Common Aircraft Fuel Myths

There are a number of fuel related myths and misrepresentations floating around the general aviation industry. While I can’t touch them all, I hope to provide some enlightenment around a couple of them in this post. Starting with one about aircraft fuel gauges.

Fuel Gauges

At some point in your past, have you ever heard a pilot or a mechanic make a statement about the accuracy of the fuel gauges? It usually goes something like this, “The only time that the regulations require the fuel gauges to be accurate is when they are empty.”

This statement is a misrepresentation of what the regulation actually said. I am putting this in the past tense because the regulation under 14 CFR Part 23 has been rewritten in the last year or so.

At first glance, the “empty” statement appeared to be true but you needed to read the whole regulation to put it into context.

The regulation in question was §23.1337 titled, “Powerplant instruments installation.” The relevant text starts at §23.1337(b):

…

(b) Fuel quantity indicator. 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. 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 [Sec. 23.959(a);]

(2) Each exposed sight gauge used as a fuel quantity indicator must be protected against damage;

(3) Each sight gauge that forms a trap in which water can collect and freeze must have means to allow drainage on the ground;

(4) There must be a means to indicate the amount of usable fuel in each tank when the airplane is on the ground (such as by a stick gauge);

(5) Tanks with interconnected outlets and airspaces may be considered as one tank and need not have separate indicators; and

(6) No fuel quantity indicator is required for an auxiliary tank that is used only to transfer fuel to other tanks if the relative size of the tank, the rate of fuel transfer, and operating instructions are adequate to–

(i) Guard against overflow; and

(ii) Give the flight crewmembers prompt warning if transfer is not proceeding as planned.

…

The often misunderstood portion of the regulation is §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 determined under [Sec. 23.959(a);]” At first glance, it could be read as “it only needs to accurate when it is at zero” but this is not the case. What this portion is trying to clarify is that it needs to read “zero” when the usable fuel is gone even though there may still be “unusable” fuel in the system. It is not saying that is the only time it needs to be accurate.

If we go back to the stem of §23.1337(b), it says, “(b) Fuel quantity indicator. 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. In addition—” This seems pretty clear if you don’t skip over it. The indicating system needs to provide an accurate indication to the pilots during flight of how much usable fuel is left in the tanks at all times. There is no specification as to how accurate but it needs to provide an accurate indication.

If we look at the more familiar “§91.205   Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements,” we’ll see a requirement there as well. When we learn to fly, parts of this regulation are often set to memory. Most student pilots that are approaching their private checkride can tell you that fuel gauges are required for VFR operations. What the regulation actually says is that a “Fuel gauge indicating the quantity of fuel in each tank.” is required. The fact that the gauge is there and showing something is not enough. It must indicate the quantity of fuel in its associated tank.

The new 14 CFR Part 23 regulation related to the fuel system and its gauges is §23.2430 Fuel systems. The “zero” statement is no longer a part of the text. The simplified regulation now states:

(a) Each fuel system must—

…

(4) Provide the flightcrew with a means to determine the total useable fuel available and provide uninterrupted supply of that fuel when the system is correctly operated, accounting for likely fuel fluctuations;

…

As the new version of the regulation indicates, the design must tell the pilots the useable fuel available.

All that said, the fuel gauges on light aircraft are notorious for having problems. Some seem to develop a kind of dead spot when refueled to the same level every time (such as full). Rocking the wings may fix that problem on the ground and a bit of bouncing in the air seems to keep it working while in flight. There are newer sensor designs which purport to alleviate this issue.

You should also not rely solely on the gauge indication to determine how much fuel you have left. If you start with a known quantity of fuel, you should calculate your expected fuel burn prior to your flight and keep track of the time while flying. If either the fuel gauges or your calculations indicate that you are starting to run low on fuel, land at the nearest suitable airport and take on additional fuel.

Fuel in the Tanks

Many a flight instructor has told his or her students a partial truism that the only time you can have too much fuel is when your aircraft is on fire. In a training aircraft with two people on board, this may have some validity. But when you start filling all the seats in an airplane or start flying higher performance aircraft, this may not be the case. Your weight and balance calculations may not allow you to take full fuel on every flight. Depending on the manufacturer, there may also be zero safety tolerance when flying the airplane over its maximum gross weight if you decide to keep adding fuel. For more about topping off the tanks, check out Bret Koebbe’s post, Top off the airplane fuel tanks? Maybe…. For more about weight and balance check out my post, Back to Basics – Weight and Balance.

Flying Safely Is No Accident

We often hear about how safe flying is when compared to driving, and in the carefully calculated and safety managed environment of the airline world, this is true.

In general aviation, safety requires a deliberate approach to risk management that includes determining the fuel required and the fuel available for every flight. It is only through appropriate training and by mitigating as many of the risks as possible that we can have a long and safe experience as a general aviation pilot.

Stay safe out there!