Fuel gauges?

[BearForce One]

Sat on an Airbus at Gatwick, now overdue by 45 mins because of a discrepancy with fuelling paperwork.

I understand it's important to know exactly how much fuel has been loaded, especially given recent events.

But it makes me wonder - does this mean the plane can't reliably tell how much fuel it's carrying?

[Bealzebub]

Yes, the aircraft fuel gauges are normally very reliable. However there are any number of situations that might cause a discrepancy. Those discrepancies can range from technical faults, to simple human errors such as what the last crew wrote in the technical log, or a refueller writing a 9 that looks like a 7. There are a series of checks that are applied to every refuelling operation that, within defined margins of error, should safeguard against gross errors. When these checks throw up a discrepancy it is obviously important that the discrepancy be resolved before the crew can sign off the refuelling log, and the captain can then sign the tech log.

Aircraft can (in defined circumstances) dispatch with an unserviceable fuel gauge, but it is clearly important that the fuel on board is confirmed prior to dispatch. The tank quantities can be read by mechanical "sticks" prior to dispatch in order to confirm quantities and it would require other measuring systems be fully serviceable.

The basic check is that the crew calculate what they consider the fuel uplift to be. The actual uplift (for many reasons) is likely to vary from that figure, but should normally fall within a limit of 5% of the calculated figure. If it doesn't then further investigation is almost certainly warranted. The likelyhood is a human error in paperwork entries or mathematical calculation, but the gross error check is also a line of defence in more serious potential issues such as leak or faulty equipment.

Most aircraft systems are very reliable most of the time, but the system redundancies, and most importantly the cross checks are in place to ensure that reliability isn't taken for granted. Most of the time a discrepancy can be properly resolved quite quickly, but irrespective of how long it takes, it is vital that it is properly resolved.

[Basil]

Good explanation by Bealzebub.
One error we had was when the old boy doing the refuelling used the wrong SG page.*
The energy in our fuel load is proportional to its mass, not its volume and the mistake on this occasion was to load the volume for a higher SG than that of the actual fuel. The refuelling system measures volume loaded.

As with so many of these things the resolution, although apparently simple, turned out to be less so resulting in a bit of paperwork for Bas.

But fear not; we departed with the correct fuel on board.

*A habit had grown amongst the refuellers of placing the most common SG page at the front and not in sequence. Someone had looked at our on-board copy, probably a pilot, and thought 'That's wrong', opened the ring-binder and placed the page in its correct order. The refueller just used the figures from the first page without looking at the SG at the top to which they related.
The RAF had large signs in the hangars which read 'Don't assume - check!' There but for the grace etc . . .