FAA Flight Instructor Airplane (FIA)Performance, Weight and BalanceHard
An engineer explains that a particular turbocharged engine can maintain its full rated manifold pressure up to 18,000 ft, but above that altitude, manifold pressure begins to decrease as the airplane climbs further. What term describes this altitude?
- AAbsolute ceiling
- BService ceiling
- CCritical altitude
- DDensity altitude
Show answer & explanationAnswer & explanation
Correct answer: C. Critical altitude
Critical altitude is the maximum altitude at which a turbocharged or supercharged engine can still maintain its rated power (manifold pressure); above this altitude, the turbocharger can no longer compensate for the thinning air, and manifold pressure and power decline.
Why the other options are wrong
- A. Absolute ceiling is the altitude where climb rate reaches zero, unrelated to manifold pressure limits.
- B. Service ceiling is defined by a specific climb rate (typically 100 fpm), not manifold pressure capability.
- D. Density altitude is a measure of air density affecting performance generally, not a specific engine power limit.
Critical Altitude
The maximum altitude at which a turbocharged or supercharged engine can still produce its full rated power/manifold pressure; above it, power output decreases with further climb.
- Applies specifically to forced-induction (turbo/supercharged) engines
- Above critical altitude, manifold pressure can no longer be maintained at rated value
- Naturally aspirated engines lose power steadily with altitude and have no critical altitude
Memory trick: Critical altitude: the turbo's tipping point where power starts to droop.