FAA Commercial Pilot Airplane (CAX)Aircraft SystemsHard
A turbocharged engine can maintain 30 inHg manifold pressure up to its critical altitude of 18,000 feet MSL. Above the critical altitude, the wastegate is fully closed and manifold pressure decreases at approximately 1 inHg per 1,000 feet of additional altitude gained. Assuming full throttle is maintained, what manifold pressure should the pilot expect at 21,000 feet MSL?
- A24 inHg
- B30 inHg
- C27 inHg
- D33 inHg
Show answer & explanationAnswer & explanation
Correct answer: C. 27 inHg
Above critical altitude, the wastegate can no longer compensate for thinning air. The airplane climbs 21,000 − 18,000 = 3,000 feet above critical altitude, and manifold pressure drops 1 inHg per 1,000 feet: 3,000 ft ÷ 1,000 ft × 1 inHg = 3 inHg drop. 30 inHg − 3 inHg = 27 inHg.
Why the other options are wrong
- A. This would result from a 6 inHg drop, incorrectly doubling the altitude difference.
- B. 30 inHg is only maintained up to the critical altitude, not above it.
- D. Manifold pressure cannot increase above critical altitude; this reflects the wrong direction of change.
Critical Altitude
The maximum altitude at which a turbocharger can still maintain its rated manifold pressure; above this altitude, manifold pressure drops with further climb.
- Wastegate fully closed at critical altitude
- Above it, MP falls roughly 1 inHg per 1,000 ft (engine dependent)
- Defines the turbocharger's performance ceiling
Memory trick: Past the critical peak, pressure leaks — 1 inch per thousand feet.