Ramifications of Cherokee Trim Jackcrew Failure

As I mentioned in a couple of other threads in RAP recently, the trim
jackscrew is very worn in our '74 Archer. The wear seems to be nearly half
of the thread depth. In other words, you can move the screw laterally
inside the drum about half of the height of a thread. It can be rocked
angularly inside the drum something like 5-7 degrees. Our A&P can't get to
it to replace it until mid July, so I'm trying to decide if we should ground
the plane, rush it done somewhere else, or wait until the next annual (nine
months). Our A&P tells us that he's never seen one fail, although he has
seen the cable break, which is pretty much of a non-event.

Of course, if the cable breaks, the trim is probably left stuck in some mid
position, not full up or down trim. I can see why this might not be
catastophic.

OTOH, a failure of the jackscrew itself might result in the trim slipping
all the way to full up or down, and this might be more difficult to deal
with. I've never tried to fly with trim at the extremes, so I don't really
know how much effort is needed to stay in control. I know we can just try
it, but that will put more stress on the jackscrew, so I'm not sure I want
to run the experiment on this airplane. Anyone out there with a Cherokee
want to run the experiment for me and let me know what you find?

Another possible failure mode is for the screw to strip so badly that some
or all antiservo function of the tab is lost. This could result in extreme
difficulty in controlling pitch. Anyone have thoughts on how bad this might
be? I have trouble imagining the screw stripping rapidly to the point that
the screw could slide freely up and down inside the drum, but it might be
possible. How can we find out how bad this would be? Is the antiservo
function essential for flight control, or is it just to improve the "feel"
of the controls?

An final concern is that with the extreme looseness we have now, the flutter
margins might be significantly compromised. Stabilator flutter could
certainly result in catastrophic failure modes! This is another potential
ramification of flying with it in the current state. Any thoughts on how to
research this possibility?

Finally, are there other failure modes I haven't though of? Anyone think
that this can wait until the annual? I'd love some thoughts. I have a lot
of respect for you folks, although I realize that free advice is worth just
what I paid for it. Thanks.

--
Bob (Chief Pilot, White Knuckle Airways)

I don't have to like Bush and Cheney (Or Kerry, for that matter) to love
America

Bob,

I'm going to expose my ignorance here but, hey, why not...?

Is this something you could do yourself and get signed off
later by your A&P?? I've never owned a certificated
aircraft, just my homebuilt RV-6 so I'm somewhat at a loss
regarding maintenance of Archers, Cessna's etc...

If you can't do the work yourself, then I'd certainly look
for another A&P to do the job. Waiting for a month and a
half to have this kind of job done is too much for me.
Especially, since you'll be missing some of the best flying
weather.


John



On Wed, 2 Jun 2004 23:57:10 -0400, "Bob Chilcoat"
wrote:

As I mentioned in a couple of other threads in RAP recently, the trim
jackscrew is very worn in our '74 Archer. The wear seems to be nearly half
of the thread depth. In other words, you can move the screw laterally
inside the drum about half of the height of a thread. It can be rocked
angularly inside the drum something like 5-7 degrees. Our A&P can't get to
it to replace it until mid July, so I'm trying to decide if we should ground
the plane, rush it done somewhere else, or wait until the next annual (nine
months). Our A&P tells us that he's never seen one fail, although he has
seen the cable break, which is pretty much of a non-event.

Of course, if the cable breaks, the trim is probably left stuck in some mid
position, not full up or down trim. I can see why this might not be
catastophic.

OTOH, a failure of the jackscrew itself might result in the trim slipping
all the way to full up or down, and this might be more difficult to deal
with. I've never tried to fly with trim at the extremes, so I don't really
know how much effort is needed to stay in control. I know we can just try
it, but that will put more stress on the jackscrew, so I'm not sure I want
to run the experiment on this airplane. Anyone out there with a Cherokee
want to run the experiment for me and let me know what you find?

Another possible failure mode is for the screw to strip so badly that some
or all antiservo function of the tab is lost. This could result in extreme
difficulty in controlling pitch. Anyone have thoughts on how bad this might
be? I have trouble imagining the screw stripping rapidly to the point that
the screw could slide freely up and down inside the drum, but it might be
possible. How can we find out how bad this would be? Is the antiservo
function essential for flight control, or is it just to improve the "feel"
of the controls?

An final concern is that with the extreme looseness we have now, the flutter
margins might be significantly compromised. Stabilator flutter could
certainly result in catastrophic failure modes! This is another potential
ramification of flying with it in the current state. Any thoughts on how to
research this possibility?

Finally, are there other failure modes I haven't though of? Anyone think
that this can wait until the annual? I'd love some thoughts. I have a lot
of respect for you folks, although I realize that free advice is worth just
what I paid for it. Thanks.

In rec.aviation.owning Bob Chilcoat wrote:
snip
: Another possible failure mode is for the screw to strip so badly that some
: or all antiservo function of the tab is lost. This could result in extreme
: difficulty in controlling pitch. Anyone have thoughts on how bad this might
: be? I have trouble imagining the screw stripping rapidly to the point that
: the screw could slide freely up and down inside the drum, but it might be
: possible. How can we find out how bad this would be? Is the antiservo
: function essential for flight control, or is it just to improve the "feel"
: of the controls?

I would think the worst-case scenario would be to have the anti-servo tab
floating, which would happen if the thing stripped completely. I also
think that this is the most likely scenario! When the remaining acme threads
on the jackscrew/trim drum get too thin to support the forces involved,
the whole mess will probably stip out all together. If you're lucky, it will
jam at some indeterminate point - if not, it'll float.

By the way, this is usually caused by grit or sand getting into the grease,
and wearing out the load-bearing surfaces. I'll probably take mine apart
and clean the grease out at annual.

