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I am creating a program to calculate the fuel needed to fly and I need to know the fuel cost of the plane. In the boeing 737-800 (PMDG), on the display that shows the plane's systems, there is the FF (Fuel Flow) field. I think that is the fuel consumption index.

enter image description here

However, using a stopwatch, the value there appears to be incorrect. Advancing the throttle to keep the FF at 5, about 10 pounds was spent in 8 seconds. I expected 10 pounds to be spent in 2 seconds (5 pounds * 2 seconds = 10 pounds).

My question is: does the FF indicate the plane's fuel consumption? If so, does it indicate pounds per second? If not, how can I know the fuel consumption?

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    $\begingroup$ Timing anything for 10 seconds & then extrapolating that rate out to "per hour" values can induce lots of error in your observed values. Timing for a minute or longer will reduce the errors. Plus, easy math, fuel used in 1 minute x 60 gives fuel consumption rate per hour, which is what your FF indications represent. $\endgroup$
    – Ralph J
    May 22, 2021 at 3:09
  • $\begingroup$ Why is there, in the picture, so much fuel in the centre tank and so little in the wing tanks? Shouldn't you always burn fuel from the centre tank first? $\endgroup$
    – Jan Hudec
    May 25, 2021 at 5:17
  • $\begingroup$ @JanHudec that is a random picture I've found on web just to show the Engine Display and the FF field. And that is the Engine Display of FSX's Boeing 737. It has a limit of about 8600 pounds in the wing tanks. $\endgroup$ May 25, 2021 at 5:40

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The FF value can show either the fuel flow or the fuel used, depending on the position of a switch on the center forward panel. If it shows the fuel flow rate, then the value shown is in thousands of kilograms or pounds per hour per engine.

FF

1 FUEL FLOW Switch (spring-loaded to RATE)

RATE – displays fuel flow to engine.

USED –

  • displays fuel used since last reset
  • after 10 seconds, display automatically reverts to fuel flow.

RESET –

  • resets fuel used to zero
  • displays fuel used for 1 second, decreases to zero, then displays fuel flow.

2 Fuel Flow (FF) Readout (digital)

[Option - kgs] Displayed (white) – fuel flow to engine with FUEL FLOW switch in RATE position (kilograms per hour x 1000).

[Option - lbs] Displayed (white) – fuel flow to engine with FUEL FLOW switch in RATE position (pounds per hour x 1000).

3 Fuel Used Readout (digital)

Displayed (white) – when FUEL FLOW switch moved to USED or RESET.

(Boeing 737 NG FCOM 7.11.13 - Engines, APU - Over/Under-Displays)

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  • $\begingroup$ With the wing tanks showing 84xx quantity, that's lbs rather than kg, so I'd assume that Fuel Flow is in PPH (x 1,000) rather than in kg/hour. Unless you've just pushed the Used/Reset switch to Used, the FF is showing flow. So, 99.8% of the time, it's displaying fuel flow. $\endgroup$
    – Ralph J
    May 22, 2021 at 3:05
  • $\begingroup$ @RalphJ Yes, it even says "FUEL LBS" in the image in the question, so that is in lbs. I just mentioned the kg for completeness. $\endgroup$
    – Bianfable
    May 22, 2021 at 10:16

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