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DavidV935

Saab Climbout on Autopilot oscillates/porpoises

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Didn't see this mentioned in posts from the past few months. After take-off in the LES Saab 340A, I hit gear up, then after 500' AGL, I engage A/P, then click CLIMB(H), then click on NAV. The climb rate oscillates from plus 2000 fpm to minus 2000 fpm or more in either direction. I don't know if I'm going to crash like a lawn dart, or climb like an F-16 and then stall out. It oscillates back and forth and I don't know how to overcome it or stabilize it. It does not happen every time, but often enough to be quite annoying. Is there a tip or trick someone can share? I've only owned this plane for 2 weeks, so it could be pilot error! Thanks.

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This usually happens when the airspeed is too high.  Are you getting close to the barber pole speed?

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2 hours ago, Goran_M said:

This usually happens when the airspeed is too high.  Are you getting close to the barber pole speed?

I did notice that, but when I initially engaged the AP and hit CLIMB(H), the airspeed was still in the 150-160 range and gaining.

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Are you remembering to set your altitude target beforehand? Sometimes I'll forget to reset it after approach and it'll be very low. Then the AP will level out and it will start to accelerate like crazy.

Also you might want to consider engaging the flight director and climb/nav modes and hand-fly it prior to engaging the AP. If you enable the AP when you're already following the FD cue you'll get smoother and more predictable actions from it. If there's a big difference then when you enable the AP you can get significant pitch moments as it tries to sort the situation out.

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I have an issue with the oscillation as well and I won't start a different post.

I'm flying level at FL170, the A/P is keeping track and altitude just fine, then there's about a 30 degree turn and the A/P loses maybe 100-150 ft of altitude during the turn. Speed is around 200 kts IAS, 45kts below the barber pole (shouldn't be too fast). The A/P compensates heavily for the altitude loss, starting off with VS +/- 2000 feet, then increasing the oscillations more and more until VS is +/- 6000ft up and down. This sort of dynamically unstable behaviour would end with a stall. Tried reducing power just in case but that had no effect. Had to disengage the A/P, manually level the aircraft and only then re-engage the A/P and the aircraft would continue flying level. 

That's probably not normal behaviour, is it?

I have screenshots of the plane at the extremes of the oscillation and logfiles, should those be necessary for investigation of the matter.

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i'll revisit the flight model some time this week and run some more tests, then come back with something.  

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This has happened to me before when after takeoff I forget to disengage AutoCoursen and CTOT.  The "150-160" range and gaining into a progressive overspeed porpoise is a tell-tale sign.  Not saying this is what happened, but this is the first thing I would check on myself.

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I've done numerous flights in the last couple of days and for me having Autocoarsen and CTOT off doesn't seem to have a specific effect. About every other flight the progressive oscillations occur - usually after the aircraft has passed a turn of more than 30 degrees or so. I always keep my speed around 40-50 kts below the barber pole, so it shouldn't be overspeed induced. It seems there are issues with damping the oscillations whenever VS goes over 1500 fpm during a turn. It can sort of stabilise when the VS is smaller but anything over that and it keeps progressing until pitch is going up and down over 20 degrees and there is no other way than to disengage the A/P, manually stabilise and re-engage the A/P. 

 

Edited by Matt_thePilot

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