View Full Version : Fail passive

2nd May 2004, 12:22
What is fail passive and fail active system

Young Paul
2nd May 2004, 15:45
I'm not sure that "fail active" is the alternative.

AFAIK, "Fail passive" means that if a system fails, it won't drastically change anything, but the system will be downgraded.

"Fail operational" means that even if a system fails, the degraded mode will still do what it is supposed to do.

So (something like this) on A320, Cat IIIB is basically "fail operational" - if an autopilot fails at any stage in the approach, the aircraft can still complete an autoland. However, if one is already not working, the system is "fail passive" - another failure will prevent the autoland from being carried out, but it won't leave the aircraft in a mess.

However, the B737 is not "fail passive" below 400'. If the autopilot fails and a go-around is initiated, then the aircraft will be horrendously out of trim, and will require fast work and some dexterity from the pilots to recover it.

Even more particularly, there is a vast difference in an engine failure in the missed approach at 50' on the B737 and the A320.

I am aware of the inadequacies of the answer, but that is a quick attempt to give some idea of the issues at stake.

3rd May 2004, 00:38
Concerning flight control systems, the following can be said:

Fail-Operational flight control system:
A flight control system is fail-operational if, in the event of a failure below alert height, the
approach, flare and landing, can be completed automatically. In the event of a failure,
the automatic landing system will operate as a fail-passive system.

Fail-Passive Flight Control System:
A flight control system is fail-passive if, in the event of a failure, there is no significant outof-trim condition or deviation of flight path or attitude but the landing is not completed
automatically. For a fail-passive automatic flight control system the pilot assumes control
of the aeroplane after a failure.

Hope it helps, DBate

Young Paul
3rd May 2004, 14:26
Yeah, well, that's what I would of said if I'd of beed able to think of the clever words.:O