What is the typical relationship between altitude and engine/propeller efficiency in piston-engine airplanes?

• A. Engine power and propeller efficiency typically decrease with altitude due to thinner air; true airspeed increases with altitude for a given indicated airspeed.
• B. Engine power increases with altitude; propeller efficiency increases; true airspeed decreases with altitude for a given IAS.
• C. There is no relation between altitude and engine efficiency.
• D. Both engine power and propeller efficiency increase with altitude.

Answer: (A)

As air gets thinner with altitude, the engine has less air to mix with fuel, so the maximum power it can produce falls off. The propeller also relies on moving a dense airstream to generate thrust, so thinner air means less thrust and a drop in propeller efficiency for a given power setting. Even though engine power and propeller efficiency decline, the indicated airspeed stays the same when you set the same throttle and pitch, but the true airspeed increases because the air density is lower. In short: engine power decreases with altitude, propeller efficiency decreases, and true airspeed rises for a given indicated airspeed.