Dual Trim Switches?

After flying Tomahawks and Citabrias, I took my first lesson in a
Cessna 172 last night. I asked many questions, but I had one question
which nobody in my flying club (including the chief pilot) knew the
answer to when I was the Why does the electric elevator trim have
two switches (both of which must be depressed) on the yoke instead of
one?

The best theory I could come up with was "to avoid runaway trim if one
switch stuck", but that is not very satisfying...

Anyone know why?

Chris

(Another question: 13 fuel drains? What were they thinking??? But I
am pretty sure the answer is "the lawyers designed that part, not the
engineers.")

Christopher Brian Colohan wrote in
:

After flying Tomahawks and Citabrias, I took my first lesson in a
Cessna 172 last night. I asked many questions, but I had one question
which nobody in my flying club (including the chief pilot) knew the
answer to when I was the Why does the electric elevator trim have
two switches (both of which must be depressed) on the yoke instead of
one?

The best theory I could come up with was "to avoid runaway trim if one
switch stuck", but that is not very satisfying...

That's it. It's the one and only reason.




Anyone know why?

Chris

(Another question: 13 fuel drains? What were they thinking??? But I
am pretty sure the answer is "the lawyers designed that part, not the
engineers.")


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

Bertie

"Christopher Brian Colohan" wrote in message
...

(Another question: 13 fuel drains? What were they thinking??? But I
am pretty sure the answer is "the lawyers designed that part, not the
engineers.")

Cessna was offered the Monarch fuel tanks when the went back into
production. But, according to Bill Barton, Cessna never even acknowledged
the offer.

There are 13 drains because they NEED them. The Monarchs need only one, ALL
water is drainable, and they never leak. Cessna has particularly poor fuel
tank engineering.
http://www.sumpthis.com/imagescourte...er17274599.htm

You should always test the trim switches according to the POH to prevent
runaway trim.

KG

On Sep 28, 9:15 am, Christopher Brian Colohan
wrote:
After flying Tomahawks and Citabrias, I took my first lesson in a
Cessna 172 last night. I asked many questions, but I had one question
which nobody in my flying club (including the chief pilot) knew the
answer to when I was the Why does the electric elevator trim have
two switches (both of which must be depressed) on the yoke instead of
one?

I assume this is a recent model 172. To avoid a stuck switch (or
shorted switch) from running the trim. In the preflight checklist that
Cessna provides one of the tests under the "trim test" section is to
ensure that the trim does not move with either of the switches is
moved by itself. Additionally you should ensure that the electric trim
does not work at all when the auto-pilot cut-off is held down.

The best theory I could come up with was "to avoid runaway trim if one
switch stuck", but that is not very satisfying...

Anyone know why?

Chris

(Another question: 13 fuel drains? What were they thinking??? But I
am pretty sure the answer is "the lawyers designed that part, not the
engineers.")

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.

-Robert, CFII

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

"Robert M. Gary" wrote in news:1190997921.453348.284760@
22g2000hsm.googlegroups.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.


Kay.

Bertie

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

"Christopher Brian Colohan" wrote in message
...
After flying Tomahawks and Citabrias, I took my first lesson in a
Cessna 172 last night. I asked many questions, but I had one question
which nobody in my flying club (including the chief pilot) knew the
answer to when I was the Why does the electric elevator trim have
two switches (both of which must be depressed) on the yoke instead of
one?

The best theory I could come up with was "to avoid runaway trim if one
switch stuck", but that is not very satisfying...

Anyone know why?

Chris

(Another question: 13 fuel drains? What were they thinking??? But I
am pretty sure the answer is "the lawyers designed that part, not the
engineers.")

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?

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.

Bob,

I note that you say "If the switches..." and "if they are...", so you
are not really saying that they are. I always thought one switch
engages the trim motor and the other engages a clutch to actually have
the motor move the trim cable. Either swith alone will not do
anything.

Gerd
T182T

"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.

On the Arrows and Bo's in our club, the Electric Trim is a single toggle on
the yoke, and there is a cutoff switch on the panel to deal with runaway
trim...

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. Still, I can see where there might be advantages to either
approach to this safety feature.