aircraft systemsFAA-published question bank (Private Pilot, 2008)
While cruising at 9,500 feet MSL, the fuel/air mixture is properly adjusted. What will occur if a descent to 4,500 feet MSL is made without readjusting the mixture?
A
The fuel/air mixture may become excessively lean.✓ Correct Answer
B
There will be more fuel in the cylinders than is needed for normal combustion, and the excess fuel will absorb heat and cool the engine.
C
The excessively rich mixture will create higher cylinder head temperatures and may cause detonation.
Explanation
As the airplane descends, the air becomes denser, so a mixture leaned for 9,500 feet has relatively less fuel for the greater mass of air at 4,500 feet and becomes too lean. PHAK advises enriching the mixture during descent; the other options describe an overly rich mixture, which is the opposite of what happens.
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