FAA Commercial Pilot Airplane (CAX)Aircraft SystemsHard
A pilot flying a turbocharged airplane advances the throttle rapidly during a go-around. Manifold pressure momentarily spikes well above the takeoff-rated limit before settling back to normal. What is the most likely cause of this transient overboost, and how should the pilot avoid it?
- AThe propeller governor failed to increase RPM quickly enough; the pilot should reduce mixture to compensate
- BTurbo lag causes the wastegate to react slower than the throttle movement; the pilot should advance the throttle smoothly and slowly
- CThe fuel injector nozzles delivered excess fuel; the pilot should switch to the auxiliary fuel pump
- DThe cabin pressurization controller opened the outflow valve; the pilot should turn off the pressurization system
Show answer & explanationAnswer & explanation
Correct answer: B. Turbo lag causes the wastegate to react slower than the throttle movement; the pilot should advance the throttle smoothly and slowly
Rapid throttle movement can outpace the wastegate's ability to respond (turbo lag), causing exhaust gas to briefly overdrive the turbine and produce a manifold pressure spike above rated limits. Smooth, gradual throttle application allows the wastegate controller to keep pace and prevents overboost.
Why the other options are wrong
- A. Propeller governor response affects RPM, not manifold pressure spikes from turbo lag.
- C. Excess fuel delivery would cause a rich mixture issue, not a manifold pressure spike.
- D. Cabin pressurization systems are unrelated to engine manifold pressure control.
Turbo Lag / Overboost
A brief manifold pressure spike above rated limits caused by the wastegate's inability to respond as fast as rapid throttle movement; avoided by smooth throttle application.
- Rapid throttle motion outpaces wastegate response
- Can exceed rated manifold pressure limits
- Smooth throttle inputs prevent overboost damage
Memory trick: Slam the throttle, spike the boost — go easy to keep it smooth.