X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Tue, 21 Oct 2008 01:07:27 -0400 Message-ID: X-Original-Return-Path: Received: from mail.skywerx.com ([206.123.212.134] verified) by logan.com (CommuniGate Pro SMTP 5.2.9) with ESMTPS id 3230059 for lml@lancaironline.net; Sun, 19 Oct 2008 22:12:55 -0400 Received-SPF: none receiver=logan.com; client-ip=206.123.212.134; envelope-from=aflyer@lazy8.net Received: from blueroom (ip-206-123-213-175.static.fasttrackcomm.net [206.123.213.175]) by mail.skywerx.com (8.12.8/8.12.8) with ESMTP id m9K21qLh020455 for ; Sun, 19 Oct 2008 20:01:52 -0600 From: "John Huft" X-Original-To: "'Lancair Mailing List'" References: In-Reply-To: Subject: RE: [LML] Re: S-Tec Autopilot Problems X-Original-Date: Sun, 19 Oct 2008 20:12:09 -0600 X-Original-Message-ID: <02fe01c93259$42771b70$c7655250$@net> MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_02FF_01C93226.F7DCAB70" X-Mailer: Microsoft Office Outlook 12.0 thread-index: AckyLkx+nxQqnRueTMOsvTctpjkqJgAKfZRQ Content-Language: en-us X-Virus-Scanned: ClamAV 0.93/6963/Sun Apr 27 06:37:52 2008 on mail.skywerx.com X-Virus-Status: Clean This is a multipart message in MIME format. ------=_NextPart_000_02FF_01C93226.F7DCAB70 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Scott, this is not an accurate picture. =20 All autopilots work as closed loop control systems. In the hangar, the = loop is open. In other words, the control system is not seeing a = response to its inputs (to the servos). So, it keeps changing things, = waiting for a response (from the gyros, indicating the airplane has = moved). It is not =E2=80=9Creal-world=E2=80=9D for the autopilot. Even = the altitude hold is not happy. =20 Even in wing-leveler mode, the stec is not happy (though the wings are = presumably level). =20 John Huft RV spy =20 =20 From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of = Sky2high@aol.com Sent: Sunday, October 19, 2008 3:05 PM To: lml@lancaironline.net Subject: [LML] Re: S-Tec Autopilot Problems =20 Jeff, =20 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. =20 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. =20 Makes one appreciate a completely independent, parameter managed, = electronic digital AP. =20 Scott =20 In a message dated 10/19/2008 5:20:34 A.M. Central Daylight Time, = VTAILJEFF@aol.com writes: 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. =20 Jeff =20 In a message dated 10/17/2008 8:04:21 P.M. Central Daylight Time, = Sky2high@aol.com writes: Wayne and John, =20 My many years of experience with the S-Tec 50 as reflected in a file = that is over 1.5" thick, provides these observations: =20 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. =20 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"). =20 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. =20 2. The S-Tec 50 occupies 3 valuable 3" holes in the panel (TC, DG and AP = Brain). =20 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. =20 4. GPSS (ARINC Roll Steering) requires the addition of another box. =20 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. =20 Scott Krueger AKA Grayhawk Lancair N92EX IO320 SB 89/96 Aurora, IL (KARR) Pilot not TSO'd, Certificated score only > 70%. =20 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 =E2=80=9Cbrain box=E2=80=9D 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 =20 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 =20 Small tail 360 with Stec50 autopilot. =20 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 =20 =20 =20 _____ =20 New MapQuest Local shows what's happening at your destination. Dining, = Movies, Events, News & more. Try = it out!=20 _____ =20 New MapQuest Local shows what's happening at your destination. Dining, = Movies, Events, News & more. Try = it out!=20 _____ =20 New MapQuest Local shows what's happening at your destination. Dining, = Movies, Events, News & more. Try = it out!=20 ------=_NextPart_000_02FF_01C93226.F7DCAB70 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable

Scott, this is not an accurate = picture.

 

All autopilots work as closed loop control systems. In = the hangar, the loop is open. In other words, the control system is not = seeing a response to its inputs (to the servos). So, it keeps changing things, waiting for = a response (from the gyros, indicating the airplane has moved). It is not = =E2=80=9Creal-world=E2=80=9D for the autopilot. Even the altitude hold is not = happy.

 

Even in wing-leveler mode, the stec is not happy (though = the wings are presumably level).

 

John Huft

RV spy

 

 

From:= Lancair = Mailing List [mailto:lml@lancaironline.net] On Behalf Of = Sky2high@aol.com
Sent: Sunday, October 19, 2008 3:05 PM
To: lml@lancaironline.net
Subject: [LML] Re: S-Tec Autopilot Problems

 

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

 

In a message dated 10/19/2008 5:20:34 A.M. Central Daylight = Time, VTAILJEFF@aol.com writes:

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 =E2=80=9Cbrain box=E2=80=9D  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 ??

 Altern= atively 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

 <= /o:p>

 

 



New MapQuest Local = shows what's happening at your destination. Dining, Movies, Events, News & = more. Try it out!



New MapQuest Local = shows what's happening at your destination. Dining, Movies, Events, News & = more. Try it out!



New MapQuest Local = shows what's happening at your destination. Dining, Movies, Events, News & = more. Try it out!

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