and make pressure (which is typically measured downstream of the engine driven pump), you're good.

Ya know, one of the advantages of answering questions is the opportunity to think about details not formerly considered. Here's one which dawned on me due to this discussion. I have an earlier AFP FM-200 with a drum type mixture valve, and thus a purge valve. When opened, the purge valve routes fuel from the FM200 back to the left tank, from a point just prior to the flow divider. Removing the flow divider's 2 psi opening pressure from the circuit means a dry boost pump should prime and pump more easily.
 
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