X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Mon, 30 Jun 2008 21:04:21 -0400 Message-ID: X-Original-Return-Path: Received: from smtp0.av-mx.com ([137.118.16.56] verified) by logan.com (CommuniGate Pro SMTP 5.2.4) with ESMTP id 2998448 for lml@lancaironline.net; Mon, 30 Jun 2008 18:34:41 -0400 Received-SPF: pass receiver=logan.com; client-ip=137.118.16.56; envelope-from=pinetownd@volcano.net Received: from DennisDell (63-170-119-88.dsl.volcano.net [63.170.119.88]) (Authenticated sender: pinetownd@volcano.net) by smtp0.av-mx.com (Postfix) with ESMTP id 698AD1BF643 for ; Mon, 30 Jun 2008 18:34:01 -0400 (EDT) X-Original-Message-ID: From: "Dennis Johnson" X-Original-To: "Lancair Mailing List" Subject: Runaway Trim X-Original-Date: Mon, 30 Jun 2008 15:33:54 -0700 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_00D8_01C8DAC6.B454DF60" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.5512 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5512 This is a multi-part message in MIME format. ------=_NextPart_000_00D8_01C8DAC6.B454DF60 Content-Type: text/plain; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable I have a Legacy with a CH Products stick grip with "hat switch" for trim = and RAC servos and an RAC relay between the hat switches and the trim = servos. I'm stubborn by nature and I'm still unwilling to agree that = runaway trim is a realistic concern in my Legacy. I'm not saying that's = true in other airplane types or in other Legacies with different trim = systems. Just to clarify, "runaway trim" means, to me, that the trim moves = without command from either pilot. Either the trim starts moving on its = own or it continues moving after the trim switch is released. =20 In contrast, "stuck trim" means, to me, that the trim servo fails to = move when the trim switch is activated. =20 I had stuck trim (not runaway trim), which I believe is a common failure = mode and each of us with trim systems like mine should confirm that the = airplane is flyable if the trim sticks in any reasonably likely = position. Mine stuck when adding power to execute a missed approach = after a practice ILS in clear weather. Lots of nose up and lots of = stick pressure, but the airplane was easily controllable at all times. = But it did get my attention at a busy time while cleaning up the = airplane for the missed approach. =20 By the way, maybe I'm just slow, but if my situation had been runaway = trim instead of stuck trim, I don't think I could have reacted in time = to find the right circuit breaker and pull it out to stop the trim = before it moved all the way to the stop. I don't remember, but I may = have had both hands on the stick, which would have made pulling the = breaker even tougher. Here's why I don't think runaway trim is a realistic concern for my = Legacy.=20 1. I have examined the hat switch and am confident that the switch = handle ("hat") is unlikely to become stuck in any position. =20 2. If the electrical contacts in the hat switch stick closed, I believe = that physically moving the switch in the opposite direction will unstick = the contacts. That's not a certainty, but the risk seems to be too low = for me to worry about. 3. If the trim relay sticks closed, I believe that moving the trim = switch in the opposite direction will exert enough force to break the = relay open. I haven't tested this, but RAC says they have and that it = will. =20 4. I have physically examined my wires in the trim circuit and believe = that a wiring fault that causes runaway trim is unlikely. =20 5. The typical measures taken to prevent runaway trim add complexity = and parts count, reducing overall reliability of the trim system, making = stuck trim more likely. If any of these assumptions are wrong, please tell me where my mistake = is! Runaway trim can be an attention grabbing experience and I don't = want to be ignoring that risk based on false assumptions. Steve's = experience is troubling because it could indicate that my conclusion is = wrong. Once I can talk to him and discover the root cause, I may change = my conclusion. One last thing. My conclusion is based on how I have defined the level = of risk that is acceptable to me and understand that reasonable people = can find the exact same risk to be unacceptable. I'm not saying that = runaway trim in my system is impossible, just that its probability is = low enough that I'm not going to worry about it. Just like flying over = the mountains in a single engine airplane, which I did this weekend from = Northern California to Portland, Oregon. Wow, what a smoky trip from = all the wildfires in California! On the flight to Portland Saturday = morning, I couldn't see the ground until Eugene. And when I landed back = home this afternoon, the airplane is covered with gritty material. But = other than that, it was a fun trip. Dennis Johnson Legacy, 145 hours ------=_NextPart_000_00D8_01C8DAC6.B454DF60 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable
I have a Legacy with a CH Products stick grip with "hat switch" for = trim=20 and RAC servos and an RAC relay between the hat switches and the trim=20 servos.  I'm stubborn by nature and I'm still unwilling to agree = that=20 runaway trim is a realistic concern in my Legacy.  I'm not saying = that's=20 true in other airplane types or in other Legacies with different trim=20 systems.
 
