Aircraft Fuel Gauges

There are a few Aviation Proverbs that come to mind, regarding fuel gauges:
“The only time an aircraft's fuel gauges are accurate is when the needle is on E.”

Another is:
“Anybody who believes an aircraft's fuel gauges could be sold the Brooklyn Bridge/Eifel Tower, etc.”

Unfortunately, these maxims are too true; a pilot should never trust his fuel gauges, but (and there is always a 'but') as seen above; the gauges are accurate, when they point to the 'E'. So could we not use this to our benefit?

This discussion will apply more to an owner/operator of a specific aircraft, rather than to a hired aircraft, although some instances will apply to both.

The first question to the reader is: Have you ever run a fuel tank dry? Hopefully, if the answer is in the affirmative, you did this on purpose and not by accident. If the answer is in the negative, then the second question is: Why not?

During a preflight, do you trust the gauges, or do you do a visual check with the aid of some form of dipstick? If you own or fly a high wing aircraft, this could be difficult and on some (C210 come to mind) nearly impossible without the aid of a stepladder of sorts. If you do trust the gauges, then you had better go to the first paragraph and start reading again. A dipstick is a relatively accurate method of determining fuel quantity, but can be misleading if the aircraft was parked on a less-than-level surface. Modern aircraft (particularly NTCA types) do come with fuel flow transducers and can be used to augment Mk 1 eyeball, dipstick, fuel log and gauges; provided you have taken the time and trouble to set the K-factor accurately. Now, if you took the trouble to read the manual that came with your EFIS or engine monitor, or fuel flow transducer; you would have seen a warning to use fuel flow information as an advisory service and not gospel truth. You see, that man Murphy is alive and well and hard at work in the aviation industry, something as small as a human hair could end up in the fuel tank and make its way through a fuel filter screen and hinder or stop the impeller of a fuel flow transducer. Nothing is impossible for Mr. Murphy.

The CAR's require that on a VFR flight, the aircraft must land with a minimum of 45 minutes fuel onboard, so if you land with less than that, then you will be in contravention of the CAR's. So if you plan to run a tank dry (highly recommended) then you need to be close to or overhead an airfield and have at least one hour of fuel in the other tank. My procedure will be done at a safe altitude (at least 2000 ft. AGL) to fly the selected tank down to a specific value on the gauge, for example, start of the red arc or a specific value. Then remain at your normal cruise power setting, and start timing until the engine quits. When that happens, do not panic, fly the aircraft. Reduce throttle position, select the other tank and then select the fuel pump on and wait for the engine to start running again. In most cases the propeller will not stop, but keep windmilling. But beware! If your aircraft is equipped with a Rotax or similar type engine, or your propeller drive mechanism is geared, the propeller will probably come to a sudden stop, will not windmill and will require a crank with the starter motor.

If the aircraft has a carburetor, it will soon be running smoothly again and power can be added so as to return to your airfield.

If the engine is fuel injected and does not have a fuel return to a fuel tank to expel air, such as in Lycoming engines, then all the air in the system needs to be expelled through the injector nozzles. This could take a few minutes as the engine splutters and coughs, but it will continue to run and moving the mixture richer, will assist with greater fuel flow to expel air in the pipes quicker. If the engine has a constant speed propeller, it is important to first reduce throttle position as the governor will command the propeller to reduce pitch to try to maintain RPM, and if too much power is added then; an over-rev is likely to happen.

If you were to do this once a year on each tank, then you will have a good idea of endurance on the tanks which might help you on that dark and stormy afternoon when you have painted yourself into a corner by adding links to the 'poor judgment chain.' Being better prepared for an emergency will allow you a better chance of a successful outcome when the fuel gauge/needle points at a specific value or position, and by reducing power to a lower value than your normal cruise power, you will be assured of attaining the endurance you had measured during your testing whilst you execute the precautionary landing you should have considered earlier.

Running a tank dry is something I believe all pilots should do, but on your first attempt ensure you are over an airfield, just in case you were to panic and end up in a forced landing. A secondary benefit of this, is you will now be able to check your useable fuel quantity in the tank and hopefully find that this is still the same as your initial testing indicated or as indicated in the aircraft's POH.

Keep it safe out there…
Chalkie Stobbart - Technical Talk
Aircraft Technical

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