FAA Commercial Pilot Airplane (CAX)Aircraft SystemsHard
A turbocharged engine has a critical altitude of 18,000 feet, at which it produces 30 inHg manifold pressure with full throttle. Above critical altitude, the wastegate is fully closed and manifold pressure decreases at approximately the standard non-turbocharged rate of 1 inHg per 1,000 feet. What manifold pressure would the pilot expect at 21,000 feet, full throttle?
- A27 inHg
- B30 inHg
- C33 inHg
- D24 inHg
Show answer & explanationAnswer & explanation
Correct answer: A. 27 inHg
21,000 ft − 18,000 ft = 3,000 ft above critical altitude. At approximately 1 inHg loss per 1,000 ft above critical altitude (wastegate fully closed, no more compensation available), manifold pressure drops 3 inHg from 30 inHg, giving 27 inHg.
Why the other options are wrong
- B. 30 inHg would only be maintained at or below critical altitude.
- C. Manifold pressure cannot increase above critical altitude with a fully closed wastegate.
- D. This assumes a 6,000-ft difference, not the actual 3,000-ft difference.
Manifold Pressure Above Critical Altitude
Once critical altitude is exceeded, the turbocharger can no longer maintain rated manifold pressure, and MP decreases roughly 1 inHg per 1,000 feet like a normally aspirated engine.
- Critical altitude = highest altitude for rated MP with wastegate closed
- Above it, further climb causes MP loss similar to normally aspirated engines
- Pilots should expect performance decay above critical altitude
Memory trick: Past the peak, lose an inch per thousand feet.