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Trim abruptly changes upon AP engagement


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It looks like the trim suddenly and abruptly increases 1 unit up upon AP engagement in SPD/VS modes. This does not happen when ALTS is on. I don't have an axis assigned to elevator trim, only buttons.

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I have this happen regularly. After takeoff and flying manually via FD I will select speed and set to 250kts. I will be clean and in a very stable climb at 230 kts., hit AP and she pitches up every time. I hadn't noticed the trim indicator changing until you mentioned it but can confirm it trims UP over 2 units but settles down to ~1 unit above what it was in the stable non-AP climb. You would think with speed set at a higher number than currently flying, that when you engage AP it would trim DOWN to capture the 250kits. 

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  • 2 weeks later...

There isn't much detail but I don't see anything wrong with that. When you engage SPD mode you are telling it to climb by modifying its pitch to maintain the speed at whatever throttle setting you have. The whole purpose of using SPD is to climb at a specific speed so it seems kind of normal for it to pitch up. After all, it is all about climbing and you can't climb without pitching up.

 

I have no issues changing the speed for the SPD mode and have it vary its pitch to then meet that new speed target. So if you do, I think some actual way to reproduce it would be useful.

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12 minutes ago, Thx1137 said:

There isn't much detail but I don't see anything wrong with that. When you engage SPD mode you are telling it to climb by modifying its pitch to maintain the speed at whatever throttle setting you have. The whole purpose of using SPD is to climb at a specific speed so it seems kind of normal for it to pitch up. After all, it is all about climbing and you can't climb without pitching up.

 

I have no issues changing the speed for the SPD mode and have it vary its pitch to then meet that new speed target. So if you do, I think some actual way to reproduce it would be useful.

 

Well if the pilot has trimmed the aircraft properly to maintain the selected speed, and then you engage to AP there shouldn't really be any change in pitch. A minor change sure but nothing like what these users are describing. I've seen exactly what they're describing after every single take off when I engage the AP. Definitely not normal.

 

Not a deal breaker by any means but definitely not normal. I should add that this only happens the first time I engage the AP after take off. After that SPD mode works flawlessly.

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The problem is that if I engage the AP close to the level off altitude (<3000ft to it), it WILL overshoot due to this ABRUPT and unrealistic nose up trim change. It’s not necessarily on SPD mode. On PITCH or VS mode it happens as well. After the aircraft goes from 1000fpm to something like 6000fpm in 2s, I disengage the AP, retrim it, manually level off and engage the AP again. Each time I re-engage the AP on the same flight the effect is less pronounced. After the 4th or so AP ON/OFF/ON cycle, it engages normally, as it should. 
This is not a discussion on how to fly it. I’m pretty sure I’m engaging the AP with the aircraft spot on the FD and well trimmed. I flew the CRJ for a regional in 2014...

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Upvoting the issue. Aprupt nose up on AP engagement on stable climb (nav + speed) happens sometimes. Yesterday it happened when flying by hand bit higher before engaging AP.

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