FAA Airline Transport Pilot (ATM)Transport Aerodynamics and High-Altitude OperationsHard
An aircraft is cruising at FL370 with Mmo of 0.82. At 1g level flight, high-speed buffet onset occurs at Mach 0.87, giving a 0.05 Mach buffet margin at the current cruise Mach of 0.82. The pilot enters a 45° bank turn, producing a load factor of approximately 1.4g. Buffet onset Mach decreases by approximately 0.01 Mach for every 0.1g increase above 1g. What is the approximate high-speed buffet margin during the turn?
- A0 Mach (no margin remains)
- B0.05 Mach
- C0.01 Mach
- D0.09 Mach
Show answer & explanationAnswer & explanation
Correct answer: C. 0.01 Mach
Load factor increase above 1g = 1.4 - 1.0 = 0.4g. At 0.01 Mach decrease per 0.1g, buffet onset Mach drops by 0.4/0.1 x 0.01 = 0.04 Mach, from 0.87 to 0.83. New margin = 0.83 - 0.82 = 0.01 Mach.
Why the other options are wrong
- A. Margin is reduced but not eliminated at this load factor.
- B. This is the original 1g margin, not accounting for increased load factor.
- D. This value incorrectly adds rather than subtracts the buffet decrement.
Load Factor Effect on High-Speed Buffet
Increasing load factor (e.g., in a turn) raises the required lift coefficient, causing high-speed buffet onset to occur at a lower Mach number and reducing buffet margin.
- Buffet margin = buffet onset Mach minus cruise Mach
- Higher g-loading reduces high-speed buffet margin
- Steep turns at high altitude risk buffet onset near Mmo
Memory trick: More g's squeeze the buffet margin tighter