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FAR 91.205 states:

§ 91.205 Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements.


(a) General. Except as provided in paragraphs (c)(3) and (e) of this section, no person may operate a powered civil aircraft with a standard category U.S. airworthiness certificate in any operation described in paragraphs (b) through (f) of this section unless that aircraft contains the instruments and equipment specified in those paragraphs (or FAA-approved equivalents) for that type of operation, and those instruments and items of equipment are in operable condition.

(b) Visual-flight rules (day). For VFR flight during the day, the following instruments and equipment are required:

...

(5) Oil pressure gauge for each engine using pressure system.

What is a "pressure system" in this regard?

Background: Our aircraft has an oil pressure gauge that erroneously reads high pressure. The A&P checks this every year and notes that the oil pressure is within the normal range.

EDIT: "I guess I should be clear about what "erroneously high" means, the gauge pegs the needle so it isn't just a little high". – Ron Beyer

The sender has been replaced and the only thing left is the gauge. We've been flying like this for years. Recently one of our CFI's has determined that we are in violation of 91.205 and has told us to ground the aircraft until it can be repaired.

Have we been flying against FAR's? Is a "pressure system" meant to mean a pressurized oil system, or does it mean an aircraft with an altitude engine? Like I said, our A&P (two different ones in fact) have signed off on this since the engine was replaced 10-ish years ago.

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EDIT: With the additional information that the oil pressure gauge is "pegged", the CFI is correct in calling it un-serviceable.

It sounds like the A&P has checked the engine oil pressure with another means and finds that the actual engine oil pressure reading is within limits. The A&P then signs it off thinking that a correct gauge reading is not required. The A&P is incorrect as a serviceable gauge is required as per FAR 91.205

Oil pressure gauges are required for engines with pressurized lubrication systems, but not for engines with "splash" oiling.

Older engine designs, and may of today's smaller engines (like lawn mowers), use a "splash" oiling system that does not supply oil under pressure to the bearing journals, or to other friction points in the engine.

In a "splash" system, oil is applied to the cylinders and pistons by rotating dippers on the connecting-rod bearing caps. As they they rotate, the dippers pass through the oil pan and slash oil into the areas that require lubrication.

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If the oil pressure gauge in your "pressure system" is reading high, outside acceptable limits, it should be replaced. It is easy to confirm using another gauge. High oil pressure can also be caused using the wrong viscosity oil, incorrect pressure relief valve on the oil pump, or bearing clearances that are to tight.

High (or low) oil pressure could cause an engine to fail. If the gauge was out of calibration, it would be impossible to tell what the actual pressure was. If you are accustomed to seeing high oil pressure, how could you possibly identify what was too high, or what was too low?

I would not want to fly any aircraft with an inaccurate oil pressure gauge because it would rob you of valuable time to deal with an engine that might fail at any time.

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  • $\begingroup$ "The A&P is correct that the high oil pressure readings should be addressed" The A&P keeps signing the aircraft off at annuals even when explicitly being asked about this issue, it's the CFI who is telling us to get it fixed... $\endgroup$
    – Ron Beyer
    Commented Mar 24, 2020 at 4:53
  • $\begingroup$ I guess I should be clear about what "erroneously high" means, the gauge pegs the needle so it isn't just a little high. $\endgroup$
    – Ron Beyer
    Commented Mar 24, 2020 at 14:18
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    $\begingroup$ @RonBeyer that means the gauge is clearly unserviceable, as in completely useless, and you really shouldn't be flying it. I could see a condition where you are told to subtract 5 PSI from the guage indication for actual, although I would say this would require a calibration placard on the panel to be valid (the information has to be available to anybody flying). At minimum the CFI would be correct to refuse the fly the plane as PIC. Whether it's grounded or not it's up to you. My mind is boggled that an A&P would sign off on an unserviceable critical item like that. $\endgroup$
    – John K
    Commented Mar 24, 2020 at 17:06
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It just means an engine using a pressurized oil system to the main and cam bearings. These systems require oil pressure to keep the metal of the bearing and the journal separated, as opposed to roller or ball bearings where there is direct rolling metal contact and the oil only needs to be splashed (radials use roller/ball bearings typically for the main journals). A pressurized oil system normally never has metal to metal contact at the moving interfaces except when starting, and if you ran such an engine without ever stopping it but just replaced the oil, the bearings would theoretically last forever. So the pressure is actually a critical item especially at takeoff power.

I think technically the CFI is right, in that the guage is unairworthy insofar as it reads incorrectly and the green arc is a go-no-go item. If the A&P added a placard with a correction factor, maybe that gets into a grey area legally speaking and could be allowed, but really it should be fixed and I'm surprised the mechanic would sign that off just saying "it's fine".

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