Aircraft fuel gauge question

[speedy_thrills]

How do aircraft (particularly sports/fighter aircraft) gauge how much fuel is left in their tanks? It must be quite difficult when the aircraft is rolling about and the fuel is sloshing back and fourth all the time (more so than a car and in directions that baffles cant really prevent).

[chim_knee]

I thought they used sponges to "hold" the fuel whilst still allowing it to drain... but I could have made it up (in which case I should patent it).

[Smart Roadster]

Not totally sure but I guess they use flow gauges. Put in X flow Y go home before X=Y. In light planes the fuel gauges are crap but you know the fuel flow at each power setting so you know you can fly for so many hours plus a bit for luck.

[wolves_wanderer]

In the R22 the gauges work in the same way as they do in your car. The result is that there is less fuel than indicated because you tilt forward in normal flight. We always go by time, you can always get 3 hours so we just work out when we next need to land.

[Le TVR]

I think that often they use sensors that act like the plates of a capacitor using the fuel as a dielectric. The plates are in various places inside the tanks and are 'read' by measuring the capacitance value with an high frequency oscillator.

But as stated you do not rely on them
Know what your uptake is, fuel flow rate or power setting rate.

Final check can be using the dripstick (or dipstick in light aircraft) on the ground.

[eharding]

Rule #1 - never trust your fuel gauges - you should know the amount you put in, your burn rate, and the amount of time flown. Light aircraft fuel gauges, in general, are notoriously unreliable. That being said, the gauge in the Pitts is just about the simplest and least prone to failure - it's a length of clear plastic tubing with graduation marks running down the side of the instrument panel, acting as a sight gauge for the amount of fuel in the tank - the graduations are 'full, 2/3 and 1/3', with an adjusted mark to show full when sitting in the three point attitude on the ground. It almost never fails, but the level indication is at best approximate, and after a few aeros you can get an airlock in there for a while which takes a while to correct itself.

The Yak has damped sensors driving twin bar-graph displays in the front cockpit, and is generally a bit more reliable than the average spamcan, but not infallible - if one of the bulbs in the bar graph fails, you can get a bit of a surprise when it looks like all of the fuel in one tank has disappeared. It's a sign that you're flying the thing in balance though if you manage to keep the consumption even on both sides.

The Extra has a more sophisticated setup, since it has a dedicated aerobatic tank and ferry tanks - you can only fully aerobat it when the ferry tanks are empty - although I know at least one competitor last season who had to break off in the middle of his sequence and land with only a few litres to spare - mistimed his take-off, spent too long in the hold, burning through the reserve in no time.

[SpencerO]

Sure one of the better qualified chaps can provide a better explanation than I can, but afaik in light aircraft they are notoriously unreliable flaoting cork on an arm type systems, which do indeed react changes in attitude / acceleration. Baffles in the tanks do minimise sloshing, but it does still happen. In addition to this, in some light aircraft the fuel guages themselves arent particularly accurate unless the engine is actually running as the meters are underpowered until then. In larger aircraft fuel flow is measured (often via an impeller system in the fuel lines) which when deducted from the known quantity of fuel uplifted provides a far more reliable reading. Hope that helps, but like i said, I'm sure a better explanation will be along shortly..

S

(lol - expert on scene sooner than anticipated heh heh)

>> Edited by SpencerO on Thursday 9th February 12:23

[esselte]

Le TVR said:

Final check can be using the dripstick (or dipstick in light aircraft) on the ground.

Might be a bit late at this point.

[Kiwi XTR2]

I saw a patent application in '87-'88 that had an ultrasonic system with three sensors. From those 3 readings it could calculate the 'free surface' and therefore the remaining volume. There were also 3 extra inputs for a 3-axis accelerometer but the fuel gauge (as the application stood) didn't actually use those inputs in the gauge calculations.

I think the applicant may possibly have been Horizon Technology but I've got no idea whether it was put into production. ???

[8]

Some newer airliners use ultrasound to measure fuel depth.

[Roop]

The guages in light aircraft as bloody awful as already mentioned. A friend of mine found out the hard way as the donk on his PA-28 stopped as he turned finals to land due to running out of fuel

[Kiwi XTR2]

8 said:

Some newer airliners use ultrasound to measure fuel depth.

If it is that system (above) I could have taken the job offer and stock options

[Smart Roadster]

The cork with the stick on sensors are better than some electric ones. At least they are reliable when straight and level. Unless you do what friend did, he put a ping pong ball on the end to make the top of the stick easy to see. Unfortunately the extra wind resistance jammed the stick in the full position. He only discovered this when slowing down turning base and the stick dropped almost to the empty mark!

[MarkK]

On more modern aircraft (usually with digital displays) you use one or more sensors which run the height of the tank and give a frequency based on the height of fuel in the tank. From this you can work out a volume and from that you can work out a mass. The sensor damps out the fuel movement so effectively you get a filtered average value. Most of the time the fuel is constrained by baffles in the tanks and the sensor is positioned so that the fuel angle rotates around it.

At extreme angles the software will revert to using the engine fuel flow to figure out fuel consumption until the aircraft recovers to a more sensible angle. Or it can freeze the display - if you are throwing the aircraft around you aren't going to be looking at your gauges.

Also the fuel quantity can be averaged over a number of seconds to filter out sudden changes.

Something else to bear in mind is that just because an aircraft is rolled on its side doesn't mean the fuel is at an angle in the tank - once positive G is applied the fuel will be at it's normal relative angle. It only hits an angle while the aircraft is unbalanced.

[Extra 300 Driver]

Kiwi XTR2 said:

8 said: Some newer airliners use ultrasound to measure fuel depth. If it is that system (above) I could have taken the job offer and stock options

What airliners?

[sjp63]

Le TVR said:

I think that often they use sensors that act like the plates of a capacitor using the fuel as a dielectric. The plates are in various places inside the tanks and are 'read' by measuring the capacitance value with an high frequency oscillator. But as stated you do not rely on them Know what your uptake is, fuel flow rate or power setting rate. Final check can be using the dripstick (or dipstick in light aircraft) on the ground.

Correct, pilot and ex aircraft engineer