Just to clarify, "runaway trim" means, to me, that the trim moves = without=20 command from either pilot.  Either the trim starts moving on its = own or it=20 continues moving after the trim switch is released. 
 
In contrast, "stuck trim" means, to me, that the trim servo fails = to move=20 when the trim switch is activated. 
 
I had stuck trim (not runaway trim), which I believe is=20 a common failure mode and each of us with trim systems like = mine=20 should confirm that the airplane is flyable if the trim sticks in=20 any reasonably likely position.  Mine stuck when adding power = to=20 execute a missed approach after a practice ILS in clear weather.  = Lots of=20 nose up and lots of stick pressure, but the airplane was easily = controllable at=20 all times.  But it did get my attention at a busy time while = cleaning up=20 the airplane for the missed approach. 
 
By the way, maybe I'm just slow, but if my situation had=20 been runaway trim instead of stuck trim, I don't think I could = have=20 reacted in time to find the right circuit breaker and = pull it out=20 to stop the trim before it moved all the way to the = stop.   I=20 don't remember, but I may have had both hands on the stick, which would = have=20 made pulling the breaker even tougher.
 
Here's why I don't think runaway trim is a realistic concern for my = Legacy.=20
 
1.  I have examined the hat switch and am confident that the = switch=20 handle ("hat") is unlikely to become stuck in any = position. =20
 
2.  If the electrical contacts in the hat switch stick = closed, I=20 believe that physically moving the switch in the opposite direction = will=20 unstick the contacts.  That's not a certainty, but the risk seems = to=20 be too low for me to worry about.
 
3.  If the trim relay sticks closed, I believe that = moving the=20 trim switch in the opposite direction will exert enough force to break = the relay=20 open.  I haven't tested this, but RAC says they have and that it=20 will. 
 
4.  I have physically examined my wires in the trim = circuit and=20 believe that a wiring fault that causes runaway trim is unlikely.  =
 
5.  The typical measures taken to prevent runaway trim = add=20 complexity and parts count, reducing overall reliability of the trim = system,=20 making stuck trim more likely.
 
If any of these assumptions are wrong, please tell me where my = mistake=20 is!  Runaway trim can be an attention grabbing experience and I = don't want=20 to be ignoring that risk based on false assumptions.  Steve's = experience is=20 troubling because it could indicate that my conclusion is wrong.  = Once I=20 can talk to him and discover the root cause, I may change my = conclusion.
 
One last thing.  My conclusion is based on how I=20 have defined the level of risk that is acceptable to me and = understand=20 that reasonable people can find the exact same risk to be = unacceptable. =20 I'm not saying that runaway trim in my system is impossible, just = that=20 its probability is low enough that I'm not going to worry about = it. =20 Just like flying over the mountains in a single engine airplane, which I = did=20 this weekend from Northern California to Portland, Oregon.  Wow, = what a=20 smoky trip from all the wildfires in California!  On the = flight to=20 Portland Saturday morning, I couldn't see the ground until Eugene.  = And=20 when I landed back home this afternoon, the airplane is = covered=20 with gritty material.  But other than that, it was a fun = trip.
 
Dennis Johnson
Legacy, 145 hours
 
 
 
     
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