Yaw oscillation in turbulence

genna

genna

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ТУ-104
Flying DA40 yesterday I was stuck in a some turbulent air for a while(VFR). It wasn't anything dangerous - i would categorize it as light - but pretty uncomfortable. The most annoying part of the experience was the side to side(yaw) swinging on the autopilot. I ended up hand-flying it which was better. This has happened before, but usually not often and momentary. This time it was almost constant.

My question is what's the cause and is there a way to deal with it other than hand-flying?
 
Was it a KAP140 autopilot? Those tend to be 2 axis. Gotta stay on the rudder pedals.
 
What autopilot are you using? I can't think of any autopilots that can induce yaw because most don't have any rudder control. In my G1K plane with the GFC700, I will put my feet firmly on the rudder pedals to reduce yaw. If you leave your feet off of the rudder pedals, this allows the rudder to swing around in the wind, effectively shortening the vertical tail surface (i.e. the rudder is doing nothing to help control yaw). Putting your feet on the pedals keeps the rudder from moving around and gives a bit more yaw control. If you try to counteract the yaw, you may induce an oscillation so steady pressure on both sides seems to work best.
 
My understanding this is a common issue for DA-40’s. I’ve only flown 5 hours in them, and occasionally peddling the rudders was necessary. Other DA-40 owners I’ve spoken with have said the same thing. It’s not chronic from my experience but occasionally does happen.

It’s a tremendous aircraft and fun to fly, but has some downsides.
 
It's an early G1000 with integrated AP. I used to think it was KAP140, but now not so sure. Probably something I should know :). I tried dampening it with pedals, but really couldn't quite time it correctly. Seems to me that AP was basically overreacting both in roll and pitch. The roll part was putting it in adverse yaw and then back and forth we went. The pitch part kept trying to be precise with the altitude, but kept swinging my IAS rather wildly(getting into yellow arch at times). After a few minutes of this, I took the control and flew it based more on attitude until conditions improved somewhat. Later I discovered the CWS button :). That helped.
 
Flying_Nun said:
If the panel looks like this then it's the KAP140. They are known to have issues in the DA-40.
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Wow... so funny to see such an ancient piece of machinery next to a G1000 PFD lol

genna said:
GFC 700
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I am surprised it would struggle that much, the GFC700 is digital and should be pretty good about mitigating that. I wonder if some feedback rheostat somewhere (or something) was sending back bad data

Having said that, on a scale of 1-10 once turbulence gets passed a 3-4 I like to handfly it anyway. Used to do the same sailing, big seas and high wind I'd rather feel the craft personally
 
Tantalum said:
Wow... so funny to see such an ancient piece of machinery next to a G1000 PFD lol
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One advantage is that the KAP140 has a separate gryo and turn/bank instrument which means you can lose the G1000 AHRS but still have your autopilot. It's still clunky compared to the GFC700.
 
Flying_Nun said:
One advantage is that the KAP140 has a separate gryo and turn/bank instrument which means you can lose the G1000 AHRS but still have your autopilot. It's still clunky compared to the GFC700.
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Cool! That's actually a great bit of "accidental" redundancy
 
Lindberg said:
I've noticed this a lot in Airbuses.
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I have always felt they ride turbulence more poorly than Boeing.. they seem to wobble and wallow around a bit before finding their footing
 
genna said:
It's an early G1000 with integrated AP. I used to think it was KAP140, but now not so sure. Probably something I should know :). I tried dampening it with pedals, but really couldn't quite time it correctly. Seems to me that AP was basically overreacting both in roll and pitch. The roll part was putting it in adverse yaw and then back and forth we went. The pitch part kept trying to be precise with the altitude, but kept swinging my IAS rather wildly(getting into yellow arch at times). After a few minutes of this, I took the control and flew it based more on attitude until conditions improved somewhat. Later I discovered the CWS button :). That helped.
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If you’re not getting the timing, try just holding the rudder centered...don’t let it move. If that doesn’t work, just a little cross-control should stop it.

Remember, too, that the inclinometer ball isn’t a yaw instrument...don’t chase that with the rudder pedals.
 
A tailwheel endorsement will help every airplane that yaws in my experience.
 
MauleSkinner said:
If you’re not getting the timing, try just holding the rudder centered...don’t let it move. If that doesn’t work, just a little cross-control should stop it.

Remember, too, that the inclinometer ball isn’t a yaw instrument...don’t chase that with the rudder pedals.
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I tried to just hold it as well. To be honest, DA40's rudder pedals are pretty tight. There is not a lot of movement of the pedals to get a rudder deflection. I did not feel any movement while it was swinging around. Not even really any pressure on my feet.
 
genna said:
The most annoying part of the experience was the side to side(yaw) swinging on the autopilot. I ended up hand-flying it which was better. This has happened before, but usually not often and momentary. This time it was almost constant.

My question is what's the cause and is there a way to deal with it other than hand-flying?
Click to expand...

I've always applied a little pressure on the right rudder while resting left foot on the opposite rudder (poor man's yaw dampener) ... pretty pronounced here when west bound between the Van Horn and El Paso areas ... not as bad east bound
 
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