Question

In a quadruplex system, when does a passive fail situation occur?

(a) When all the channels fail

(b) When 3 channels fail and none is disconnected

(c) When 2 channels fail none is disconnected

(d) When 1 channel fail none is disconnected

This question was posed to me during an internship interview.

This intriguing question comes from Redundancy and Failure Survival in FBW in division Fly by Wire Flight Control of Avionics

Answer 1

Correct answer is (c) When 2 channels fail none is disconnected

The explanation is: When two channels fail and are not disconnected, two good channels versus two failed hard over channels so that the control surface does not move resulting in a ‘fail passive’ situation. When one channel fails and is not disconnected, the other three good channels overrides the failed one.