X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Sun, 19 Oct 2008 17:04:37 -0400 Message-ID: X-Original-Return-Path: Received: from imo-m24.mx.aol.com ([64.12.137.5] verified) by logan.com (CommuniGate Pro SMTP 5.2.8) with ESMTP id 3219530 for lml@lancaironline.net; Sun, 19 Oct 2008 08:59:52 -0400 Received: from Sky2high@aol.com by imo-m24.mx.aol.com (mail_out_v39.1.) id q.c54.3921dc38 (39329) for ; Sun, 19 Oct 2008 08:59:45 -0400 (EDT) From: Sky2high@aol.com X-Original-Message-ID: X-Original-Date: Sun, 19 Oct 2008 08:59:45 EDT Subject: Re: [LML] Re: S-Tec Autopilot Problems X-Original-To: lml@lancaironline.net MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="-----------------------------1224421185" X-Mailer: Unknown sub 5000 X-Spam-Flag:NO -------------------------------1224421185 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Content-Language: en Jeff, =20 I think another way to get a "slow turn" is as a result of the loss of a th= e=20 guidance signal (DG, VOR, LOC or GPS) and the inability to precisely adjust= =20 the "roll centering". This can be observed during an in-the-hangar ground=20 check by engaging the AP and watching the stick slowly moving one way or th= e=20 other. The roll centering adjustment screw is behind one of the face plate= =20 screws. =20 Let's say that some airplane is configured like mine - vacuum AI and DG, =20 electric TC assisting the AP where such an AP is considered the sole backup= to a=20 vacuum system failure (i.e. S-Tec 50) - that is, backup for the loss of the= =20 AI whilst in the clouds. One can simulate this failure in the hangar by=20 firing up the avionics, performing an AP reset and engaging the AP in DG gu= idance=20 mode. Wait for the electronics to stabilize - note that the static pressur= e=20 is constant (good for altitude hold) and, since the engine is not running,=20 there is no vacuum for the DG or AI. Watch the stick drift left or right (= if=20 the roll centering is not centered). Gulp! Well, at least the altitude h= old=20 will work and I guess I must consider the Garmin 430W as part of the backup=20= =20 for course guidance (GPS, VOR or LOC) and wing leveling. =20 Makes one appreciate a completely independent, parameter managed, electroni= c=20 digital AP. =20 Scott =20 =20 In a message dated 10/19/2008 5:20:34 A.M. Central Daylight Time, =20 VTAILJEFF@aol.com writes: Another failure mode of the STEC system (and IMHO has resulted in a fatal =20 accident) is a loss of ground signal to the TC that results in the AP =20 continuing to "operate" without any attitude reference. This results in a sl= ow turn=20 left or right depending on if you are 28 v dc or 14 v dc system. If you onl= y=20 have a single attitude indicator and a TC in your cockpit and are IMC you c= an=20 have a heck of a time sorting our what has failed. OBTW the TC has a terrib= le=20 history of stuck flag failures. The FAA issued an SAIB on this two years ag= o=20 but so far STEC has kept this under the wraps. =20 Jeff =20 =20 In a message dated 10/17/2008 8:04:21 P.M. Central Daylight Time, =20 Sky2high@aol.com writes: Wayne and John, =20 My many years of experience with the S-Tec 50 as reflected in a file that i= s=20 over 1.5" thick, provides these observations: =20 1. The pitch board (regardless of version) is a dangerous, but interesting,= =20 device: a. One type of pitch board failure results in the TC tach test pin to=20 always "true" even if the TC has failed or is electrically off. A possible= =20 preflight Pilot checklist test should be to pull the TC circuit breaker and= =20 reset the AP. If it becomes operational (green ready light), the board is=20= bad. =20 Note, S-Tec test procedures will not detect this failure. If the AP is use= d =20 during this failed condition whilst in flight, the resulting death spiral i= s=20 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=20 avoided by leaving the sensor open to the cockpit pressure (if the cabin is= not=20 pressurized). Since altitude hold is used in stable cruise flight, it is=20 responsive to the consistent cabin pressure, merely changing no more than 5= 0 feet=20 when the cabin air vents are opened or closed (thus demonstrating lack of=20 "lag"). =20 c. In my thick file is a page describing 34 different component=20 configurations on the pitch board for modifying pitch sensitivity. That is= ,=20 simultaneous modifications to 7 or eight components, not just a single resi= stance. =20 2. The S-Tec 50 occupies 3 valuable 3" holes in the panel (TC, DG and AP=20 Brain). =20 3. The TC gyro is tilted (as they are in all TCs) and turbulence results in= =20 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=20 like a pig after a truffle. Too much work to switch to something else now. =20 =20 Scott Krueger AKA Grayhawk Lancair N92EX IO320 SB 89/96 Aurora, IL (KARR) Pilot not TSO'd, Certificated score only > 70%. =20 =20 In a message dated 10/17/2008 1:26:25 A.M. Central Daylight Time, =20 aflyer@lazy8.net writes: =20 One thing I do know about the S-Tec autopilot is that the pitch algorithms=20 are extremely sensitive to any lag or delay in the static system. On certif= ied=20 installations the =E2=80=9Cbrain box=E2=80=9D is required to be mounted ve= ry close (3=20 feet or less) to the static ports, and the autopilot must have its own stat= ic=20 ports, and not share them with any other instrument. Also not allowed are t= he=20 little bottles that are supposed to catch moisture in the tubing.=20 Pitch oscillations are the main symptoms of too much static lag.=20 John Huft=20 RV spy=20 =20 From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Wayn= e=20 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 =20 AP works OK to maintaining heading - but pitch enters oscillations=20 eventually becoming so severe requires disconnection of AP after about 30 -= 40 =20 seconds. Incredibly there is no 'gain' adjustment in the pitch axis of th= e STEC=20 50. I have read about replacing resistor on board to modify gain - but a=20 little uncomfortable with this option. No slop in the linkage. =20 Have not dug into the servos yet - but hoping there is a way to mechanicall= y=20 reduce the 'gain' by extending the servo attachment point radially outward=20 on bellcrank - or ?? =20 Alternatively thinking about installing BMA EFIS Gen 4 system with new serv= o=20 actuators. Anyone have any experience with BMA gen 4 EFIS on a close=20 coupled LNC2? I do know there is gain on both axis. =20 Thanks again for the help. =20 Wayne =20 =20 ____________________________________ New MapQuest Local shows what's happening at your destination. Dining,=20 Movies, Events, News & more. _Try it out! _=20 (http://pr.atwola.com/promoclk/100000075x1211031713x1200669822/aol?redir=3Dh= ttp://local.mapquest.com/?ncid=3Demlcntnew00 000002)=20 =20 ____________________________________ New MapQuest Local shows what's happening at your destination. Dining, =20 Movies, Events, News & more. _Try it out! _=20 (http://pr.atwola.com/promoclk/100000075x1211031713x1200669822/aol?redir=3Dh= ttp://local.mapquest.com/?ncid=3Demlcntnew00 000002)=20 **************New MapQuest Local shows what's happening at your destination.= =20 Dining, Movies, Events, News & more. Try it out=20 (http://local.mapquest.com/?ncid=3Demlcntnew00000002) -------------------------------1224421185 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Content-Language: en
Jeff,
 
