Big Watches, Fuel Gauges, and Annunciator Lights

The notion that engines run on air is no more absurd than attempting flight with insufficient fuel in a tank connected to the engine. Yet hardly a week goes by that somewhere a pilot is attempting to go where none have gone before.

The technology of measuring fuel has gotten much better than the wire and cork device used on the Piper Cub. The Cub’s design was stone simple—mount the fuel tank directly in front of the pilot (not such a great idea from a crashworthiness perspective, but only a short hose run to the engine) and attach a cork to the end of a wire rod. The length of rod protruding through the fuel cap indicated how much go-juice remained. Only two things could go wrong: The cork could become saturated and sink, giving the pilot a sinking feeling that no fuel remained, or worse, the rod could become slightly bent and jam, falsely indicating more fuel on board.

We paid little attention to any of that and always used our big pilot watches to determine how long to stay aloft. Time is one of the best indications of what’s left in the tank(s), provided you know what was on board to begin with and are familiar with the engine’s burn rate—both highly recommended procedures.

New pilots are often told that the fuel gauges on light aircraft are unreliable and that the FAA only requires them to be accurate in one condition—empty. Let’s dig into that a bit more. As usual, there seems to be some gray!

Under CAR 3.672 Fuel Quantity Indicator, the Civil Air Regulations by which most legacy aircraft are certificated, “Means shall be provided to indicate to the flight personnel the quantity of fuel in each tank during flight…Fuel quantity indicators shall be calibrated to read zero during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply as defined by § 3.437.” [ital. added]

Under the new regs, FAR 23.1337 Powerplant Instruments Installations—(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. [ital. added] An indicator calibrated in appropriate units and clearly marked to indicate those units must be used. In addition: (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 determined under § 23.959(a)…” It’s not appreciably different from the earlier rule.

It could be reasonably argued that quantity indications should be accurate but sometimes the hardware is suspect. The FAA goes on to say in a safety publication, Time in Your Tanks—P-8740-03, “Fuel gauges are subject to malfunctions and errors. Therefore, unless restricted by the gross weight or center of gravity limits, it is considered good judgment to ‘top off’ the tanks at fuel stops. If the fuel load must be limited, you should endeavor to get an accurate measurement of fuel quantity by using a dipstick calibrated for the aircraft.” Dipsticks are a good measure…IF…they are properly calibrated…AND…the aircraft is sitting level, which allows Murphy’s law to slip in.

With the “rebirth” of GA in the ’90s, most manufacturers added low-fuel annunciator lights to new production aircraft (a.k.a. fuel “idiot lights”—they have been installed on most automobiles and larger aircraft for decades) separate from the gauging system to make it really hard to run the beast out of fuel or a tank dry. In general, it’s worked very well although one young CFI and his student managed to run a new production Cessna, equipped with lights, out of fuel. No need to belabor the point where I’m sure both participants will do better in the future. The fuel accident numbers of new tech aircraft compared to the classics is much better and it’s not because pilots suddenly got smarter.

But suppose you own a classic, and installing annunciator lights is prohibitively expensive. A highly recommended upgrade, second only to installing shoulder harnesses, is to install a fuel flow transducer/totalizer which will accurately measure fuel flow to within a few tenths of a gallon. As for quantity, it must be properly programmed, so it’s not quite as goof-proof as the low fuel lights. Used on every flight, it provides great peace of mind—either more fuel is required or it isn’t. As far as not running a tank dry, a timer or that stopwatch function on your chronograph or some other clock on board should help.

Back to the FARs—Do the gauges need to be accurate or not? In my view, it’s irrelevant. The Air Safety Institute’s “Golden Hour” approach of always landing with one hour of fuel on board elegantly solves the problem.



  1. “FAA only requires them to be accurate in one condition—empty.”

    At least that old wives’ tale can be dismissed; both the old and new FARs require the “quantity of [usable] fuel” to be displayed. That text doesn’t specify precision, but accuracy is right there (“quantity”).

  2. This is an interesting statement by the FAA….”Therefore, unless restricted by the gross weight or center of gravity limits, it is considered good judgment to ‘top off’ the tanks at fuel stops.” It is way too simplistic in my opinion.

    First of all, it only considers the legality of your maximum takeoff weight and your CG. It makes no consideration for performance. There can be a lot more to think about when going on a flight depending on the circumstances. And if you crash in the trees at the end of the short runway because you filled up based on an FAA recommendation, you are not going to get a chance to end up running out of fuel later on in the flight. There are times when filling up and having the weight and CG within limits is a very dangerous thing to do.

    Next, there is the idea of only filling up to your maximum weight for what is a long cross country with minimal reserves. Of course, the FAA will never say or recommend to fly a little bit overweight but think about it…If you are 25 pounds overweight in a Cessna 150, is it dangerous? Some people will say yes but on the long runways we normally use, a 1600 pound aircraft 25 pounds over max gross is really not noticeable.

    But, that is an extra full hour of fuel. Of course the legal(and depending on the conditions) possibly safer thing to do is to make an enroute stop somewhere and then take on the extra fuel. But in the real world of convenience where it may actually be dangerous to proceed to that marginal weather airport in the hills to get some gas(and admittedly saving time).

    I have tended to err on the side of having more gas which may put me a few pounds overweight. It can be very difficult to measure a fuel tank with no fuel dipstick and hoping the fuel guage is correct. Of course some will label me as dangerous. But I ain’t running out of gas and a lot of people seem to do it. They get lost, have to go to a further than planned alternate or have some other unplanned event.

    In the end, it is what is known as a calculated risk. Thinking you are a safer pilot because you have thirty minutes fuel reserve and are therefore legal and I am dangerous because I am 25 pounds overweight but have an hour and thirty minutes fuel reserve is not necessarily good judgement.

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