The fuel-injected engine has six cylinders, so it's not a "per-cylinder" thing nor is it related to a carburetor.

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P.S. That screen is the entirety of the air filter. Apparently it's meant to keep out small birds, and not much else.


1 Answer 1


Those are the impact tubes for the fuel metering for the RSA-type fuel injection system:

enter image description here

Above is the face of the fuel servo. If you had more light you would have also seen the throttle valve as shown above. When the throttle opens, air rushes in, and the higher pressure in the impact tubes would deliver more fuel. See schematic and description below:

enter image description here

... As a result, the diaphragm moves to the left, opening the ball valve. Contributing to this force is the impact pressure that is picked up by the impact tubes. [Figure 2-32] This pressure differential is referred to as the "air metering force." This force is accomplished by channeling the impact and venturi suction pressures to opposite sides of a diaphragm.

Source: FAA-H-8083-32A. Aviation Maintenance Technician Handbook-Powerplant Volume 1.

  • $\begingroup$ How neat. I guess this is how the mechanical fuel injection regulates the amount of injected fuel. I hadn't really before considered how this would work, I just assumed the injectors were open for longer, much like in a EFI setup. $\endgroup$ Commented Apr 16, 2021 at 1:52
  • $\begingroup$ @KennSebesta: I'm not familiar with all the types, but this is the Bendix RSA type. Neat indeed. $\endgroup$
    – user14897
    Commented Apr 16, 2021 at 1:57
  • 1
    $\begingroup$ The airplane FI systems like the Bendix are a very old fashioned constant flow type, similar to the old Bosch mechanical fuel injection on cars back in the 60s/70s before EFI came along. There are no injector units at the cylinders that open and close; fuel is coming out of the injector nozzles full time. That's why you prime these engines just by turning on the fuel boost pump and moving the mixture to full rich; as soon as the mixture goes in the system starts spraying fuel. You have to start with the mixture at idle cutoff and move it up after it catches, or you'll have a nice fuel flood. $\endgroup$
    – John K
    Commented Apr 16, 2021 at 3:06
  • $\begingroup$ Seems like this design owes far more to traditional carburettor principles than to electronic fuel injection as we know it now. $\endgroup$
    – MikeB
    Commented Apr 16, 2021 at 8:38
  • $\begingroup$ Mechanical fuel injection has been around since the 30s. They were all constant flow until EFI was developed in the 70s. EFI never really made it into aviation due to the cert hurdles (you have to create a fully dual redundant electronics control system) and the only incremental improvement over constant flow in the case of airplanes, so they just stuck with tried and true until recently. $\endgroup$
    – John K
    Commented Apr 16, 2021 at 13:13

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