Question

The flight path is unstable during __________

(a) the aircraft flying at airspeed less than minimum drag speed

(b) the aircraft flying at airspeed more than minimum drag speed

(c) the aircraft flying at airspeed less than maximum drag speed

(d) the aircraft flying at airspeed more than maximum drag speed

I have been asked this question in an interview.

This intriguing question comes from Descent Performance in Aircraft Operations topic in chapter Climb and Descent Performance of Aircraft Performance

Answer 1

Correct choice is (a) the aircraft flying at airspeed less than minimum drag speed

The explanation: The flight path is unstable during the aircraft flying at airspeed less than minimum drag speed. In this case the minimum rate of descent occurs at the minimum power speed and minimum gradient occurs at the minimum drag speed.