FAA Airline Transport Pilot (ATM)Transport Aerodynamics and High-Altitude OperationsHard
A transport aircraft in cruise inadvertently exceeds Vmo/Mmo, triggering the overspeed warning ('clacker'), requiring the crew to reduce speed immediately. What primary structural/aerodynamic hazard does the Vmo/Mmo limit protect against, distinct from stall or low-speed buffet concerns?
- AFlutter, control surface reversal, and structural loads/divergence as speed approaches the design dive speed margin
- BLoss of cabin pressurization caused by increased dynamic pressure on the fuselage
- CInsufficient engine thrust available to sustain the higher speed in level flight
- DExcessive fuel consumption resulting from operating above the optimum cruise speed
Show answer & explanationAnswer & explanation
Correct answer: A. Flutter, control surface reversal, and structural loads/divergence as speed approaches the design dive speed margin
Vmo/Mmo are set with a margin below the design dive speed (Vd/Md) to protect against aeroelastic phenomena such as flutter, control surface reversal, and structural divergence, as well as excessive structural loads—hazards unrelated to stall or buffet at the low-speed end.
Why the other options are wrong
- B. Cabin pressurization is controlled independently and is not affected by dynamic pressure in this way.
- C. Thrust availability is a performance issue, not the reason for the Vmo/Mmo structural limit.
- D. Fuel economy is unrelated to the structural certification basis of Vmo/Mmo.
Vmo/Mmo Structural Protection
Vmo/Mmo are certified speed limits providing margin below the design dive speed (Vd/Md) to prevent flutter, control reversal, and structural divergence.
- Vd/Md is the design dive speed used in flutter/structural certification
- Vmo/Mmo include margin below Vd/Md for safety
- Exceeding Vmo/Mmo risks aeroelastic instability, not stall
Memory trick: Clacker warns of the cliff edge before flutter takes the wing.