As far as continuing to fly it, for the straight-wing version, Piper says
the maximum free play at the tab trailing edge is 0.150 inches (page 2A12
in service manual). If your free play is smaller than this, it's probably
OK to fly it someplace to get it fixed. I wouldn't wait 9 months, though,
I would fix it right away. I don't like to worry about the flight controls.

--
Aaron Coolidge (N9376J)

On Wed, 2 Jun 2004 23:57:10 -0400, "Bob Chilcoat"
wrote:

As I mentioned in a couple of other threads in RAP recently, the trim
jackscrew is very worn in our '74 Archer. The wear seems to be nearly half
of the thread depth. In other words, you can move the screw laterally
inside the drum about half of the height of a thread. ///
I'd love some thoughts ///

Phone round for an A&P that can do it immediately at a reasonable
price.
Before the delivery flight, apply the thickest grease you can find.
That will mitigate the bump load that is the way that the screw will
fail.

Brian W

I don't have specific knowledge of the failure scenarios, but I think when
it comes to control-system problems, you need to err on the side of being
fairly conservative. This sounds like waiting or getting another mechanic.

- Mark

Brian Whatcott wrote:

Phone round for an A&P that can do it immediately at a reasonable
price.

Try the new shop at Old Bridge. He's not overloaded yet. 732 792-0220.

George Patterson
None of us is as dumb as all of us.

Wasn't it a elevator ballscrew that failed on that Alaskan Airlines flight a
few years ago and killed everyone on board ? It sounds like it's time to
replace it.

You could probably get additional info from the Piper Owners chat site
if you're a subscriber to the magazine.

I think this makes you a test pilot for your aircraft. Personally, I
hate being a test pilot.

"Bob Chilcoat" wrote snip

Another possible failure mode is for the screw to strip so badly that some
or all antiservo function of the tab is lost. This could result in extreme
difficulty in controlling pitch. Anyone have thoughts on how bad this might
be? I have trouble imagining the screw stripping rapidly to the point that
the screw could slide freely up and down inside the drum, but it might be
possible. How can we find out how bad this would be? Is the antiservo
function essential for flight control, or is it just to improve the "feel"
of the controls?

An final concern is that with the extreme looseness we have now, the flutter
margins might be significantly compromised. Stabilator flutter could
certainly result in catastrophic failure modes! This is another potential
ramification of flying with it in the current state. Any thoughts on how to
research this possibility?

Bob, I'm kinda worried about this. On my snub-nose Aztec the
stabilator has a similar "antiservo" surface that "drives" the entire
stabilator. By pulling on the yoke you are first moving that tab
which in result "flys" the entire tail to a new pitch command
position. Loss of control of that tab suface (i.e. trim jackscrew
unexpectedly strips) at least on my bird, will not just loose trim,
but could, if it shears past the limit, loose all longitudinal pitch
control. I had a seperate A&P look at it and he was of the opinion
that it wouldn't shear past the limit even if all the threads stripped
off. To this day I'm not so sure. Let's face it. This is test pilot
****. My airplane has almost 5000 airframe hours on it and was used
in bush ops for a long time (gritty environment.) Seperately, I had
bad flutter in this airplane in 90 degree banks with a neighbor on
board. (This airplane had tail damage in the 60's before I bought it
and rearmost bracket had been repaired.) We landed and discovered
during the next annual that the Jesus bolt that connects the pitch
control rod to the stabilator tab bracket had worn an egg-shaped hole
in it, and that the hole's edge on the control rod mounting bracket
was paper thin. A little more flutter and we would have lost all
pitch control (i.e. kiss your ass goodbye.) The flutter occured at
about 140mph IIRC, shook the yoke and sounded like somebody shaking a
metal garage door at the bottom. IMHO, you shouldn't screw around
with this (pardon the pun.) As the poster said above, if it's close
to max play; ground the thing.

pacplyer

p.s. did I say 90 degree banks? I meant 60 degree banks. Pushed the
wrong button again. I hate it when that happens. ;-)

If the stabilator on the Aztec is the same as that on the Cherokee, the
antiservo tab isn't the sole thing driving the stabilator, although it looks
like it. If you take the top half of the tail cone off, you can see a large
tubular rod extending forward into the tail from the main pivot for the
stabilator. I'm told that this has a counterweight on the end inside the
tail, and that the main control (don't know if it's a rod or cables) from
the yokes attach to this. The only cables going to the trim jackscrew are
the ones from the trim control between the seats. If the trim jackscrew
stripped out, you would not lose the ability to move the stabilator with the
yoke, but you might lose all antiservo action of the tab, depending on the
failure mode. This may or may not be a complete disaster. The reason there
is antiservo action is so that the forces on the stabilator don't result in
positive feedback as you move the stabilator away from the neutral position.
Since the center of pressure moves with AOA, without the antiservo action
the normal aerodynamic action would be to push the stabilator farther from
neutral the farther you move it, the opposite of what you want. At least
that's the way I understand it. Can you fly with it like that? Don't know.
Don't want to find out.

We're rapidly reaching the conclusion that we should get it sooner rather
than later.

--
Bob (Chief Pilot, White Knuckle Airways)

I don't have to like Bush and Cheney (Or Kerry, for that matter) to love
America

"pacplyer" wrote in message
om...
Bob, I'm kinda worried about this. On my snub-nose Aztec the
stabilator has a similar "antiservo" surface that "drives" the entire
stabilator. By pulling on the yoke you are first moving that tab
which in result "flys" the entire tail to a new pitch command
position. Loss of control of that tab suface (i.e. trim jackscrew
unexpectedly strips) at least on my bird, will not just loose trim,
but could, if it shears past the limit, loose all longitudinal pitch
control.