Originally Posted by
43Inches
Looking at the fuel system schematics on the Chipmunk they do seem to have solved this by having the fuel selector position lower than the tanks. So there is still a gravity feed to where the lines join and the selector, from the sump outlet, as well as built in non return valves in the tank outlets.
I think the fuel selector of the PA-28-180 is also below the tank outlets in level flight but it certainly is not at the highest sustainable pitch attitude.
I have found the discussion interesting and informative and wondered what addition guidance had been written by FAA. I found AC 23-16A which has some discussion of the subject in section "23.951 Fuel System General (Amendment 23-43)" which starts on page 43.
It includes the following -
"(Many in-service airplanes were certified under the Civil Air Regulations (CAR). For background, CAR § 3.430, which preceded § 23.951(b), required the fuel system arrangement to “permit any one fuel pump to draw fuel from only one tank at a time.” In the past, this has sometimes been interpreted (incorrectly) as “a fuel pump may only draw fuel from one tank at a time.” The CAR regulation is actually more permissive than § 23.951(b) as it did not address the possibility of introducing air into the fuel system.)"
ref
https://www.faa.gov/documentLibrary/.../AC_23-16A.pdf
edit to add text of CAR 3.340 -
"§ 3.430 Fuel system arrangement. Fuel systems shall be so arranged as to permit any one fuel pump to draw fuel from only one tank at a time. Gravity feed systems shall not supply fuel to any one engine from more than one tank at a time unless the tank air spaces are interconnected in such a manner as to assure that all interconnected tanks will feed equally. (See also § 3.439.)"