Topic-icon ATA 28-41, Fuel Quantity

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10 months 4 weeks ago - 10 months 3 weeks ago #540 by Mark
The 767's (and from what I've seen, the 757 also) uses a somewhat different approach for obtaining fuel levels than most conventional fuel quantity measuring systems.

Aircraft such as the 727, older 737's, DC-9's, DC-10's, and A300 use a total of three wires (tank to processor/indicator) for tank measurement. The tank probes are combined at the tank spar plug and only LoZ, HiZ, and Comp wires are needed beyond the tank itself.




Each individual probe on a 767/757 has its own LoZ input excitation wire. The HiZ output wires are all connected together. This does include the compensator output signal.

The fuel quantity processor unit looks at each probe separately by using millisecond "shots" down each LoZ wire. Only one probe receives excitation at a time, so the output HiZ signal can be measured for just that probe alone.

If a probe is lost, the processor uses a "sister probe" method for estimating the missing probe's value.

The good thing about this approach is that faults can be isolated down to the exact probe vs using a breakout box for tank probe troubleshooting on the aircraft mentioned above. The bad thing though........ many more chances of wires breaking.


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Last edit: 10 months 3 weeks ago by Mark.

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10 months 3 weeks ago #541 by Mike Forti
Replied by Mike Forti on topic ATA 28-41, Fuel Quantity
That's why this site is so cool, because I would have never known this otherwise, thanks Mark!!

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10 months 3 weeks ago #542 by Mike Forti
Replied by Mike Forti on topic ATA 28-41, Fuel Quantity
Does this system use a REF voltage for the LoZ side?

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10 months 3 weeks ago #549 by Mark
Replied by Mark on topic ATA 28-41, Fuel Quantity
The LoZ excitation is usually a fixed A/C voltage. I've never actually read what it is, nor have I tried to measure it with a meter.

In older fuel quantity measuring systems, all probes received the same input reference voltage.

I'd have to assume it is the same for the 757/767 for all probes.

Because we're dealing with fuel...... whatever the voltage is, its small.

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4 months 4 days ago - 4 months 4 days ago #590 by Mark
Replied by Mark on topic ATA 28-41, Fuel Quantity
767 tank rear spar fuel quantity connectors and HiZ bussing plug.




The standard (from old 727, 737, and 747 days) three pin connectors are used. The LoZ and Comp pins are not used. The wider (shielded) HiZ pin is.

All LoZ and Comp wiring go through the upper plug.

As stated above..... more wires have advantages for usage (tank quantity reliability) and troubleshooting (isolation of single bad probe), but...... there is a whole bunch more crap that can break.
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Last edit: 4 months 4 days ago by Mark.

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