I think another way to get a "slow turn" is as a result of the los= s of=20 a the guidance signal (DG, VOR, LOC or GPS) and the inability to precisely=20 adjust the "roll centering".  This can be observed during an=20 in-the-hangar ground check by engaging the AP and watching the stick=20 slowly moving one way or the other.  The roll centering adjustment= =20 screw is behind one of the face plate screws.
 
Let's say that some airplane is configured like mine - vacuum AI and DG= ,=20 electric TC assisting the AP where such an AP is considered t= he=20 sole backup to a vacuum system failure (i.e. S-Tec 50) - that=20 is, backup for the loss of the AI whilst in the clouds. = One=20 can simulate this failure in the hangar by firing up the avionics, performin= g an=20 AP reset and engaging the AP in DG guidance mode.  Wait for the=20 electronics to stabilize - note that the static pressure is constant=20 (good for altitude hold) and, since the engine is not running, there is= no=20 vacuum for the DG or AI.  Watch the stick drift left or right (if the r= oll=20 centering is not centered).  Gulp!  Well, at least the altitu= de=20 hold will work and I guess I must consider the Garmin 430W as part of the ba= ckup=20 for course guidance (GPS, VOR or LOC) and wing leveling.
 
Makes one appreciate a completely independent, parameter managed,=20 electronic digital AP.
 
