FMC fuel indicating more than Actual Fuel Quantity

[Ops Normal 57]

Hi guys,

My FMC over-reads the actual fuel in the Tank by about 200lbs (0.2). With the fuel quantity set to zero through the fuel loading Page, the FMC still shows there is about 200lbs of fuel in the aircraft, but looking through fsx fuel menu and the aircraft gauges the tanks are empty.

 

The FCOM says

 

"PLAN/FUEL

[Option – With PLAN FUEL line]

Fuel on board is automatically displayed as received from the airplane fuel quantity indication system."

 

Am I missing something or has this been addressed before?

 

 

[downscc 1,327]

The difference between total fuel and total usable fuel. The remaining unusable fuel is still weight so it is a good idea to include it in FMC for performance consideration.

[Ops Normal 57]

I thought as much. Thanks for the reply. Another thing on my list to confirm with my TRI. :smile:

[scandinavian13 5,192]

 

 

Am I missing something or has this been addressed before?

 

This is something mentioned even in the tutorials. You should really have a look at them.

[Ops Normal 57]

Thanks Kyle. I found it in the vol 2 of the tutorials. I didn't see it in any of our company training manuals. There isnt much written in the tutorial about "why" the fmc operates like that. So I'll ask our TRI.

[downscc 1,327]

I think you mean Vol 2 of the FCOM, which are Boeing created.  PMDG created an Introduction manual, single volume, that focuses on how to use the product and some FAQs about the B737 in general. This is or should be required reading.

[Ops Normal 57]

I havent seen it mentioned in any of the FCOM's VOL 1 or 2 yet. but its mentioned briefly in PMDG-737NGX-Tutorial-2, page 39.

 

 

Note 2 – the FMC’s total fuel number is often overstated by an average of
200lbs or so versus the true amount displayed on the upper engine DU
fuel gauge. This is a realistic quirk of the real airplane that we’ve modeled
in the PMDG 737NGX. It is not a concern in these calculations however
because all of the FMC fuel numbers are overstated by the same factor –

[Driver170 42]

"Regarding the FMS quantity, the anomaly you described can be explained by the fact that the fuel upstream of the fuel pumps, downstream of the spar valve is taken into account when calculating the remaining fuel on board, hence always reading a slightly higher value. This is substantiated by closely observing the fuel gauges after the engines have been shut down and the fuel in the lines returns to the tanks, which is seen by a slow increase in fuel tank quantity in the approximately 1 minute after shutdown. Actual arrival fuel read from the gauges should therefore never be done prior to this time, as fuel is still returning to the tanks."

 

Basically people we need to look at the difference between the fuel gauges which indicate what the fuel quantity is in the tanks and the FMS which calculates fuel on board.

 

Fuel gauges read the fuel tank quantity only, when engines are shut down, some fuel pumped towards the engine with engines running returnes to the tank, increasing it quantity after shutdown in the first ~60 seconds. This amount is expressed to rounded nearest 10.

 

FMS calculates the quantity on board, taking into account fuel in tanks and fuel pumped towards but not yet used by the engines, thus always indicates a higher value then fuel quantity gauges. This amount is rounded to nearest 100 as expressed in tonnes.

 

The two are fundamentally different, with the gauges always under reading actual total fuel on board until engines have been shut down and system returned to its rested state when fuel has returned to the tanks.

 

Best way to check fuel used is the using the fuel used indicator, expressed to nearest 10. Using fuel on board before departure reduced by fuel used gives most accurate reading to 10.

 

And as this accuracy is not really ever needed the FMS quantity should be used as it calculates exactly the fuel on board and the fuel used and the fuel not in tanks anymore but not yet used by the engines. It gives accuracy to 100, close enough on an aircraft burning ~37kg per minute, as closer accuracy to the 10 would change every 20 seconds!

[Ops Normal 57]

Thanks Vernon for the very detailed explanation. That explains it very very clearly.

 

Not talking specifically about PMDG's NGX,

It means if the tanks are ever empty on ground like in the second picture, the fuel lines should be empty and there is theoretically no fuel in the aircraft since the fuel lines are depressurized on shutdown/ fuel pumps off, but the FMS is "calibrated" or designed to take into account the eventual fuel in the lines so it will indicate 0.2?

 

 If fuel is eventually put into the aircraft, and the pumps are turned on, the fuel quantity indicator should reduce by 0.2 (200lbs), and the FMS will read fuel from the tanks and adjust it by 200lbs.

[downscc 1,327]

Sorta.., realistically you never get every pound of fuel out of the tanks and fuel lines. There is always some slop left even when "empty."  Same goes for your automobile, but these tanks are much larger.