Wayne and John,

 

My many years of experience with the S-Tec 50 as reflected in a file that is over 1.5" thick, provides these observations:

 

1. The pitch board (regardless of version) is a dangerous, but interesting, device:

    a. One type of pitch board failure results in the TC tach test pin to always "true" even if the TC has failed or is electrically off.  A possible preflight Pilot checklist test should be to pull the TC circuit breaker and reset the AP.  If it becomes operational (green ready light), the board is bad.  Note, S-Tec test procedures will not detect this failure.  If the AP is used during this failed condition whilst in flight, the resulting death spiral is interesting and demonstrates the usefulness of an AP disconnect switch.

 

    b. I know nothing about static lag, but a bad static hookup can be avoided by leaving the sensor open to the cockpit pressure (if the cabin is not pressurized).  Since altitude hold is used in stable cruise flight, it is responsive to the consistent cabin pressure, merely changing no more than 50 feet when the cabin air vents are opened or closed (thus demonstrating lack of "lag").

 

    c. In my thick file is a page describing 34 different component configurations on the pitch board for modifying pitch sensitivity.  That is, simultaneous modifications to 7 or eight components, not just a single resistance.

 

2. The S-Tec 50 occupies 3 valuable 3" holes in the panel (TC, DG and AP Brain).

 

3. The TC gyro is tilted (as they are in all TCs) and turbulence results in AP turns - sometimes as much as 5 degrees off the course.

 

4. GPSS (ARINC Roll Steering) requires the addition of another box.

 

Otherwise, the thing tracks the necessary 430W over-the-ground course line like a pig after a truffle.  Too much work to switch to something else now.

 

 

One thing I do know about the S-Tec autopilot is that the pitch algorithms are extremely sensitive to any lag or delay in the static system. On certified installations the “brain box”  is required to be mounted very close (3 feet or less) to the static ports, and the autopilot must have its own static ports, and not share them with any other instrument. Also not allowed are the little bottles that are supposed to catch moisture in the tubing.

 Pitch oscillations are the main symptoms of too much static lag.

 John Huft

RV spy

 

From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Wayne Bailey
Sent: Sunday, October 12, 2008 6:49 PM
To: lml@lancaironline.net
Subject: [LML] Blue Mountain Avionics Autopilot

 

Small tail 360 with Stec50 autopilot.   AP works OK to maintaining heading - but pitch enters oscillations eventually becoming so severe requires disconnection of AP after about 30 - 40 seconds.   Incredibly there is no 'gain' adjustment in the pitch axis of the STEC 50.  I have read about replacing resistor on board to modify gain - but a little uncomfortable with this option.  No slop in the linkage.  Have not dug into the servos yet - but hoping there is a way to mechanically reduce the 'gain' by extending the servo attachment point radially outward on bellcrank - or ??  Alternatively thinking about installing BMA EFIS Gen 4 system with new servo actuators.  Anyone have any experience with BMA gen 4 EFIS on a close coupled LNC2? I do know there is gain on both axis.  Thanks again for the help. Wayne