# Is the FF (Fuel Flow) the fuel consumption index?

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.

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?

• 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.
– Ralph J
Commented May 22, 2021 at 3:09
• 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? Commented May 25, 2021 at 5:17
• @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. Commented May 25, 2021 at 5:40

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.

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).