View Full Version : Ultrasonic FQIS

30th Oct 2007, 23:05
B777 is the only aircraft with an ultrasonic FQIS, and it looks like future types are back to capacitance.

How do the systems compare from an accuracy/reliability/maintainability point of view?

1st Dec 2007, 08:41
Digital capacitance and ultrasonic are equally accurate. It's the software that provides the accuracy and that it is constantly evolving.

As far as maintenance goes there is a fundamental difference which the industry hasn't managed to grasp.

In capacitive system it is the in-tank wiring looms that degrade. They really need replacing after 5-8 years as their capacitive properties change. The probes themselves are just tubes of aluminium, so if kept clean rarely fail. Where a probe is replaced it is likely to be the diode block, or the connector. But maintenance crews normally change the probe as a way of avoiding getting in the tank to repair a connector or change a loom.

In an ultrasonic system, the looms are not capacitive and shouldn't degrade. But the probes are the active devices and do fail. If a probe fails it is removed from the system by software, and each cell of probes can tolerate 3 probe-failures before you need to enter the tank. It is therefore the fact that harnesses changes are less frequent that should be the advantage of ultrasonics.

2nd Dec 2007, 04:25
from operational experience, the B744 which uses the capacitive system, seems to give a more accurate volumetric readings. however, seems like its the hydrometer that frequently gives erroneous SG readings.

whereas, on the B777 which uses the ultrasonic system, the probes frequently gives erroneous fuel height. as a result, the fuel volume is erroneous. strangely, the SG on the B777, is very consistent throughout all phases of flight.

now, if only boeing would combine both systems, then we would be rid of erroneous fuel indications in the real world ;)


2nd Dec 2007, 08:24
744 - more accurate volume readings - how are you measuring that? Not against the dip-sticks I hope.

I suspect that your B777 system is carrying some faults that are not being properly managed by maintenance crews.

2nd Dec 2007, 10:56
744 - more accurate volume readings - how are you measuring that? Not against the dip-sticks I hope.

:uhoh: what i meant is fuel height from capacitors thus volume.

to elaborate, on the b744, when we get an 'imbalance', when we check the CMC, it's usually the SG that 'causes' the imbalance. whereas, on the b777, when we check the CMC, SG of both tanks are usually the same, the variance is with the fuel height thus volume.


3rd Dec 2007, 11:35
The "SG" is not directly detected (at least on the 777). There is a densitometer in each tank, which is only used on the ground and really gives fuel type. Density is actually derived from speed of sound which is detected using probes with rivets in them at known heights, from which a return is detected.

There is also a water detector in each tank (2 in centre on -200), which is basically another ultrasonic probe lying on the bottom of the tank.

Thanks for the replies. From an ultrasonic perspective, it is obvious that the reflected return is stronger when the surface is smooth and horizontal wrt the probe. If the surface is at a high angle, or if it is shaken up, the return will be degraded.

As regards probe failure, the problem is that fuel appears to be corrosive for solder joints. This clearly affects both systems, although I think that with a bit of imagination, a dry channel containing the ultrasonic transducers could be constructed along the bottom of the tank, which would reduce the vast weight of the armoured pink cables that have to be used in fuel tanks since TWA800.

5th Dec 2007, 03:46

fascinating! (regarding how density is detected!)

so, you reckon the 'inaccuracies' (for the b777) is due to fuel surface at high angle or shaken up? occasionally, we get blanks or XXXs on certain probes. why's that? usually, it clears up when we parked! :ugh:


6th Dec 2007, 12:57
As with all systems, the guys building the system have no contact with the pilots (or refuellers) that use it. I know what the system is meant to do, but don't really know how you guys use it. That's why I was interested in what the perspective from the cockpit was.

Problems we can replicate in the lab, we can generally address, although, unless they are serious, they usually have to wait for a Boeing driven update, as the cost of producing a new release is itself high. Its obviously a lot harder to resolve a problem if we cannot replicate it. Labs are, by their nature, static, and although some early testing was performed using a road fuel tanker and speed bumps.

As regards individual probes reading xxx, the system is designed to check probes against their neighbours and blank out any that are suspect. As for why they are suspect, you have my guess, but it is only a guess.

11th Dec 2007, 08:58
xxx - mostly like a crack in the solder joint in the base of the probe. Thermal cycling and vibration is what causes the faftigue. Attitude, aeration etc affects all systems, and is usually corrected by software, although it isn't perfect.

If probes are displaying, or have displayed xxx - remove them at the next opportunity, even if they work when you look at them on the ground.