5
$\begingroup$

I went on my first flight yesterday (Cessna 152), and I was told not to trust the fuel gauges and to check fuel levels manually. The instructor said that the only fuel gauge reading that I can trust is a 0.

Why is that exactly, and what can be done (or what has been done/studied) to improve their accuracy? Is there some fundamental technical limitation that makes better accuracy difficult to achieve?

Do automobiles have more or less the same issue, or is the inaccuracy more prevalent in aircrafts?

$\endgroup$
  • 3
    $\begingroup$ The following may not be 100% accurate for aircraft. Usually, fuel gauges use a buoyancy to determine the level of the fuel. With changes in attitude, there may be inaccuracies due to fuel sloshing to one side or another. Also, because there are moving parts, they can fail more easily than, say, a pitot tube. $\endgroup$ – CourageousPotato Jul 30 '19 at 0:44
6
$\begingroup$

The problem is small airplanes use automotive style rheostat (variable resistor) fuel senders, with the float in the tank, and these just aren't reliable enough to be trusted with something potentially life or death like fuel quantity, and they aren't particularly accurate in the first place.

Here's the thing... In a car you just drive around going past gas stations constantly until the gauge is low. Imagine you were operating a car in some remote place where the gas stations were 300 miles apart and if you run out of gas short of the next gas station, the road is full of hungry alligators equipped with Slim Jims, making your door locks useless, and you, dinner.

Well, your tank is good for 600 miles and it's half full on the gauge. Maybe good for 300 miles, maybe not. Are you willing to trust the automotive gauge now? This is what faces you as pilot any time the tanks are not full, in an airplane with 40 year old fuel senders, that maybe sits around for extended periods.

Bigger airplanes use fuel quantity measurement systems that use "Capacitance Probes", hollow tubes with a center rod, and a current is passed through and the capacitance measured by the fuel level in the tube that completes the circuit between the outer tube and the inner rod. A computer will monitor the levels from several probes arranged around the tank and they provide a very precise and reliable fuel reading.

There are capacitance probe senders available for homebuilt aircraft, but I'm not aware of any certified units that can be installed in something like a 152. If available, that would be a good solution, although I'd probably still not be willing to trust it 100%.

Best thing is to use a dipstick and be aware of your fuel consumption/time. During training and renting airplanes, people get into the habit of always going somewhere with full tanks, but later on, especially if you are flying commercially, you are going to have to fly a lot with intermediate quantities, excess fuel being unwanted ballast, so you need to get into the habit of knowing and tracking fuel quantity.

Impress your instructor by showing up with something like this and you will always know exactly how much fuel you have. If you're cheap and refuel the plane yourself you can make your own with a wood stick if you have a way to get all the useable fuel out of the tank, then add 5 gal at a time and marking your stick appropriately at each level.

| improve this answer | |
$\endgroup$
  • 1
    $\begingroup$ Re last paragraph; just don't drop it into the tank by accident. Yes it is possible if it starts sliding sideways out of your hand. Been there, done that. Came back with two dowel rods and used them like chopsticks to fish it out but it sure took a good long while. $\endgroup$ – quiet flyer Jul 30 '19 at 2:39
  • $\begingroup$ I used to use a wooden ruler and finally glued a cross piece to it to prevent that after I nearly dropped it in lol. Nowadays I use a stick that is a tube in a piece of alum channel and I hold my finger over the top of the tube to get the level, and it's simply too long to fit in the tank that way. $\endgroup$ – John K Jul 30 '19 at 2:48
  • $\begingroup$ I'm guessing that senders is a typo for sensors in this answer? $\endgroup$ – Terran Swett Jul 30 '19 at 3:29
  • $\begingroup$ Newer planes use magneto-resistive sensors. The float arm uses a magnet in the fuel tank, an externally mounted sensor reads the magnet position to sense the fuel level. No electricity running thru the tank, and the signal can be calibrated for more accurate readings. These are retrofittable into older planes as well (I did); the cost to repair original resistor float guages are as much as new modern equipment, so it makes sense to change over and get way from 45 year old corroded floats. ciescorp.net/documentation/comparative-fuel-sensor-technology $\endgroup$ – CrossRoads Jul 30 '19 at 3:41
  • $\begingroup$ @TannerSwett no actually fuel sender is a term that used to be used back in olden times. And... when I looked at CrossRoads' link, it says "fuel sender", so not so olden I guess lol. $\endgroup$ – John K Jul 30 '19 at 4:17
3
$\begingroup$

