A flight crew encounters moderate-to-severe turbulence at FL350 while cruising at Mach 0.80. The AFM specifies a turbulence penetration speed lower than normal cruise speed. What is the primary reason for reducing to this speed?
- ATo comply with a universal ATC-mandated speed restriction during turbulence reports
- BTo increase only the stall margin, since drag is not a concern during turbulence
- CTo balance the margin between low-speed (stall) buffet and high-speed (Mach) buffet, limiting structural loads from gust-induced load factor changes
- DTo reduce fuel consumption for the duration of the turbulence encounter
Show answer & explanationAnswer & explanation
Correct answer: C. To balance the margin between low-speed (stall) buffet and high-speed (Mach) buffet, limiting structural loads from gust-induced load factor changes
Turbulence penetration speed is selected to provide roughly equal margins to both low-speed buffet and Mmo/high-speed buffet, since gusts can momentarily increase or decrease effective angle of attack and speed. This minimizes the risk of stalling or overspeeding and reduces structural loads from gust-induced load factor excursions.
Why the other options are wrong
- A. There is no single universal ATC-mandated turbulence speed; it's an AFM performance value.
- B. Turbulence speed also protects against high-speed buffet/overspeed, not just stall.
- D. Fuel efficiency is not the reason for turbulence penetration speed selection.
Turbulence Penetration Speed
A recommended reduced speed used in turbulence that provides balanced margins from both stall buffet and high-speed buffet/overspeed, minimizing structural gust loads.
- Typically below normal cruise Mach/speed
- Balances low-speed and high-speed buffet margins
- Reduces risk of structural overstress from gust load factors
Memory trick: Split the difference between stall and screech (overspeed) to survive the bumps.