Fuel is carried in the wings or in tanks inside of the fuselage. High wing aircraft have the advantage that gravity will always make sure that fuel flows to the engine mounted on the nose of the aircraft.
With a low wing aircraft, fuselage tanks or wing mounted engines the fuel will need some help to get to the engine. A well designed system must make sure that the flow will not be interrupted at any time.
Each tank must have a coarse filter and a drain at the lowest point so that water and residue can be removed. Each tank ideally should have an ON-OFF valve. With twin engine aircraft the tanks should be able to feed the other engine, this is called cross flow.
Backup pumps and a fuel pressure, quantity and flow indicators are also helpful items for the pilot.
Below we are going to discus the Rotax fuel system items as you will find on most small aircraft powered by Rotax engines. Intimate knowledge by the pilot is required for safety reasons.
Fuel flows from the tank (which, as already mentioned, should contain a coarse filter and a water drain) to a fuel valve/cock, gascolator with drain at the lowest point in the aircraft, fine filter before the mechanical fuel pump on the engine near the gearbox. A return line is fitted to avoid formation of vapor lock. See the image to the right.
From that point the fuel line is connected to both Bing carburetors. This is the system for all 912 and 914 engines. Note that the iS engines have a fuel injected system.
The optional electric fuel pump should be installed after the fine filter (to protect the pump) and before the mechanical pump. The pump should then be operated just before engine start and during maneuvers, takeoff and landings. In case of engine failure you will switch the pump on too as part of the engine failure checklist.
This engine has no mechanical pump (due to the fact that fuel pressure must be 0,25 bar higher than air box pressure) so fuel must be brought to the engine by an higher pressure electrical fuel pump. This pump must be backed up by a second pump for fail safe reasons. But how do you connect them? In series or parallel?
Early model 914 engines and old installation manuals show a parallel setup of the fuel pump. See the image. But there was no check valve preventing fuel to flow through the non operating pump (back flow), thereby starving the engine from fuel in some cases.
With this arrangement the pumps are placed in series and each pump has a parallel check valve. See image. This makes sure that a non operating pump can be bypassed through its parallel check valve so that fuel flows parallel to the pump.
This should work as long as the pumps can guarantee flow through the pump without it being operated (transparency). Which is not the case with positive displacement pump. This is easy to test in a quick setup if you are not sure about the pumps you have.
Prevents vapour lock and the fuel can keep cooling the pumps, as they can run warm. With multiple tanks on the aircraft you will need a header tank with enough capacity to contain the returned fuel or a selector valve capable of switching the return line so that fuel is returned to the tank in use and thus will not overflow the wrong tank.
If such a header tank is not available make sure you know which tank receives the return fuel and use fuel from that tank first as to prevent the tank from overflowing. Return fuel is usually not much but any fuel leak may constitute a fire hazard.
Make sure that you do not raise fuel pressure to high for the mechanical fuel pump as a massive fuel leak could occur and a possible fuel starvation situation. See FAA AD-2010-18-14.
Rotax discovered a deviation in the pump attachment bolts where oil leaks could occur and a possible un-commanded inflight engine shutdown, more information in: EASA-EAD-2012-0019-E.