Turbulence penetration speed is defined as the lower of which two values?

• A. 280 knots
• B. Mach 0.75
• C. Lower of 280 knots or Mach 0.75
• D. 260 knots

Answer: (C)

Turbulence penetration speed is chosen to protect the aircraft structure while staying within published speed limits, so you don’t push into areas where the airplane isn’t designed to handle gusts. It is defined as the lower of two reference values: 280 knots indicated airspeed or Mach 0.75. The reason for taking the smaller of these is that at lower altitudes the airspeed limit is dominated by IAS limits (Vmo), while at higher altitudes the speed constraint shifts to a Mach limit (Mmo). By using the lesser of 280 knots and Mach 0.75, you ensure you remain within both the IAS and Mach limits across the flight envelope, regardless of altitude. Using only one value would either risk exceeding the airframe’s IAS limit at low altitude or violate the Mach limit at high altitude, while the chosen rule keeps you safely within both. The other numbers don’t adapt to changes in altitude in the same way, making them unsuitable on their own.