hetfield
11th Sep 2008, 09:21
Anybody who knows where I can find some info on this (large jet/transport aircraft)?
What over-reading is acceptable?
AVOdriver
11th Sep 2008, 09:45
Can only give you figures from the aircraft maintenance manual for the type I work on: (for main tanks) Empty + - 0.6%; Half Full + - 2%; Full + - 3%
For feed tanks Empty + -7.25%; Half Full + - 7.5%; Full + - 7.5%.
These would be percentage of indication full scale deflection.
I would expect regardless of type that the figures would not be much different from the above.
Centaurus
13th Sep 2008, 13:35
In the 737-200 the accuracy of the fuel gauges was plus or minus three percent of the full gauge reading. It means if you were down to (say) 1000 kgs indicated total fuel - 500 kgs in each main tank, the worst actual contents could be as much as 500 kgs minus 3% of full gauge (4500 kgs) which is 135 kgs. Meaning you could have as low as 365 kgs per tank and not as you hoped 500 kgs in each main tank. If flight planning right to the bitter end of 30 minutes fixed reserve (some call it holding fuel) then it was wise to add a bit more for possible worst case gauge error.
Conan The Barber
13th Sep 2008, 13:45
Anybody who knows where I can find some info on this (large jet/transport aircraft)?
The relevant AMM.
SNS3Guppy
13th Sep 2008, 15:48
Hetfield,
I don't know your background, but it may help to understand that fuel gauges in large airplanes are calibrated based on weight, or fuel mass, rather than simply gallons. Fuel expands and contracts with temperature, so quantity (gallons or liters) isn't particularly important, but weight is (pounds, kilos). Fuel indicators are modified by a temperature input to give the most accurate reading possible, and calibrated by multiple capacitance probes to ensure accuracy. Additionally, fuel is monitored throughout each flight and compared to predicted burns, starting fuel, ending fuel, fuel uplift, etc, to ensure everything matches...as well as continuing fuel logs or other means of tracking fuel consumption in flight.
We have a 2.5% error potential, depending on the system installed (we have several), and plan accordingly. As other posters have noted, you can find the tolerances for the fuel system in the aircraft maintenance manuals or in the aircraft operations or flight manuals.