The other two answers are very good but leave out a key point, airplane fuel tanks are not always level and since liquid will slosh around and level its self (un an unaccelerated situation) the gauge can only be accurate under certain conditions. In a car you are always more or less on level ground often going a constant speed. In an aircraft for a given speed you pitch will change based on load configuration. You also need to bank to run and pitch up/down to climb and decent. Often the sender is put somewhat in the middle of the tank to mitigate the issue a bit but that is only so effective.

enter image description here

(source)

Turbulence, small course corrections, and pitch/power changes will all slosh fuel around and ultimately make the gauges inaccurate.

| improve this answer | |
$\endgroup$
  • $\begingroup$ Sooo few airplanes have that kind of system anymore. Certainly not anything with an electrical system. $\endgroup$ – CrossRoads Jul 30 '19 at 15:56
  • $\begingroup$ @CrossRoads it was merely the best picture i could find to show how floats work. Most GA planes still have a similar float that operates a rheostat. $\endgroup$ – Dave Jul 30 '19 at 18:03
0
$\begingroup$

A sendor takes the sensor reading and sends it to a device like a JPI EDM900 which is calibrated with the sending unit. Upon filling the tanks, the pilot can tell the EDM900 how much fuel is in each tank to start a flight. The EDM900 than displays the fuel level as the flight progresses, can do the math to display fuel used, fuel remaining, show fuel flow from an appropriate sensor (in my case, feeding into the carburator), can send fuel information to a GPS to display a flying range available, can warn of an empty tank, and can tell you how much to an empty tank. Much more info than just an old float gage that was only accurate when it was empty.

The info I see is similar to this, with the addition of carburator throat temperature, and just 4 cylinders vs 6.

I would say the fuel management/totalizer functions are similar to what modern cars have, with real info provided vs just an idiot light for many of the functions.

enter image description here

| improve this answer | |
$\endgroup$
  • $\begingroup$ Do you trust it enough to no longer dip the tanks if going somewhere with close to minimum fuel requirements? $\endgroup$ – John K Jul 30 '19 at 12:31
  • 5
    $\begingroup$ I'm not sure how this answers the question of "what causes fuel gauges to be untrustworthy". Am I missing the obvious? $\endgroup$ – a CVn Jul 30 '19 at 12:41
  • $\begingroup$ Nope. I still dip the tanks before a long flight just to be sure, or at least look to be sure the level is where I last filled it to. I did a 3 hour flight Saturday with ~4.4 hours in the tanks. Upon getting close, I was seeing warnings about low fuel. Dipping the tanks before re-filling, I had 3 gallons on one tank and 9 on the other, so an hour+ still available. I still have some learning to go on resetting it upon filling the tanks or filling to a known quantity. Seeing the fuel flow and the cylinder temps is great for leaning the engine out, better than a single stock exhaust temp gage. $\endgroup$ – CrossRoads Jul 30 '19 at 12:44
  • $\begingroup$ It answers the 2nd question of What can be done to improve their accuracy. $\endgroup$ – CrossRoads Jul 30 '19 at 12:50
  • $\begingroup$ In my bush flying days in a C-180 on floats the trips were around 1 to 1.5 hours round trip + 30 min reserve, and payload"s everything,so I rarely left base with more than 2 hours total, maybe 20 imperial gal, usually between 1/4 and 1/2 on the gauges. On the way back getting close to base, if I'd had to deke around weather, the gauges would be showing the sender floats bouncing on the tank bottoms with ~5 imp gal per side remaining and I'd be saying to myself "trust your dipstick". I'd land and dip the tanks at the dock just to see and sure enough I still had 30 min to an hour's fuel. $\endgroup$ – John K Jul 30 '19 at 16:29

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.