FMC page: PROGRESS 1/3

[Hardy Heinlin]

Hello,

as you know, the first line on that page shows the fuel remaining at the last waypoint. The other three lines indicate the predicted calculated fuel remaining at the active, at the next, and at the destination.

I just noticed there was a software update in 1997: it says the fuel remaining at the last waypoint is a value referring either to the calculated fuel or to the totalizer fuel, whichever is lower.

I can insert this criterium in five seconds, no problem.

But I wonder if this software update is still valid. The difference between calculcated and totalizer fuel can be very high, especially in the final segment of the flight *. It looks strange when the first line displays a fuel value that is so unusually off the predicted fuel progress values.

What's the advantage of this feature? Pilot confusion? If the totalizer is the more reliable reference, why not use the totalizer reference for the predicted fuel values as well? The progress page would then give a better picture of the progress, no?

In the old days, when the fuel progress was recorded on paper only, what did the crew enter at the last waypoint? Totalizer fuel or calculated fuel?


Cheers,

|-|ardy


* See Ief's comment below.

[Jeroen Hoppenbrouwers]

Blunt guess: suppose you rewrite the requirement as "The fuel remaining display at the last passed waypoint shall never exceed the actually measured totalizer fuel at the last passed waypoint", would that shine light on the intention?

I agree that it makes sense to project from measured fuel, but it is possible that a measured fuel level isn't correct, and in this way the crew has the chance to build awareness of the situation. At the very least they can make a split calculation.

Don't know.


Hoppie

[Hardy Heinlin]

Maybe. But the crew has this chance only if the crew knows this feature. I have seen this feature in only one AOM so far (LH). It is not mentioned in other manuals.


|-|

[IefCooreman]

If it's a company option, it's a company option, although I wouldn't know many companies using this option :-).

You make a preflight check (based on "on-block fuel", reduced with any fuel consumed by maintenance for whatever reason, add the uplift converted to kgs with density provided by fuelling company, and that value has to be within certain limits of the totalizer shown).

Anything from that point on (wheels up let's say), calculated is the reference. This is because less than full fuel reduces the accuracy of the totalizer fuel to the limits (every fuel measuring is giving a measured value, hence giving you an error as well)

Totalizer comes back into the game with really low fuel levels because a lot of fuel measuring devices are not really measuring (wingtips way up, center tank empty,...) hence they are not giving you a value with an error in it. It is simply 0 for a lot of them. So the total accuracy of the totalizer fuel increases with decreasing fuel. (that's what I've been told, and it sounds reasonable)

On cargo flights, you are frequently close to your max landing weight. In these cases you do take totalizer into account to be safe side. Diversions as well. This can also be if totalizer is more than calculated: there are cases where diversions have been questioned by the authorities because the pilots declared "minimum fuel" based on calculated, while on arrival the on block totalizer fuel suddenly appeared to show extra time to spend waiting in a hold. But the journey logs at the end of the day are based on totalizer fuel, so go and explain you use calculated...  It's part of the grey zones we are confronted with and it's stuff that doesn't make our life easy. :-)

[Jeroen Hoppenbrouwers]

I was told that, indeed, the only reliably calibrated fuel quantity readout is zero. And even this is questionable as there are so many nooks and corners where fuel can hide, you always get a few gallon more out of the thing by rocking it.