Scott
 
In a message dated 10/19/2008 5:20:34 A.M. Central Daylight Time,=20 VTAILJEFF@aol.com writes:
<= FONT=20 style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size= =3D2>
Another failure mode of the STEC system (and IMHO has resulted in a f= atal=20 accident) is a loss of ground signal to the TC that results in the AP=20 continuing to "operate" without any attitude reference. This results in a=20= slow=20 turn left or right depending on if you are 28 v dc or 14 v dc system. If y= ou=20 only have a single attitude indicator and a TC in your cockpit and are IMC= you=20 can have a heck of a time sorting our what has failed. OBTW the TC has a=20 terrible history of stuck flag failures. The FAA issued an SAIB on this tw= o=20 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,=20 Sky2high@aol.com writes:
Wayne and John,
 
My many years of experience with the S-Tec 50 as reflected in a fil= e=20 that is over 1.5" thick, provides these observations:
 
1. The pitch board (regardless of version) is a dangerous, but=20 interesting, device:
    a. One type of pitch board failure results=20= in=20 the TC tach test pin to always "true" even if the TC has failed or=20= is=20 electrically off.  A possible preflight Pilot checklist test=20 should be to pull the TC circuit breaker and reset the AP.  If it=20 becomes operational (green ready light), the board is bad.  Note, S= -Tec=20 test procedures will not detect this failure.  If the AP is us= ed=20 during this failed condition whilst in flight, the resulting=20 death spiral is interesting and demonstrates the usefulness of= an=20 AP disconnect switch.
 
    b. I know nothing about static lag, but a b= ad=20 static hookup can be avoided by leaving the sensor open to the cockpit=20 pressure (if the cabin is not pressurized).  Since altitude hold is= =20 used in stable cruise flight, it is responsive to the consistent cabin=20 pressure, merely changing no more than 50 feet when the cabin air vents=20= are=20 opened or closed (thus demonstrating lack of "lag").
 
    c. In my thick file is a page describing 34= =20 different component configurations on the pitch board for modifying pitc= h=20 sensitivity.  That is, simultaneous modifications to 7 or eight=20 components, not just a single resistance.
 
2. The S-Tec 50 occupies 3 valuable 3" holes in the panel (TC, DG a= nd=20 AP Brain).
 
3. The TC gyro is tilted (as they are in all TCs) and turbulen= ce=20 results in AP turns - sometimes as much as 5 degrees off the course.
 
4. GPSS (ARINC Roll Steering) requires the addition of another=20 box.
 
Otherwise, the thing tracks the necessary 430W over-the-ground= =20 course line like a pig after a truffle.  Too much work to switch to= =20 something else now.
 
Scott=20 Krueger AKA Grayhawk
Lancair N92EX IO320 SB 89/96
Aurora, IL=20 (KARR)

Pilot not TSO'd, Certificated score only >=20 70%.
 
In a message dated 10/17/2008 1:26:25 A.M. Central Daylight Time,=20 aflyer@lazy8.net writes:

One=20 thing I do know about the S-Tec autopilot is that the pitch algorithms= are=20 extremely sensitive to any lag or delay in the static system. On certi= fied=20 installations the =E2=80=9Cbrain box=E2=80=9D  is required to be=20= mounted very close=20 (3 feet or less) to the static ports, and the autopilot must have its=20= own=20 static ports, and not share them with any other instrument. Also not=20 allowed are the little bottles that are supposed to catch moisture in=20= the=20 tubing.

 Pitch=20 oscillations are the main symptoms of too much static=20 lag.

 John=20 Huft

RV=20 spy

 

From: Lancair= =20 Mailing List [mailto:lml@lancaironline.net] On Behalf Of Wayne=20 Bailey
Sent: Sunday, October 12, 2008 6:49 PM
To:=20 lml@lancaironline.net
Subject: [LML] Blue Mountain Avionics=20 Autopilot

 

Small tail 360 with Stec50 autopilot. = =20

 AP works OK to maintaining headi= ng -=20 but pitch enters oscillations eventually becoming so severe=20 requires disconnection of AP after about 30 - 40=20 seconds.   Incredibly there is no 'gain' adjustment in= the=20 pitch axis of the STEC 50.  I have read about replacing=20 resistor on board to modify gain - but a little=20 uncomfortable with this option.  No slop in the=20 linkage.

 Have not dug into the servos yet - bu= t=20 hoping there is a way to mechanically reduce the=20 'gain' by extending the servo attachment point radially outward=20= on=20 bellcrank - or ??

 Alternatively thinking about installi= ng BMA=20 EFIS Gen 4 system with new servo actuators.  Anyone have an= y=20 experience with BMA gen 4 EFIS on a close coupled LNC2? I d= o=20 know there is gain on both axis.

 Thanks again for the=20 help.


Wayne

 

&n= bsp;

 
<= /DIV>








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