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 :ok:

fbw380

[barit1]

Instantly crash and burn?

:E

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 (http://www.airdisaster.com/reports/ntsb/AAR73-14.pdf) 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]

He deflates.
Correct!

http://i521.photobucket.com/albums/w334/capnbloggs/FlyingHighAP.jpg

[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 (http://www.pprune.org/members/108480-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!:E

[rudderrudderrat]

Hi PENKO,
Well, how much feel do you have in a 737 with its hydraulics and artificial feel?
About the same as the Airbus Rudder. You know how much deflection is required by the "feel" of your displaced feet.

How much aileron deflection can you "feel" is being applied in Normal Law?
In Alternate Law, you can "feel" how much aileron is required by you side stick deflection.

[Microburst2002]

In alternate law in roll, the feel is always the same for the same sidestick angle, irrespective of speed or anything else. No feedback at all. There is a relation between angle and roll rate, still, wich varies with circumstances, unlike in normal law. But I don't regard that as a way to feel.

[Yankee Whisky]

I guess this depends on the AP system and can vary a great deal.

My question would have been " Can the manual trim settings be changed with the AP engaged ?"

If so; disconnecting the AP in flight may cause minor to major changes in attitude, depending on trim settings.

If not, there should be only minor attitude changes initially.

A great deal also depends on the natural design in flight stability of the aircraft and the C/G

In any case, pilots should be "hands on" when changing the AP setting
just in case something has, nevertheless, hit the fan !.

[bungeeng]

@Bloggs

"Say what? What is "neutral"??"

I guess he is referring to the trimmed 0 point of the control wheel's roll / aileron axis. Aileron trim can be used to offset this datum, but the AP will never use aileron trim and you should never touch it while the AP is engaged, as otherwise, as you disconnect the AP, the control wheel will 'jump' to this new datum and initiate a roll!

[fmgc]

Can I suggest that you guys RTFQ!!

The question was about Airbuses, deliberate disengagement of the AP and I read it that the OP was not suggesting that there were any failures.

Therefore, at the risk of sounding arrogant, my answer is the only relevant one on this thread!

[Ka8 Flyer]

I disagree :)

[fmgc]

teeeheeeee:rolleyes:

[FLEXPWR]

Here's my 2 cents.

I fly the A320, and the 1G value is calibrated, not matching actual local acceleration. Typically, the force of gravity would be measured less than 1G near the equator, and I don't know the value one would read sitting at the poles, since I did not go there. In subtropical latitudes (East and South East Asia), the g displayed on the FMGC is less than 1, EVEN ON THE GROUND. Could it be due to slingshot effect as we get closer to the equator?

Typical values in flight always showed me less than 1G, so assuming that the Airbus books are correct and precise, disconnecting the AP would indeed revert to maintaining 1G instead of ALT, thus causing a very slight but noticeable climb.

If this does not happen on 330/340, maybe this minor flaw was corrected by software, but I don't have any experience on those types.

Should not be much of an issue anyways, how long do you want to have the aircraft flying hand off without an autopilot?

Flex