Fuel flow test fail

[marcusofcotton]

^^^ Very well presented analysis DPearson!!! ^^^

[Bcone1381]

I wish we could probe this deeper over a cup of coffee at Oshkosh, Mark!

I once believed pump suction feed in BOTH from one tank empty in a gravity feed system would surely result in an engine failure. I also erroneously believed it did happen.

I don't believe it is a risk anymore (in a MFI system that has no malfunctions like a tank venting issue.) and the accident did not happen due to it.

I belive that the L & R tanks are joined at the TEE of the fuel selector, and a high tank never lets the lower tank's fuel line get empty. (if a system has no malfunctions.) The full tank always fills the empty fuel line via gravity through the TEE. This prevents a pump from ever sucking air (in a MFI system with no malfunctions).

We started to question air sucking pump theories years ago. An accident happened to a Four Place. Fuel Levels in the tanks were uneven tank, so I believed erroneously that it failed due to a pump sucking air. That accident hit home. It made me feel unsure about the fuel system with MFI. We as group in good faith believed wrongful data. The owner/builder of the accident aircraft discovered the accident was a result of the wrong fuel pump installation after the repair and subsequant flight and another engine hicup. (I hope I recalled this correctly.) I no longer see any data that shows a pump was able to suck air from an empty tank.

I also don't think this forum is a good platform to debat it. Maybe a campfire, a zoom call, or an authority writing a article.

[Bcone1381]

I keep questioning what I wrote....the Dpearason post was excellent, and his background is robust...mine is not. I'm just a pilot and lack understanding. I still have questions, but in this post I gave my input then said "This is not a good platform to debate it." I fear I shut down discussion with me having the last word. Thats wrong.

[kestrel]

Dpearson points out that there should be a gravity feed test to the pump and then a pump test to the engine. I agree and would add that I would like to see the gravity flow to the fuel pump be achieved with a single fuel tank for the case where one tank is empty or sloshed to the other end of the tank. Flow tests on "both" seem inadequate unless you will always fly on both (or at least always when climbing or might need high power such as possible go-around)

There is another element for any fuel system that includes "return fuel" to a header tank or the fuel tanks. On these systems the fuel system is going to flow more fuel than the engine needs. It seems that in this case the gravity feed test to the fuel pump should be based on the flow rate of the pump/fuel system rather than the engine consumption. This is not a concern for typical Lycoming MFI, but EFII flows 40 GPH and I think the Conti systems may get close to that.

I'll let others decide if the gravity portion of the flow tests should be based on 125% or 150% as I see good argument for either.

[Dpearson]

rodsmith, I’m curious: did you ever get to the bottom of this?

[rodsmith]

I replaced the tubing/hose from the fuel selector to the engine driven pump with half inch. Also changed the routing to allow sweeping bends and eliminate a 90 deg fitting. Bought the airflow performance pump and an Andair gascolator with 1/2 inch fittings and about triple the filter area of the 3/8th version. Result was double the original flow rates, about 10% above required rate. Downside besides the cost and time spent is now I need to fabricate a composite cover for the gascolator. It sticks through the belly two inches while the original fit inside.