FAA Airline Transport Pilot (ATM)Transport Aerodynamics and High-Altitude OperationsMedium
A turbojet-powered transport produces 24,000 lb of static thrust per engine at sea level. As the aircraft climbs to FL350, where air density is roughly 30% of sea-level density, thrust available decreases substantially even though the engine RPM/N1 is maintained. What is the primary reason for this thrust reduction with altitude?
- AColder temperatures at altitude directly reduce the engine's mechanical efficiency
- BReduced air density lowers the mass airflow through the engine, reducing thrust output
- CThe autothrottle intentionally reduces thrust at altitude to conserve fuel
- DReduced dynamic pressure on the wings requires the engines to throttle back automatically
Show answer & explanationAnswer & explanation
Correct answer: B. Reduced air density lowers the mass airflow through the engine, reducing thrust output
Jet engine thrust depends heavily on mass airflow, which is proportional to air density. As altitude increases and density decreases, less mass of air is available for the engine to accelerate, so thrust available decreases (thrust lapse) even at constant N1, requiring higher N1 settings at altitude to produce a given thrust value, and ultimately limiting maximum cruise thrust at high altitudes.
Why the other options are wrong
- A. Colder temperatures actually can increase thrust due to higher density, so this is not the driver here.
- C. This is a real operational technique but not the fundamental aerodynamic/thermodynamic cause of thrust lapse.
- D. Wing dynamic pressure and engine thrust are unrelated; engines don't throttle automatically for this reason.
Thrust Lapse with Altitude
Turbojet/turbofan thrust available decreases with altitude primarily because mass airflow through the engine decreases as air density decreases.
- Thrust ∝ mass airflow × velocity change
- Air density decreases roughly exponentially with altitude
- Higher N1 is required at altitude to produce a given thrust value
Memory trick: Thin air, thin thrust