What happens when you disconnect the autopilot?

[flybywire380]

All,

Can anyone answer this question ....

If you have an Airbus, for example, an A330 in straight & level cruise with the autopilot & autothrust engaged, what happens when you disengage the autopilot, with no sidestick input - does the a/c maintain straight & level, climb or descend?

Thanks

fbw380

[barit1]

Instantly crash and burn?



It actually depends on how well-trimmed the airplane is. If trimmed to fly hands-off, it makes little difference in the short term whether autopilot is on or off.

If it's NOT well trimmed, it might start to roll or turn or anything else.

If you read the accident report on the EAL401 TriStar Everglades accident, the A/P was accidentally disconnected at low altitude, and the resulting deviation was so subtle that no one noticed.

[dixi188]

Part of the certification requirements for Autopilots is that when disconnected the aircraft shall be in trim so that no significant deviation from the flight path will occur.
So in the A330 or any other airliner, the aircraft should continue straight and level until external forces or control inputs cause a deviation.

[darkroomsource]

I think that it is not part of the certification requirements for ALL autopilots that the plane be in trim when the autopilot is disengaged.
I have flown a few planes where turning the AP off will find the plane out of trim, and I have read numerous accounts of where a pilot turned of the AP and found that the plane rolled left because they had not changed fuel tanks for the entire flight.

[Microburst2002]

It will tend to maintain flight path (v/s zero in this case) and roll rate zero.

Tend to is mot the same as maintain

[dixi188]

Ho Hum.

Maybe I got it wrong, but that was my understanding of things when I did a bit of work on A/P installation and certification about 35 years ago.

Thinking about it if you wind on Aileron trim, or burn to a fuel imbalance, with the A/P engaged, then when you disengage, the aircraft will roll.

I think perhaps the bit about being in trim may just apply to Approach and Land modes, so you don't get any surprises near the ground.

[fmgc]

A330/340 maintain straight and level.

A320 will go into a slight climb.

With no input on the sidestick you are commanding 1 g. At altitude g is less than 1 so to maintain 1 g it has to climb slightly. A330/340 the software accounts for this but not in the a320 family.

[Airmann]

Your company will call you and tell you that you are not paid to fly planes, only to monitor the autopilot,

[grounded27]

He deflates.

[EEngr]

He deflates.

I was going to guess: The dog bites you.

[Microburst2002]

the airplane maintains straight level flight, for a while... then slightly diverts in pitch and roll.

airbus normal law is not an ALT mode nor a HDG mode, nor a LOC and G/S mode, nor anything like that. It is stable in path, which means that it tends to maintain fligh path.

[Capn Bloggs]

Originally Posted by Grounded27

He deflates.

Correct!

[sidestick stirrer]

It's a trick question, Gentlemen.
You cannot disconnect the autopilot in an Airbus if it is in normal or alternate law.
The only time you can actually fly it in a condition approaching " flying" is in direct law.
Any other time, it's just CWS...

[grounded27]

Otto the pilot.... Great catch and pic Capn.

[EEngr]

Microburst2002:
"the airplane maintains straight level flight, for a while..."

So, to slightly change the initial conditions of the question: If the autopilot has commanded a turn (roll and yaw) while in normal mode and is then disconnected, will it maintain the turn (as if the sidestick was in the neutral position mid-turn) or return the airplane to a zero g lateral/1g vertical condition?

[Ka8 Flyer]

Guess that depends on the aircraft...

Airbus FBW family: aircraft will maintain bank angle and zero vertical acceleration (your 1G analogy).

Conventional aircraft: the control wheel is spring-loaded to neutral and thus will return to the center as the A/P drops off. The aircraft will slowly return to 0 bank angle due to the positive dihedral of the wings. As to pitch, the aircraft should start to climb as it rolls back to 0° bank angle...

[Capn Bloggs]

Conventional aircraft: the control wheel is spring-loaded to neutral and thus will return to the center as the A/P drops off.

Say what? What is "neutral"??

[Microburst2002]

In the Airbus, the airplane tends to maintain bank and level flight.

In conventionals, I think they do pretty much the same.

Or they should. I guess it depends on the particular AP system. Some models may put the airplane in an out of trim situation

[Microburst2002]

Sidestick Stirrer said

It's a trick question, Gentlemen.
You cannot disconnect the autopilot in an Airbus if it is in normal or alternate law.
The only time you can actually fly it in a condition approaching " flying" is in direct law.
Any other time, it's just CWS...

Mate, your nickname suggests you fly a fbw airbus but you seem not to know its FCS.

And direct law is not a condition approaching flying, not more than normal law is, at least.

[PENKO]

I always wonder why there is this small minority of Airbus pilots who don't feel in control of their machine. They say there is no 'feel'. Well, how much feel do you have in a 737 with its hydraulics and artificial feel? No more than in an Airbus!

CWS? Oh please!