The other day I overheard the following conversation:
- Pilot #1 —
- The gauges in that C152 are completely useless. The reading dropped from 1/3rd to zero all at once, when there was still 1/3rd of a tank of gas.
- Pilot #2 —
- Yeah, I never look at the fuel gauges, and you shouldn’t look at them either. They should just rip out all fuel gauges entirely. You should just rely on your watch and your preflight measurement of the fuel level.
- Pilot #1 —
- But isn’t there a regulation that says you have to have fuel gauges?
- Pilot #2 —
- The regulation is a joke. The gauge is required to read zero when the tank is empty, but otherwise it’s allowed to read any crazy thing it wants.
The view expressed by pilot #2 is wrong in so many ways that it’s hard to know where to start. What’s worse is that such views are fairly common in the aviation community. This is a safety problem, and needs to be corrected.
Here are the facts:
This fuel-gauge requirement can be seen as a corollary of a more general and more fundamental point, namely the importance of taking a layered approach to safety. For any important task, you want to have multiple independent ways of dealing with the task, so that each way can serve as a check and a backup for the other(s). For example, the right magneto is a backup for the left magneto.
When we apply this idea to fuel, it means you should have multiple independent sources of information about the fuel quantity. Good sources include (a) the preflight measurement (using pipette or dipstick) in combination with timing, and (b) the gauges. Each source should be a check on the others, and a backup for the others.
Of course you should not rely on method (b) to the exclusion of method (a) ... or vice versa. Instead, you should use them both, crosschecking one against the other.
Remember that having two magnetos doesn’t just make the engine twice as reliable; it makes it thousands of times more reliable, for reasons discussed in reference 1. By the same reasoning, using gauges as a backup to the pipette makes fuel exhaustion vastly more unlikely. Maybe you are losing fuel due to a leak. Maybe one tank isn’t feeding because of a blockage. Maybe you made a mistake with the pipette. You really need multiple independent sources of information, so that each can serve as a backup for the others.
The FAA has said repeatedly that the intent of FAR 23.1337(b) and FAR 91.205(b)9 is to prevent fuel-exhaustion accidents. If you have a fuel gauge that doesn’t give you a useful indication of the amount of fuel, it is not doing its job.
In particular, if the gauge is so inaccurate that you prefer not to look at it, that’s a violation of the letter and spirit of the regulations.
Tolerating defective fuel gauges is directly antithetical to the most basic principles of safety. You want layers of safety.
First of all, we should be clear about the principle that things are supposed to be reasonably accurate even if the FARs don’t spell out exactly how accurate they need to be. For example, FAR 91.159 mandates certain VFR cruising altitudes. It does not specify how accurately you must maintain the altitude ... but that lack of specificity does not give you a license to fly some willy-nilly altitude. You’re allowed some “reasonable” tolerance.
FAR 23 doesn’t spell out the fuel-gauge tolerance, but still the gauge must reasonably accurate. (This is true at the bottom of the gauge and everywhere else, so the myth that says “FARs require the gauge to be very accurate at zero and allow it to be wildly inaccurate elsewhere” is wrong twice over. The myth overstates what is required at the low end, and understates what is required elsewhere.)
I don’t know of any official guidance on what is a reasonable tolerance for general-aviation fuel quantity indicators. Instead, I offer some unofficial common-sense suggestions.
Class I: ±4 percent of indication, ±2 percent of full scale Class II: ±2 percent of indication, ±0.75 percent of full scale Class III: ±1 percent of indication, ±0.5 percent of full scale.
The Class II spec is even tighter than TSO-C55. It means that the reading can be off by less than 3% at the high end of the scale, and less than 1% at the low end. Class III is tighter still.
23.1337 Powerplant instruments installation. .... 23.1337(b) Fuel quantity indication. There must be a means to indicate to the flightcrew members the quantity of usable fuel in each tank during flight. An indicator calibrated in appropriate units and clearly marked to indicate those units must be used. ....
That seems pretty clear to me. A gauge that reads zero when the tank is still 1/3rd full does not meet this requirement.
The regulation goes on to say:
.... In addition: 23.1337(b)(1) Each fuel quantity indicator must be calibrated to read "zero" during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply....
Again, that seems pretty clear. The purpose of 23.1337(b)(1) is to reiterate that the gauge is supposed to indicate usable fuel, not total fuel. This does not supersede the main stem of 23.1337(b), but merely clarifies it. The requirement to not include unusable fuel is plainly and explicitly “in addition” to the basic requirement, not instead of it.
The basic requirement remains to “indicate to the flightcrew members the quantity of usable fuel in each tank during flight.”
Another relevant regulation is:
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: .... (9) Fuel gauge indicating the quantity of fuel in each tank.
Among other things, that means you are not allowed to placard the fuel gauge as “inoperative” in accordance with 91.213(d). The gauge is required to be there, and it is required to work.
Some other tangentially-relevant regulations are:
91.7 Civil aircraft airworthiness. (a) No person may operate a civil aircraft unless it is in an airworthy condition. (b) The pilot in command of a civil aircraft is responsible for determining whether that aircraft is in condition for safe flight. The pilot in command shall discontinue the flight when unairworthy mechanical, electrical, or structural conditions occur.
FAR 91.7(a) is tricky. The FAA has all sorts of counterintuitive, legalistic interpretations of what is meant by “operate” and what is meant by “airworthy”. I don’t want to get into the technicalities of this.
FAR 91.7(b) is more straightforward. The meaning of “condition for safe flight” is relatively intuitive and commonsensical (especially when compared to the FAA notion of “airworthy”).
91.405 Maintenance required. Each owner or operator of an aircraft -- (a) Shall have that aircraft inspected as prescribed in subpart E of this part and shall between required inspections, except as provided in paragraph (c) of this section, have discrepancies repaired as prescribed in part 43 of this chapter;
In the case of small discrepancies, I’m not sure how promptly they are supposed to be repaired. In the case of major discrepancies such as an untrustworthy fuel gauge, I reckon they need to be repaired before the next flight.
43.11(b) Listing of discrepancies and placards. If the person performing any inspection required by part 91 or 125 or section 135.411(a)(1) of this chapter finds that the aircraft is unairworthy or does not meet the applicable type certificate data, airworthiness directives, or other approved data upon which its airworthiness depends, that persons must give the owner or lessee a signed and dated list of those discrepancies. ....
An untrustworthy fuel gauge is a “discrepancy” within the meaning of FAR 43.11(b) and must be repaired in accordance with FAR 91.405.
In general, you always want to have multiple layers of safety.
It is unsafe – and contrary to regulations – to fly the airplane if the fuel gauges aren’t indicating how much usable fuel remains.
Don’t let anybody tell you otherwise.
The owner/operator is responsible for getting the fuel gauging system fixed. The pilot is primarily responsible for not flying the plane until it is fixed.