Unreliable Altitude Preselect

I’ve been generally distrustful of my altitude preselect, as it seems to sometimes shoot on through the selected altitude. Today I had a calm flight to take a look at it, and there seems to be a problem.

It appears to be intermittently reading the wrong altitude, generally 400’ low. This causes it to climb right through selected altitudes. But then later, it will be indicating the correct altitude, and will climb/descend correctly. The Pressure Altitude indicated on the transponder is correct throughout this time, and ATC is getting the correct mode C altitude. Has anyone seen anything like this?


When I first picked up my SR22 the autopilot and preselect worked fine. After a few hours the preselect did not function and if I was climbing using the preselect it would sail right through the selected altitude. Same for descents. The autopilot itself was OK on heading and Nav modes but altitude hold did not function.
I’m not sure what the problem was but the entire unit was sent back to S-Tec and was repaired (a new circuit board). It has worked flawlessly since. My suggestion is to take the aircraft to a Cirrus or S-Tec dealer and have them look at it. ALL autopilot malfunctions should be taken very seriously.
Jerry Seckler N1970

I had the same problem in my Archer ( which I set up like my 22 which is due in feb.) I was on a nite flite from oxc to 1n4 (woodbine nj) which is very dark and very flat. I was single pilot IFR and using my garmin’s and STEC 55 to manage the work. ACY approach cleared me down to 2000 to hit the IAF and I set the pre- select on the number and double checked it. As i was running my pre-landing chklist, the controller called and told me I was cleared to 2000 ft…I thought the alt/vs had failed to engage so I told him I was working my way down…He told me to check my altimeter because he showed me at 1600 ft !..i dumped the Autopilot and flew an un-eventful approach and landing… I called the tower that nite and thanked him for his vigilance and called my shop the next day.
The owner asked me when the was the last time I WASHED THE AIRPLANE ?..It was the day before the flite ! Seems a small bubble can form in the static ports that will change the pressure differential enough to cause a 4- 600 ft excursion. I had the local A+P pull the hoses and blow toward the back and the problem was fixed…Not sure if this will fix your issue, but i was told that this is the cause most of the time…hope this helps

Curt - Did you also enter the current altimeter setting as well as the altitude? As I recall there is also a requirement for an altimeter setting. Hope that’s it.

If this is true (the water was presumably blocking the static ports), then that would mean that not only would the A/P think it was at the wrong altitude, but so would you! In other words, the altimeter would also show 400-600 ft off, if the static port was partially blocked, wouldn’t it?

Scary stuff! Especially night IMC as you were!


Yah, in my case the altimeter and the transponder have the correct altitiude. I believe the transponder reads the same analog source as the preselecter, so it seems like a preselecter problem in my case.


The Archer has 2 static ports…my altimeter appeared OK…I surmise they were not inter-connected

Oh yes. I can watch the altitude jumping back and forth from 400’ low to accurate. Of course that’s when its acting up. As quantum theory predicts yesterday when I was closely monitoring it, it was very well behaved!