Rotax Fuel Systems
Fuel is carried in the wings or in tanks in 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 fuel system must make sure that fuel flow will not be interrupted at any time.
Each fuel tank must have a coarse filter in the tank and a drain at the lowest point so that water and residue can be removed. Each tank ideally should have a on off fuel valve. With twin engine aircraft the tanks should be able to feed the other engine.
Backup fuel 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 knowlegde by the pilot is required.
Basic Rotax Fuel System
Rotax Fuel System
Fuel flow from the fuel tank (which, as mentioned, should contain a coarse filter and a water drain) to a fuel cock, gascolator with drain at the lowest point in the aircraft, fine filter to the mechanical fuel pump on the engine near the gearbox. A return line is fitted to avoid formation of vapour lock. See image to the right.
From that point the fuel line is connected to both Bing carburettors. This is for all 912 and 914 engines.
Electric pump
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, take off and landings. In case of engine failure you will switch the pump on too.
Rotax 914 System
This engine has no mechanical fuel pump (due to the fact that fuel pressure must be 0,25 bar higher than airbox 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 fuel pump for fail safe reasons. But how do you connect them? In series or parallel?
Parallel
Early model 914 engines and old installation manuals show a parallel setup of the fuel pump. See image. But there was no check valve preventing fuel to flow through the non operating pump (back flow), thereby starving the engine from fuel.
Series
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.
Series no check valve
This should work as long as the pumps can guarantee flow through the pump without operating it (transparency). Which is not the case with positive displacement pump. Easy to test in a quick setup if you are not sure about the pumps you have.
Fuel return
Prevents vapour lock and the fuel can keep cooling the pumps, as they can run warm. With multiple fuel tanks on the aircraft you will need a header tank with enough capacity to contain the returned fuel or a fuel selector valve capable of switching the return line so that fuel is returned to the tank in use.
Issues
Make sure that you do not raise fuel pressure to high for the mechanical fuel pump as a fuel leak could occur. See FAA AD-2010-18-14.
Rotax discovered a deviation in the fuel pump attachment bolts, more in: EASA-EAD-2012-0019-E.
