Keith,

 

I have had an S-TEC 50 installed and "operating" since 1997 in my 320.

 

1. I also had to reverse the pitch operation because the servo was mounted on the right side of the elevator push rod tunnel and the servo/push rod connector was aft of the servo.  STEC standard is different.  Before reversing things, first test of altitude hold was in the air (ignorance is bliss) and the more altitude that was lost, the more down elevator was commanded (it thought it was commanding up). Oh well......... 

 

2. Before I relocated the static port to the Lancair recommended position, I also had problems with altitude hold.  That was primarily because of static pressure fluctuations and the solution was to vent the transducer to the cabin.  Note that there will be slight altitude changes the cabin air vents and or the cabin heat valve conditions are altered.  If you had such a problem connected to the static port, your altimeter and airspeed should have been erratic also.

 

3. RG 58 coax is well known to be an RF leaker.  If your GPS antenna connections were bad, you may want to examine the comm antenna connections also.  Another way may be to move or reroute the STEC harness or the antenna leads (although that may cause other problems such as all temperature indications going awry).  Many sensitive wires have grounded shields - remember that a shield should be grounded only at one end to avoid ground loops.  If an antenna shield ground is bad, there is no shield.  Testing wires on the ground is one thing, but remember the first rule when considering whether something will work in flight - the effect of constant vibration.

 

Scott Krueger

 

The altitude hold function on my LNC2 never worked since the day I bought the airplane (coming up on 12 months ago).  I'll leave out all of the things we tried to fix it that had no effect, and limit it to the relevant steps that actually had a positive change.  Note, the alt hold is now working very well (with one caveat, which I'll note at the end).  The purpose of posting this is to have it in the archive, in case anyone else runs into similar issues with their AP in the future.

- trim indicators (ie, add up trim, add down trim) were reversed
- pitch servo was reversed
- pressure transducer (pin 16) was apparently wired with someone counting the pins from the wrong part of the connector, so no static port data was reaching the AP

All of those things were fixed, resulting in an altitude hold that almost worked perfectly. It was a good 1/2 - 1 second 'behind' the airplane. 

The final fix was to disconnect the static line from the pressure transducer, venting it to the cabin.  For an STEC-50, which only has altitude hold (doesn't have the ability to maintain a specified altitude), this was a great solution.

Now it works like a champ!

The only hitch now is that when I transmit on radio, the alt hold (and hdg hold for that matter, but it's less noticeable) functions are significantly disrupted.  My guess is that the transmission from the com radio is hurting the signal from the pressure transducer back to the AP. I am going to research the possibility of adding some shielding to that cable.

Keith