Why does a fuel injected airplane have mixture control?

Answer 1

I wouldn't think you'd find a mixture control, of the type that would be used on a carbureted engine, on a fuel injected engine. You certainly would have a fuel control lever that would turn fuel on or off, with perhaps an idle range selection, but not in the sense that it would actually allow you to adjust the fuel/air mixture. Maybe one of the aeronautical engineers on this site could add their thoughts...?

You are right in saying that it would not be the same type of fuel metering as found on a carbureted engine, but both types of aircraft engines require specific fuel metering for one basic reason: air density changes with altitude. To maintain the approximate weight ratio of 14:1 (14 lbs of air for every 1 lb of fuel), the mixture control allows the pilot to manually reduce the amount of fuel so that the correct mixture (i.e. not too lean or rich) can be found.

Carburetors are equipped with a mixture needle, an adjustable needle located inside the float chamber that regulates the amount of fuel mixed with a given amount of air. Fuel-injected engines have an fuel/air control unit that essentially does the same job as a carburetor, just better and without all the unexpected carburetor fires. :P The mixture control (red knob in cockpit) adjusts the amount of fuel within the fuel/air control unit, this fuel/air mixture is then sent to the fuel manifold valve to be distributed through fuel lines to each cylinder.