Fuel Leaning at Altitude

[Big Kev]

Hello Everyone,

I'm a student pilot in training at the moment and my instructor and books tell me to lean the mixture on climb through altitude due to the air density decreasing.

But my book says that fuel drawn into my fuel injected engine is determined by how much the throttle valve is open which then determines the air flowing through the venturi which then determines the amount of fuel drawn out.

Because there is less air at altitude, I would of thought that if the throttle valve is left in the same position at altitude as it was at sea level, then the amount of fuel being drawn out would automatically decrease due to the less volume of air passing through the venturi, which in turn would have a decreased pulling effect on fuel through the venturi system, thus in effect causing its own leaning!

Could someone who knows this topic well please correct my understanding? - thanks in advance.....


BK

[High Wing Drifter]

The difference is that the fuel drawn is dependant on the volume not the density. The volume of air be sucked through the venturi does not change as the density is decreasing.

[capt.sparrow]

just remember throttle = air, mixture = fuel. adjusting the mixture control does not affect the air drawn etc.

[FlyingForFun]

HWD is correct, but to clarify further:

As you climb, the volume of air you draw in (if you leave the throttle alone) is near enough un-altered. But because the air is less dense, the mass of air is less.

The amount of fuel you want is determined by the mass of air. But the amount of fuel you get depends on the volume. Which is why you need to adjust the mixture.

I'm surprised your books tell you to lean the mixture in the climb. I'd only suggest that for high-altitude operations. At the lower altitudes which most of us fly at most of the time, I would suggest leaving the mixture rich during the climb. The extra fuel will help keep the engine cool. I would only lean the engine once I've levelled off at the top of the climb. But the definitive guide on how to operate your specific aeroplane will be the aeroplane's POH (Pilots Operating Handbook), so please don't take my advice without checking what the POH says first.

FFF
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[High Wing Drifter]

Recalling the heady days of Aircraft General Knowledge class the general wisdom was that you generally climb rich, to aid cooling.

However, the CAA wanted you to know that predetonation is likely with an over-rich mixture and high manifold pressures.

[Big Kev]

Thanks guys and gals for replying - that explains it - volume and mass - it all makes sense to me now.....thank you once again

:ok: