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Date: Fri, 03 May 2013 09:30:01 -0400
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Subject: Re: [LML] Re: Subtle bug in backup gyro installation
From: Charles Brown <browncc1@verizon.net>
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Colyn, wish I knew.  The Mid-Continent guys have probably figured it out =
by now -- but even they were taken a bit by surprise.  I've copied Doug =
Edwards in case he has an explanation. =20

The clamping action is progressive as you pull the knob -- it's not =
fully caged until the knob's fully out -- so a partially pulled knob is =
partially limiting travel, that's clear.  It's equally clear that the =
partially pulled knob either affects an erecting vane or couples rotor =
torque into pitch somehow.  There was no damage to gears that I'm aware =
of, just stiff bearings.


On May 2, 2013, at 2:27 PM, Colyn Case wrote:

Charley,

thanks for this info.

Can you please say a little more about what exactly happens inside the =
gyro if the knob sticks?   Sounds like you are saying certain gears get =
worn out prematurely.

Colyn

On May 2, 2013, at 8:02 AM, Charles Brown wrote:

A professionally designed and built panel and gyro installation had a =
subtle bug:  the gyro cage knob released *most* of the way to its stop, =
coming to rest on the panel...  which caused latent problems with gyro =
bearings.  A glance at your panel will tell you if you do/not have a =
potential issue.

Pic 1:  Gyro with cage knob.  You can see that the knob rests against =
the gray instrument panel.  Note that the altimeter, to the right, does =
not have this problem because the panel has been cut out to accommodate =
the altimeter knob.

Pic 2:  Gyro without the cage knob.  The notch cutout for the cage knob =
is just big enough to accommodate the post, but not the knob.

This bug was subtle enough to pass functional check at Lancair Avionics =
and to allow the gyro to function fine for over 200 hours.  During this =
time the cage function worked fine, and when the knob was released the =
gyro tracked attitude perfectly.  Around 215 hours the gyro started =
drifting.  Mid-Continent Instruments found some bearings binding and =
overhauled the unit for free.  Both the loaner that they provided in the =
interim, and the overhauled original gyro, when installed, would cage =
normally but when released, would gradually go to a max pitch up =
attitude.  Cage/uncage looked normal and only after every other =
possibility was exhausted did we think to install without the cage knob, =
allowing the cage post to release fully, and the problem went away.  An =
easy hog-out with a dremel solved the problem permanently.

I suspect this could happen with any gyro that has a cage knob; this =
particular unit is the 2-inch electric Mid-Continent Instruments =
4200-10.  It has an advertised MTBF of 2500 hours, a 1-year warranty, =
Lancair recommended it, and all the buzz on it is positive, although at =
$4K new / $2K overhauled it ain't cheap.  I toured the fabrication and =
repair shops and found high quality standards, thorough testing, and =
experienced people who were forthright in their answers to just about =
any questions.  They were mystified by this problem until we finally =
figured it out.  Their response to my initial email was a phone call =
back within 2 hours and they fixed it for free because they thought it =
should have lasted longer.  And when it was revealed to be an =
installation issue, they still didn't charge me anything.

Final side note:  Due to the recent dialog on LML about lightning and =
electrical systems, I asked about susceptibiity to lightning.  The =
4200-10 has discrete components (resistors and caps) to power LEDs for =
lighting, and the rotor motor.  Turns out they have installed features =
on the 4200s (and, I assume, their other electric instruments) to block =
high current pulses, and they pointed me to design and test specs, which =
I will go track down.  So hopefully it is not as susceptible as =
integrated circuits or even unprotected discrete circuit boards. =20

Charley Brown
Legacy #299  230 hours

<With cage knob.jpeg><without knob.jpeg>


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