Dual Trim Switches?

Bob Moore wrote in
46.128:

Bob Gardner wrote
If the switches are in parallel and one of them develops an internal
short circuit, you will still have trim runway; if they are in series,
both must be closed simultaneously. Sounds like a dumb idea to me, but
check with a mechanic or radio shop for a knowledgeable answer...I'm
not sure that I would expect the Chief Pilot to be the expert on
electrical questions.

Bob...if they are anything like I flew in the Boeings, one switch
powers the trim motor, and the second engages a drive clutch.

They're not the same. Actually, if you remember, if you got an out of
sequence operation of the two pickle switches (never flew the 70-, but the
others are lke this) you got the trim wheel brake to engage with a clatter.

Bertie

"Robert M. Gary" wrote

No one really knows. Some believe that there is one for every lawsuit
someone brought against Cessna because they forgot to drain the tanks.
Few actually believe that Cessna designed the tanks to collect water
at that many points. In anycase, we just drain them all.

From the pictures provided (by a link) in an earlier post, it is obvious
(with the help of some added color agent) that there are, or could be, some
very big problems if the tank is not drained at all of the provided drains.

Everyone knows that the airplane is a group of compromises flying in close
formation. It seems as though the tank design is a _big_ compromise, for
some reason.

It is hard for me to understand how Cessna could come up with such a poor
design. I do realize that a wet tank has to have internal structures in
place, with weight and strength being two of the most important design
criteria, but it does seem like it should be possible to leave some pass
through areas to let gas and water flow freely from bay to bay.

It might have cost a little more, but when you are spending, what, over 200
AMU's to buy an airplane, should it not be possible to include some design
improvements that costs a little more, than take the easy way out with
nearly a dozen added (should be) unnecessary drains?

If a tank design like that is Cessna's normal way of dong things, I would
think Columbia's future owners have some genuine concerns.
--
Jim in NC

"Judah" wrote

It's not unusual for one to push the trim switch up or down and nothing
happens, but I can't recall a situation where the button got stuck in a
movement position.

Usually when a switch fails closed, the movement is not the issue. The
contacts weld themselves together, and it is possible for the switch to
still be moveable in either direction.
--
Jim in NC

My memory isn't what it used to be, and though I flew a lot of planes with
trim switches I don't remember there being two. I must have a "two separate
actions" thing going on in my thinking machine.

Bob

"tjd" wrote in message
ups.com...
On Sep 28, 1:30 pm, "Bob Gardner" wrote:
If the switches are in parallel and one of them develops an internal
short
circuit, you will still have trim runway; if they are in series, both
must
be closed simultaneously. Sounds like a dumb idea to me, but check with a
mechanic or radio shop for a knowledgeable answer...I'm not sure that I
would expect the Chief Pilot to be the expert on electrical questions.

Bob, I'm curious why you think it's a dumb idea? As far as I know
they are in series and it seems like a fairly clever idea to me since
it greatly reduces the possibility of a malfunctioning switch causing
runaway trim. I'm not sure if it's a solution looking for a problem
or if that's a legitimate concern, but given how simple and cheap it
is to implement, why not?

Bob Moore wrote in
6.128:

Bertie the Bunyip wrote
They're not the same. Actually, if you remember, if you got an out of
sequence operation of the two pickle switches (never flew the 70-, but
the others are lke this) you got the trim wheel brake to engage with a
clatter.

Nope! To get the brake to engage, the trim had to be moving in one
direction and you had to be pulling/pushing the yoke in the other
direction. One was never aware that there were two switches, they
were mounted side by side in a space that one would normally find
one larger switch.

That's right, but you could also get the brake to engage if the pickle
switches ****ed up. After all, it was runaway protection.
If you still have access to one, even a sim, if you push one one way and
the other the other way, the brake will engage.

I've stil got my 72/73 manuals lying around somewhere, though of course
they're pilot manuals and not engineering manuals and as you know thye
wrote the flight manuals like they were trying to tell a four year old what
to do (for some obscure reason )

I'll look it up to be sure, though.

The 74, 75 and 76 had a hydraulic motor instead of an electric one so the
protection was a bit different, though it was still based upon both
switches being operated simultaneously, of course. No brake on those.

Bertie

On Sep 28, 7:52 pm, "Bob Gardner" wrote:
My memory isn't what it used to be, and though I flew a lot of planes with
trim switches I don't remember there being two. I must have a "two separate
actions" thing going on in my thinking machine.

In the G1000 172SP I flew, the natural thing is for your thumb to move
both switches simultaneously; moving them individually (during
preflight) takes conscious effort. Kind of like a split master
switch, it's much easier to turn both sides on than either one by
itself. You can see the arrangement pretty well in this pictu

http://www.aeroclub-tsl.gr/files/depanel.jpg

Like I said, I'm not sure how often a stuck switch is the cause of
runaway trim but this design all but eliminates that possibility at
very little cost. Of course it's almost twice as likely for a switch
to fail open and leave you with no electric trim, but I'd much rather
have that happen than having the trim run away and having to fight it
or pull a breaker at an inopportune moment.

todd.

On 28 Sep 2007 18:48:03 GMT, Bob Moore
wrote:

Bob Gardner wrote
If the switches are in parallel and one of them develops an internal
short circuit, you will still have trim runway; if they are in series,
both must be closed simultaneously. Sounds like a dumb idea to me, but
check with a mechanic or radio shop for a knowledgeable answer...I'm
not sure that I would expect the Chief Pilot to be the expert on
electrical questions.

Bob...if they are anything like I flew in the Boeings, one switch
powers the trim motor, and the second engages a drive clutch.
Either one by itself will not change the trim. Of course, as you
state, a series connection would also prevent a runaway trim.

Ding ding ding, we have a winner.

On Fri, 28 Sep 2007 19:28:42 GMT, Judah wrote:

"Bob Gardner" wrote in
:

If the switches are in parallel and one of them develops an internal short
circuit, you will still have trim runway; if they are in series, both must
be closed simultaneously. Sounds like a dumb idea to me, but check with a
mechanic or radio shop for a knowledgeable answer...I'm not sure that I
would expect the Chief Pilot to be the expert on electrical questions.

Bob Gardner

Although it's been a while since I flew a 172SP, IIRC, they're just series
switches. I don't even think it is the "one controls the clutch, the other
controls the motor" theory that a few people have responded with.

Look at the wiring diagram sometime. That's exactly how it's wired.

That is exactly what I was taught learning the Mooney M20C meny years ago!

Ron Gardner

"Robert M. Gary" wrote in message
ups.com...
On Sep 28, 9:26 am, Bertie the Bunyip wrote:


There's 13 places where water gets stuck in that system.

I seriously doubt that.

-Robert, CFI

It might have cost a little more, but when you are spending, what, over 200
AMU's to buy an airplane, should it not be possible to include some design
improvements that costs a little more, than take the easy way out with
nearly a dozen added (should be) unnecessary drains?

They're not just unnecessary -- they're dangerous. That is 13
separate points of failure that should not be in that wing.

I've had several quick drains leak over the years -- one quite badly
-- now imagine 13 of them! All plumbing fittings, by their very
nature, will eventually leak. These will, too.

There's really no other way to put it: Cezzzna really screwed the
pooch with their wing design.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"