Simviation Forums

1. Why does fuel flow increase with airspeed? I was climbing to 4500 feet in a Cessna 172 with full throttle. I already leaned the mixture after 3000 feet. My climb speed was 75 knots. At 4400-4500 feet, just before I started leveling off my fuel flow was 7-8 gph. I leveled off at 4500 feet and I left the throttle at full to increase my airspeed and the fuel flow increased to 10 gph at 100 knots. Why if I didn't touch the throttle or mixture control after leveling off?

I know that the propeller RPM increases with airspeed but if I left the throttle valve the same before and after leveling off, shouldn't the fuel flow be the same?

2. Also why is the fuel flow above 3000 ft MSL lower when the mixture control is at rich than when I lean it a bit? Shouldn't the fuel flow always be higher when the mixture is full rich.

I'm refering to FS2004 up here, I don't know is it like this only in the sim or also in real. It seems that in FS2004 fuel flow increases with engine RPM and not only with the throttle and mixture.

None of this sounds right... in the sim or real life, 10 gph is pretty high for a 172, even at full throttle.

How are you determining your fuel flow?


Fuel flow should not increase with airspeed- shut the engine off in flight, and even wait for the prop to stop. Then shove the yoke all the way forward. You will see an increase in airspeed but not fuel flow, believe me.
Even with the engine running, you should not see an increase in fuel flow just because the airspeed is higher.



FWIW, the throttle does not control fuel flow directly, the mixture control does. The throttle actually controls the volume of air entering the carb, and only indirectly the amount of vaporized fuel entering the carb. But fuel flowing out of the tank (and past the fuel flow gauge) is controlled by the mixture control.

This points out one of the major flaws in MSFS (both FS9 and FSX)..

Fuel flow is just bizzare. I can only imagine that it is a cheat way to get max power tied to proper leaning. The MSFS alorithm for all that suff MUST tie fuel flow to HP.

In other words.. Takeoff in the C172 full rich and don't touch the mixture until you're at 6000msl (I'll pay for you plugs). You'll notice power falling of as you pass 3000ms (take note of the fuel flow too).

Then, at 6000msl, start leaning and you'll see fuel flow INCREASE as you lean... it will peak, and as you keep leaning it will drop off again. That peak is actually you maximum power mixture setting. So, in a sense... the MSFS fuel-flow gauge is a wierd fuel-flow/EGT hybrid gauge (I've been using fuel-flow for leaning since FS2002)(kinda like a makeshift EGT gauge).

That's the best explanation I can come up with.. and why more accurate and responsive gauges (and algorithms for all the interactions), are number one on my FS11 wish-list..

How are you determining your fuel flow?

With the fuel flow gauge.

In other words.. Takeoff in the C172 full rich and don't touch the mixture until you're at 6000msl (I'll pay for you plugs). You'll notice power falling of as you pass 3000ms (take note of the fuel flow too).

Then, at 6000msl, start leaning and you'll see fuel flow INCREASE as you lean... it will peak, and as you keep leaning it will drop off again. That peak is actually you maximum power mixture setting. So, in a sense... the MSFS fuel-flow gauge is a wierd fuel-flow/EGT hybrid gauge (I've been using fuel-flow for leaning since FS2002)(kinda like a makeshift EGT gauge).

Yes, that's exactly what happens when leaning. Good to know it's only a MSFS bug. 8-)

This is odd. In our jets (L-39) the fuel flow decreases with altitude, rather than increasing with airspeed.

This is odd. In our jets (L-39) the fuel flow decreases with altitude, rather than increasing with airspeed.

Jet engines are a completely different beast than piston engines.