Which statement best describes engine power and propeller efficiency with altitude for piston engines?

• A. Engine power and propeller efficiency increase with altitude; TAS decreases for a given IAS.
• B. Engine power and propeller efficiency decrease with altitude; TAS increases with altitude for a given IAS.
• C. No relation between altitude and engine efficiency.
• D. Both increase with altitude.

Answer: (B)

As you climb, air becomes thinner, so the engine can burn less fuel per cycle and produce less power. The piston engine relies on the mass of air it ingests; with lower density, the power output drops and cooling is less effective, further reducing usable horsepower. The propeller’s ability to convert that shaft power into thrust also falls in thinner air because the blade experiences less air pressure and lift, so thrust drops for a given engine power. At the same time, when you keep an indicated airspeed constant, the true airspeed must rise with altitude due to the lower air density (dynamic pressure is the same, but density is lower). That combination—lower engine power and lower propeller thrust for a given power, with TAS increasing for a fixed IAS—leads to the observation that engine power and propeller efficiency decrease with altitude, while TAS increases for a given IAS.