RV-9A weird pitch up

[Adam Weiss]

I just noticed some strange behavior on a long XC.
Trimmed out at altitude, cruise speed, autopilot on.
All of a sudden the plane quickly pitched up, just for a second.
I asked my daughter if she bumped the stick. Nope.

10 min later, I’m getting really hot, so I open the air vent wide open, and it pitches up again.

Then, I played with the vent, and sure enough, every time I quickly opened it wide, it’d pitch up.
I’m trying to go though the physics of what is happening.
Any ideas?

 

[Arnold]

In both cases the pitch returned to normal on its own?

 

[Adam Weiss]

Yes, although hard to say exactly because altitude hold was on.
It really was less than a second.

 

[Adam Weiss]

Yes, although hard to say exactly because altitude hold was on.
It really was less than a second.

Thinking about it, I ought to try this again without the autopilot.

 

[flyingron]

What autopilot? Is it connected to the static system? Do your other static instruments bump?

 

[wanttaja]

What is your static source? Could it have somehow disconnected in the cockpit? You open the vent, cockpit pressure increases from the ram air, autopilot thinks you've descended.

Ron Wanttaja

 

[Bill Watson]

What is your static source? Could it have somehow disconnected in the cockpit? You open the vent, cockpit pressure increases from the ram air, autopilot thinks you've descended.

Ron Wanttaja

Along that same line, you might have a pneumatic switch in the static line (to bypass blockage of the external ports) that has been opened. You might not notice except or this effect on the AP.


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[Adam Weiss]

Along that same line, you might have a pneumatic switch in the static line (to bypass blockage of the external ports) that has been opened. You might not notice except or this effect on the AP.


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Great questions.
I don’t have an alternate static. so I suppose it could be a disconnected line in the cabin.
Next time I’m out, I’ll do a little more investigation.
Not a big deal, and doesn’t affect performance.
Just really unexpected behavior!

 

[FORANE]

As others have said, the autopilot will behave this way when it is sampling cabin air pressure. Either it isn't connected to static or static has a leak. Less likely differentials are water in your static line, corrupted software on the autopilot (reset to factory after copying your settings for re-entry after factory reset) or a kinked static line.
By the way, this isn't necessarily a bad way to connect the autopilot. If the autopilot is not connected to static, it will not be affected by a frozen static port. It may lag with altitude changes. It may fly slightly lower than indicated, but it won't freeze with a frozen static system.

 

[Let'sgoflying!]

mine is open to cabin and does something similar, mine happens when I pull cabin heat on

 

[Adam Weiss]

mine is open to cabin and does something similar, mine happens when I pull cabin heat on

Problem solved, I think.
I also noticed it when pulling cabin heat.

 

[Let'sgoflying!]

here’s another ’weird pitch-up’, another plane (Bo) I fly, if you press on the upper instrument panel a little, the plane pitches up.
The reverse, if you pull on it.
You are shifting the gyro slightly, the airplane responds to the perceived change in attitude.
Century.

 

[Dr. O]

My Apache altimeter rises 75 feet in altitude when the alternate static air is pulled on. Always has, always will. Nothing wrong. Instrument panel has a placard to that effect for correction if using alternate air in IFR/IMC flight.