What is the minimum speed for cruise flight above 15,000 ft?

What is the minimum speed for cruise flight above 15,000 ft?

• A. Mach 0.70
• B. Driftdown speed, whichever is higher
• C. Mach 0.75
• D. Mach 0.68

Answer: (B)

In high-altitude cruise the airplane must stay above a speed that guarantees safe controllability and the ability to descend if an engine fails. The minimum cruise speed is not a single fixed Mach number; it’s the higher of two limits: the driftdown speed and the published minimum enroute speed. Driftdown speed is the lowest airspeed at which the aircraft can remain controllable while descending with one engine inoperative to reach a suitable landing altitude, taking into account weight, configuration, and altitude. Because engine-out performance worsens with altitude and weight, the driftdown speed often becomes the controlling speed, and it can be higher than any fixed Mach value. So the required minimum is the driftdown speed, whichever is higher.

In high-altitude cruise the airplane must stay above a speed that guarantees safe controllability and the ability to descend if an engine fails. The minimum cruise speed is not a single fixed Mach number; it’s the higher of two limits: the driftdown speed and the published minimum enroute speed. Driftdown speed is the lowest airspeed at which the aircraft can remain controllable while descending with one engine inoperative to reach a suitable landing altitude, taking into account weight, configuration, and altitude. Because engine-out performance worsens with altitude and weight, the driftdown speed often becomes the controlling speed, and it can be higher than any fixed Mach value. So the required minimum is the driftdown speed, whichever is higher.