FAA Airline Transport Pilot (ATM)Transport Aerodynamics and High-Altitude OperationsEasy
A pilot reviewing performance data notes the aircraft's critical Mach number is 0.74 while the aircraft is currently cruising at Mach 0.70. What does the critical Mach number represent?
- AThe Mach number below which the yaw damper is required
- BThe maximum Mach number certificated for the airframe (Mmo)
- CThe Mach number at which the entire aircraft exceeds the speed of sound
- DThe Mach number at which airflow over some point on the airframe first reaches Mach 1.0
Show answer & explanationAnswer & explanation
Correct answer: D. The Mach number at which airflow over some point on the airframe first reaches Mach 1.0
Critical Mach number (Mcrit) is the free-stream Mach number at which local airflow acceleration over a curved surface, such as the upper wing, first reaches sonic speed (Mach 1.0), even though the aircraft as a whole is still flying below the speed of sound.
Why the other options are wrong
- A. Yaw damper requirements are unrelated to critical Mach number.
- B. Mmo is a certificated operating limit, distinct from Mcrit.
- C. The aircraft itself does not need to exceed Mach 1.0; only a local point reaches sonic flow.
Critical Mach Number (Mcrit)
The free-stream Mach number at which airflow over some point of the aircraft first reaches the local speed of sound.
- Occurs before aircraft TAS reaches Mach 1.0
- Local acceleration over curved surfaces (e.g., wing upper surface) exceeds free-stream speed
- Exceeding Mcrit leads to shock wave formation and possible buffet
Memory trick: The wing 'speaks sonic' before the jet does