MML:
Cheers all, Just started to fly an A23-24 HP Musketeer. Can someone shed some light on the fuel pressure guage? What should a normal indication look like, what is the most prevalent use for the fuel pressure gauge, and other comments anyone might have?
Steve Robertson:
The fuel pressure gauge has only limited usefulness. It will rise and fall with changes in RPM and mixture.
It’s main use is to tell you something is wrong. If the pressure begins to drop below what is “normal” for the flight regime you are in, it is telling you that there is a fuel starvation problem. That would be a tank running dry, fuel valve positioned off or in between tank detents, or a failing fuel pump. If the pressure reads higher than “normal” for the flight regime you are in, it usually indicates a clogged injector.
As to what readings are “normal” for different flight regimes, you will have to fly for awhile and determine that for yourself. These things aren’t really calibrated such that the readings on your plane would be the same as on my plane all operating conditions being equal. FWIW, mine barely registers anything at idle, about 8 on full power full rich climb-out, and somewhere +/- 5 at 65% cruise power. But as I said, it varies with RPM, altitude, and leaning. My Super has a fixed pitch prop. A CS prop would induce additional effects on the “normal” readings.
Editor’s Note:
Keep in mind that these “pressure gauges”, like the “fuel flow” gauge on the Sierra or CSP-equipped planes, are not measuring primary fuel pump pressure. They are measuring the pressure in the flow divider, or “spider”, on top of the engine, which delivers the fuel to the fuel injection lines. This is metered fuel that has passed through the Bendix fuel servo; not fuel from the fuel pump to the servo. That’s why the “pressure” reads so low at idle; there is very little fuel flow at idle, therefore very low pressure in the spider. The gauge simply shows that fuel is being sent from the servo into the engine, while enabling some other diagnostics as described by Steve.