FQI vs Fuel used accuracy

[alosaurus]

Was a flight test engineer on 125 back in the 1980s and seem to remember that the fuel quantity indicating system had an accuracy of +/- 0.5% but the fuel flow meter accuracy was considerably worse.


I can't remember the accuracy figures for these fuel flowmeters and am unaware of any technical developments which may have changed the situation.

Currently working with the 737 and would appreciate any "ball park" accuracy figures for FQI vs Fuel Used indications on a modern jet aircraft?

Thanks in advance

[safetypee]

alosaurus … aah de Havilland. May have flown with you during guest appearances at Chester.
IIRC the 125 fuel flow meter had a peculiarity associated with the engine where a small ‘leak path’ fuel-flow was not registered as it by passed the main burner flow where the detector was located. I can no longer recall the purpose of the bypass, but at the time it seemed to be an important engine design feature, but a nuisance when determining accurate fuel usage.

I haven’t seen this feature on other engines, but that’s not to say some don’t have similar problems; otherwise my experience suggests that the fuel gauging and flow meter accuracies are similar.

[Mad (Flt) Scientist]

I have vague - but only vague unless I go do some checking - recollection of fuel gauging accuracy being quoted as a proportion of total/max fuel, not of current fuel. So it can be pretty poor at low fuel states.

I also seem to think that fuel flow accuracy suffers similar problems at low fuel flow rates.

So either might be good for a heavy aircraft, but a light one burning little gas will be a pest whichever means is adopted.

[Mechta]

Could it be that some of the fuel which passes through the flow meter gets recirculated by the fuel metering unit at lower throttle settings? A flow meter may not take account of fuel density changes with altitude/temperature either.

This document has some info about ways to achieve fuel savings on B737 & 757:

mdh.diva-portal.org/smash/get/diva2:121168/FULLTEXT01

I worked at Smiths Industries on the B777 ultrasonic gauging system (FQIS), and recall that one of its claimed advantages over capacitance units was its increased accuracy. Nevertheless, it has yet to be adopted on any other type as far as I know.

[jettison valve]

From the top of my head (A330-200/A340 classic):

FQI at high quantities:
+/- 0.5% of total capacity (approx. 110t) plus +/-0.5% of actual FOB
narrowing down to no overread a low quantities and -1% (?)

fuel flow transmitters:
Don^t have figures at hand, but was told that they have max. accuracy at cruise fuel flow, and are really bad at ground idle and max. thrust.

A few years ago, the Trent 500s on the A3456 had quite an issue when the flight crews complained a lot about discrepancies, like:

- 100t t/o fuel
- 90t Fuel used
- 15 (or 5t) fuel remaining

From an engineering point, the comparison is a bit like apples and peaches...
But from a flight deck perspective, certainly understandable...

Tried to dig out the ATA 73 TFU at home - but didn^t succeed...
The issue then got solved by recalibrated / new FF transmitters. If I recall correctly.

Best Regards, J. V.

[dixi188]

Had a recent event on an A300-B4.
Tank 2 fuel gauge was giving + - 2000lbs in flight so at next stop we decided to use the mag sticks.
This still gave discrepancy between known uplift in litres, stick figure and gauges, so we put on another 2000lbs for mum.
Next stop we tried it from empty tank with a known amount from bowser and found sticks still gave errors of up to 1000lbs. (This was using charts for Attitude)
So I think the only time you know how much you have is when it's empty or full and overflowing.

Incidentally, with these errors, when we land at Max Landing Weight, which is quite often, has the manufacture taken account of the likely overweight case with gauge errors.
Actually with a MLW of 295,410lbs, I don't suppose 1000lbs or so is enough to worry about in line operations.