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gord96

Aggressive climb with alt armed

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Hi all. I read through the forums and manual to find out about this. From what I have read when I press the alt button, the plane should hold the altitude when the button was pushed. It works the first time, but if I turn it off and descend a bit, when I turn it back on it will put the pitch at like 30 and shoot straight up like its trying to get to the old altitude. Does the alt feature only remember the first altitude it was set at? 

 

Also, since manual mode will try and hold the altitude as well, what is the advantage of using alt? Thanks very much for your time. I know similar questions have been asked but I couldn't find a answer for my particular issue.

 

thanks!

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3 hours ago, gord96 said:

but if I turn it off and descend a bit, when I turn it back on it will put the pitch at like 30 and shoot straight up like its trying to get to the old altitude. Does the alt feature only remember the first altitude it was set at? 

 

What do you mean by 'turn it off'? If you turn off the AP master switch while ALT is still selected, it remembers the altitude you were holding. Hence if you descend and then turn the AP back on, it tries to reattain the original altitude. You need to deactivate the ALT mode by pressing it.

 

Manual mode doesn't hold altitude, it only holds the aircraft attitude (pitch and bank).

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Thanks Mark for the reply.

 

I just meant I switch off the ALT button but keep the AP (and HDG sometimes) on. Example is last night I was at 9000 feet, switched off ALT, descended to about 5000, then switch on ALT again and my plane went straight up. Checked trim and it was at 30. Could manually bring it down by clicking the trim button but seemed odd that the ALT puts it that high right away.

 

gord

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Something just came to me. I have flown the plane 10 times or so and always went by the checklist, but the last 2 times I flew it, and didn't keep a good eye on the checklist (and is also when I noticed the issue) I forgot to unlock the controls (not the control looks but when you pull the yoke back from it's position) until I was taxing. Now would this somehow set the default pitch trim at some crazy angle because the aircraft reads the trim 'neutral' before taxi and if the yoke was still forward it would throw that reading way off?

 

 

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That didn't seem to make a difference. Issue still persists but it's no big deal as I know how to work around it, or maybe it's working as intended.

 

The issue only happens when I want to descend. As soon as ALT is pressed (either manually or with alt alert) the pitch jumps anywhere to 20-40 and the plane goes vertical. To prevent this before pressing ALT, i turn off the AP completely, make sure trim is normal then turn AP back on and the press ALT so no big deal. :)

 

Like I said, only happens when descending. No issues when climbing.

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13 hours ago, gord96 said:

The issue only happens when I want to descend. As soon as ALT is pressed (either manually or with alt alert) the pitch jumps anywhere to 20-40 and the plane goes vertical. To prevent this before pressing ALT, i turn off the AP completely, make sure trim is normal then turn AP back on and the press ALT so no big deal. :)

 

I can't reproduce this behaviour. Perhaps you can give some more info on what exactly you are doing so I can have a go? For example, I don't understand how are you starting your descent? Maybe you could decsribe a sequence of events from AP off, through initiating a climb and ALT hold, then a descent, with all the steps you do.

 

The only thing I can think of that would reproduce something like the behaviour you mention is if you have an analog axis mapped to the elevator trim. Then when you turn off the AP, as soon as you touch the trim control you may potentially get a large trim change.

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Thanks Mark!

 

Ok so let's say I am at 12000ft and want to go down to 8000ft. I turn off the ALT button and pitch forward a bit. Plane descends to 8000ft, ALT is reactivated and the plane starts to shoot up. Look at trim and it's climbing. Usually at 20 quite quickly. Climbs to as high as 40 with no input from me. Turn off ALT again. Make sure pitch is normal and turn it on again and I shoot up again. Turn off AP altogether, make sure pitch is normal, turn on AP and ALT and all is well. Only happens when descending. If I am at 8000 and climb to 12000 ALT is reactivated with no issues. 

 

Still learning the plane though and found a better way to descend may be to use IAS as slow down a bit. 

 

 

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1 hour ago, gord96 said:

I turn off the ALT button and pitch forward a bit

 

 

What do you do to 'pitch forward a bit'? If you push on the yoke, the autopilot fights you by trimming out your inputs. If you fight it all the way down to your desired altitude and then let go, you're going to get a catastrophe whether you re-engage ALT mode or not. You are not supposed to do this! When AP is engaged the only legitimate way to pitch down is to use the pitch wheel on the centre console.

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2 hours ago, MarkHurst said:

 

 

What do you do to 'pitch forward a bit'? If you push on the yoke, the autopilot fights you by trimming out your inputs. If you fight it all the way down to your desired altitude and then let go, you're going to get a catastrophe whether you re-engage ALT mode or not. You are not supposed to do this! When AP is engaged the only legitimate way to pitch down is to use the pitch wheel on the centre console.

 

Hehe. Then that's my problem! Thanks Mark. Figured it was 'user error'. :D

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On February 22, 2016 at 9:24 AM, gord96 said:

Hi all. I read through the forums and manual to find out about this. From what I have read when I press the alt button, the plane should hold the altitude when the button was pushed. It works the first time, but if I turn it off and descend a bit, when I turn it back on it will put the pitch at like 30 and shoot straight up like its trying to get to the old altitude. Does the alt feature only remember the first altitude it was set at? 

 

Also, since manual mode will try and hold the altitude as well, what is the advantage of using alt? Thanks very much for your time. I know similar questions have been asked but I couldn't find a answer for my particular issue.

 

thanks!

Been flying this cool plane and learning the A/P.

What do you mean when you say "manual" mode?  What is manual mode?

Thanks.

 

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Hi,

it´s verry important to understand the different AP modi, the open and selected modi.

You write from Alt off and descend a bit / when i turn back - all verry crazy.

 

You write, descend a bit - what you have adjusted at the FCU, what was the indications on the FMA (on top of the PFD) and on the FCU?

When i start an descend in this lower airspace (because ATC instructions), i adjust the new alt in the window (for example from 9000 to 5000) and pull the V/S knob for choose the vertical speed, perhabs adjust the speed.

If i read your message, i think, you are implaying false the alt knob. Select an new alt and push the knob - the autopilot of the airplane beginns to climb or to descend at this new alt and level off automaticly. This is an managed mode and all flightplan restriction are followed. Pull the alt knob - it´s the same, but this is the open mode, means the airplane climb or desend direct to the new alt, without compliance any flightplane restrictions. After start the climb or descend you must not pull or push the alt knob at the new alt for level off! With this you change in an new mode.
 
Frank
 
In additional, you have use the EXPED mode? The big pitch of 30° can result of this, but with pictures from your FCU, PFD and complete report of your actions, we can say more.
 
Wrong plane!

 

 

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26 minutes ago, FraPre said:

FCU / FMA / PFD / v/s knob

 

Are you in the right forum?! The Twin Otter doesn't have any of those things :)

 

2 hours ago, PaulHiggC72014 said:

What do you mean when you say "manual" mode?  What is manual mode?

 

Manual mode is the default mode when you have AP engaged but no lateral or vertical mode selected. It holds the aircraft attitude, within certain limits.

 

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:lol: No, thanks. With glasses that would not happen :lol:

 


 

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Thank you Mark, I couldn't find it in any of the manuals unless I missed it but I did do a search on the manuals which I have placed on my iPad.

 

Fine plane, not good on gas on long hauls though but GREAT for FSEconomy!

 

Thank you.

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