Copyright © 2007 jsd

1  Aircraft Fuel Gauge Accuracy

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. Similarly, pilots are trained to never overemphasize or underemphasize any particular instrument, but rather to scan all the instruments, cross-check them, and use all available information to build an understanding of the overall situation.

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 an estimate of the fuel-burn rate and the elapsed time, and (b) the gauges. No source should be overemphasize at the expense of the others. Instead, each source should be cross-checked against the others, so as to build an understanding of the overall situation.

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.

For all of these reasons, you really need multiple independent sources of information, so that each can serve as a backup for the others.

Fuel-exhaustion accidents make up a significant fraction of the total accident rate ... and they are almost entirely preventable. For safety reasons alone, pilots should insist on having accurate fuel gauges in any airplane they fly. When tires get worn out, they need to be replaced. When the fuel sensors in the tanks get worn out, they need to be replaced.

As the saying goes, it’s not just a good idea, it’s the law.

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.

2  How Accurate is Accurate Enough?

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.

Similarly, FAR 23 doesn’t spell out just how accurate the fuel gauge needs to be, but still it needs to be reasonably accurate. The gauge needs to be accurate across the whole range. See reference 2.

There is a widespread myth that says “FARs require the gauge to be very accurate at zero and allow it to be wildly inaccurate elsewhere”, but this is wrong twice over. The myth overstates what is required at the low end, and understates what is required elsewhere. See reference 2.

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.

3  The Regulations

Working Fuel Gauges — it’s not just a good idea, it’s the law.
  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....

That also 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 main requirement is to “indicate to the flightcrew members the quantity of usable fuel”. The requirement to not include unusable fuel is plainly and explicitly “in addition” to the basic requirement, not instead of it.

Again: The main requirement is 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

  (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

  (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.

4  Checking Your Gauges

Rule #1: In the course of checking your gauges, do not do anything that would put you in jeopardy of running out of fuel in flight.

Suggestion: Your local hardware store almost certainly carries plastic jugs suitable for holding fuel. Actually you want two of them: a five-gallon jug to hold lots of fuel, plus a one-gallon jug to use as a standard of measurement. You should also buy a siphon designed for siphoning fuel. These siphons include a bellows that serves to start the siphon.

Rule #2: Never siphon by mouth. Never suck on a tube that contains (or could possibly contain) fuel. Fuel in your lungs is likely to cause “chemical pneumonia” – very dangerous.

So, the plan goes like this:

Note that some types of fuel gauges are essentially little voltmeters, so they will read differently with the engine running (battery charging at 14 or 28 volts) as opposed to engine stopped (battery discharging at 12 or 24 volts).

5  Summary and Conclusion

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.

6  References

“Pilot-In-Command Decisionmaking” (Chapter 21 of See How It Flies) http://www.av8n.com/how/htm/decision.html

Federal Aviation Regulations http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfrtpl=/ecfrbrowse/Title14/14tab_02.tpl

Abstract of MIL-G-26988C, http://aero-defense.ihs.com/document/abstract/MOFZDAAAAAAAAAAA

Moritz Aerospace, TSO-compliant fuel quantity indicator system, http://www.moritzaero.com/Products/FuelSystem.aspx

“CiES Digital Fuel Sender Facts”

Copyright © 2007 jsd