FAA Commercial Pilot Airplane (CAX)Aircraft SystemsHard
A normally aspirated engine produces 29.5 inHg manifold pressure at full throttle at sea level. Assuming manifold pressure available decreases at approximately the same rate as atmospheric pressure (about 1 inHg per 1,000 feet), what full-throttle manifold pressure should a pilot expect at 7,500 feet density altitude?
- A29.5 inHg
- B15.0 inHg
- C26.5 inHg
- D22.0 inHg
Show answer & explanationAnswer & explanation
Correct answer: D. 22.0 inHg
Manifold pressure available from a normally aspirated engine falls with ambient pressure, roughly 1 inHg per 1,000 feet. 29.5 inHg − 7.5 inHg (for 7,500 ft) = 22.0 inHg. This illustrates why normally aspirated engines lose power with altitude, unlike turbocharged engines below critical altitude.
Why the other options are wrong
- A. This ignores the altitude-induced pressure loss entirely.
- B. This subtracts too much (about 14.5 inHg), more than the correct loss.
- C. This only subtracts 3 inHg, less than the correct rate-based loss.
Normally Aspirated MP Loss with Altitude
A normally aspirated engine's full-throttle manifold pressure drops roughly 1 inHg per 1,000 feet of altitude gain because it cannot compress air above ambient pressure.
- No compressor to maintain sea-level MP at altitude
- Approx. 1 inHg loss per 1,000 ft
- Contrasts with turbocharged engines, which hold MP up to critical altitude
Memory trick: Every 1,000 feet up costs about 1 inch of manifold pressure.