Fuel Quantity: Source of truth?

[Will]

It seems the FMC's "CALC" value for fuel quantity is supposed to be the usual source of truth for the airplane, right? Meaning: that's what's used at the outset, and the crew will switch the FMC over to the FQIS totalizer value only if there's a disagreement. (Right?)

What about variations between the upload and the CALC value seen before departure?

Here's a hypothetical:

+ From the sum of fuel onboard prior to the upload plus uploaded volume x density, we get 300,000 lbs onboard.
+ The FQIS totalizer shows 298,000 lbs.
+ The "CALC" value in the FMC shows 302,000 lbs.

In that scenario, do we go with CALC? Or update it on PERF INIT to a manual value that matches either the upload or the totalizer?

Thanks.

[Hardy Heinlin]

As we know, the CALC value is derived from engine fuel flow; there's nothing to calculate when engine FF is zero. And it needs a start sum. So, as far as I recall, the CALC operation only begins after engine start before which the internal CALC value is just a copy of the totalizer value anyway, and after start the copy is unlinked. This also assures that APU fuel has been copied to the start sum. Or maybe the CALC system even gets APU fuel; I can't remember at the moment ...

Does your hypothetical scenario happen after engine start?

[Will]

It started out as a thought experiment, with the Gimli incident in my mind. How do we really know how much fuel is on the aircraft? We could dip the tanks, I suppose, but without that, the flight crew will have the amount of fuel that gets uploaded, the amount of fuel on the totalizer, and the FMC calculated value. So with three sources of truth, which one does the crew rely on?

Then I got confused and edited my post a few times, because Basic 001 starts with the FMC reading "SENSED" and I can't get it to turn to CALC until all 4 engines are started. Fair enough; at first, I thought CALC wasn't even available before engine start.

But if you start Basic 000, and switch to a passenger variant before powering up the aircraft, then CALC is present before engine start and you can overwrite it with a manual value, which I guess may come in handy someday. (Or in reality maybe you'd just call the maintenance guys.)

I can't figure out why one situ starts with "SENSED" and the other starts with "CALC." It seems the FQIS has power... I know the answer is easy but I can't see it; something to do with the ground services bus on the freighter, or its electrical system?

[John H Watson]

CALC is shown on the real aircraft (freighter or pax) with all the busses powered on the ground prior to engine start. However, it is a totaliser value even though it shows CALC.



I don't know why Basic 001 shows SENSED. I can't say I've noticed any difference, say with the aircraft being refuelled. I have watched the FMC when the FQIS CBs are pulled and reset, and watched the quantity appear, but I can't recall if CALC or SENS is shown during the power up process.

When the fuel is uplifted, the refuelling paperwork is done and if there is a significant difference (say over 2 tonnes for a given load), the paperwork is reviewed. I can't say I've seen any discrepancies other than from APU fuel burn or paperwork errors. The FQIS itself is self-monitoring and should tell the engineers/pilots if there is a problem. Usually the sticks are only used if a tank indication goes blank. Each tank has numerous level sensors (e.g. the HST has 10 sensors as well as a densitomer and a compensator).

[Hardy Heinlin]

That "SENSED effect" has been introduced in PSX 10.88; it's a consequence of modification 88.02:

Version 10.88 - 27 September 2019

88.02. PROGRESS 2/3 page: When fuel disagreement exists, just one USE prompt is now displayed instead of two, and the non-selected fuel quantity method no longer blanks; it now remains selectable at any time ("fuel used" indications will blank as usual when totalizer method is selected).

This modification causes the SENSED/CALC display on the PERF INIT page to show the true mode instead of the fake "CALC" word.

Guess I'll have to re-introduce that fake word in the next update ...

It would be nice to know whether the NG FMC also shows "CALC"; the NG shows the USE prompts all the time and not only during a fuel disagreement. NG pilots need to select a USE prompt during preflight.

In PSX, in the NG FMC, before engine start, when I select CALC on PROG 2 page, PERF INIT will show CALC; when I select TOTALIZER on PROG 2 page, PERF INIT will show SENSED. Is that correct?

[Will]

If you update PSX to show the fake word "CALC" before engine start, will the crew still be able to overwrite it with a manual value?

It would seem to me (no 747 experience) that the overwrite feature would still be available when "CALC" is displayed pre-engine start, regardless of whether the CALC word refers to CALC or SENSED.

[John H Watson]

Yes, the override feature is still available with the fake CALC. That's how you enter a manual fuel value if a fuel value is not available due to faulty tank sensors prior to departure. DELETE clears the manual entry.

[Hardy Heinlin]

To get a clean logic without using the word "fake", we could call the "fake calc" a submode of the calculator; that submode just uses the totalizer value. Therefore the CALC title can be considered correct as the calculator main mode is actually selected; the calculator's submode just keeps copying the totalizer value until the engines are started after which the calculator will automatically begin to run its own program. When the SENSED title is shown, the totalizer main mode is selected and the totalizer will remain selected after engine start.

Does that make sense?


|-|

[Will]

That makes sense to me (as a user, not as a Boeing engineer--I can't comment from the latter perspective).

As long as the crew can overwrite any mode or submode of CALC with a manual value, I'll be happy.