Question

What is HREF in the control law used for auto flare?

(a) Height when flare is initiated

(b) Height where flare should be initiated

(c) Maximum height where flare can be initiated

(d) Small negative height

The question was asked at a job interview.

My doubt is from ILS/MLS Coupled Landing System and Automatic Landing topic in chapter Autopilots and Flight Management Systems of Avionics

Answer 1

The correct option is (d) Small negative height

Easiest explanation: In the control law for auto flare, HREF is a small negative height, or bias, which ensures there is still a small downwards velocity at touchdown. This avoids the long exponential ‘tail’ to reach zero velocity and enables a reasonably precise touchdown to be achieved.