Jeff,

 

I think another way to get a "slow turn" is as a result of the loss of a the guidance signal (DG, VOR, LOC or GPS) and the inability to precisely adjust the "roll centering".  This can be observed during an in-the-hangar ground check by engaging the AP and watching the stick slowly moving one way or the other.  The roll centering adjustment screw is behind one of the face plate screws.

 

Let's say that some airplane is configured like mine - vacuum AI and DG, electric TC assisting the AP where such an AP is considered the sole backup to a vacuum system failure (i.e. S-Tec 50) - that is, backup for the loss of the AI whilst in the clouds. One can simulate this failure in the hangar by firing up the avionics, performing an AP reset and engaging the AP in DG guidance mode.  Wait for the electronics to stabilize - note that the static pressure is constant (good for altitude hold) and, since the engine is not running, there is no vacuum for the DG or AI.  Watch the stick drift left or right (if the roll centering is not centered).  Gulp!  Well, at least the altitude hold will work and I guess I must consider the Garmin 430W as part of the backup for course guidance (GPS, VOR or LOC) and wing leveling.

 

Makes one appreciate a completely independent, parameter managed, electronic digital AP.

 

Scott

 

Another failure mode of the STEC system (and IMHO has resulted in a fatal accident) is a loss of ground signal to the TC that results in the AP continuing to "operate" without any attitude reference. This results in a slow turn left or right depending on if you are 28 v dc or 14 v dc system. If you only have a single attitude indicator and a TC in your cockpit and are IMC you can have a heck of a time sorting our what has failed. OBTW the TC has a terrible history of stuck flag failures. The FAA issued an SAIB on this two years ago but so far STEC has kept this under the wraps.

 

Jeff

 

In a message dated 10/17/2008 8:04:21 P.M. Central Daylight Time, Sky2high@aol.com writes:

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.

 

Scott Krueger AKA Grayhawk
Lancair N92EX IO320 SB 89/96
Aurora, IL (KARR)

Pilot not TSO'd, Certificated score only > 70%.

 

In a message dated 10/17/2008 1:26:25 A.M. Central Daylight Time, aflyer@lazy8.net writes:

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

 

 

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