107583:

I have balanced a stabilator and it does not need to be removed from the aircraft.  The procedure is outlined in the service manual.  In short, you place the aircraft in the proper position, remove the cables from the balance arm, and adjust the weights on the end of the stabilator balance arm until it stays in a certain position by itself.  Your mechanic probably also wanted to inspect the bearings and horn attachment and that does require removal.

This type of movement could also be caused by bad bearings, the stabilator pillow block attachments, or the horn itself could be cracked. All of these should probably be inspected carefully.  

I recommend that you get that fixed by someone who knows exactly what they are looking at.  Heritage Aero or Johnston Aircraft Service, and there are others that could also help.  Failing that being possible, then a good read of the applicable AD and a disassembly of the tail and inspection are in order.

The work involved is a few days and the parts are a few thousand in the worst case.  It isn't much for peace of mind on such a critical aspect of the aircraft's control system.

The access panel at the rear side of the fuselage gives a good view of the balance arm weights, and you can also see into the pillow block attachment area from there.  Perhaps some photographs would be possible?

There is an AD warning about speed and vibration leading to catastrophic in-flight failure that is preceded by this symptom.  Flying the aircraft with vibration that you can feel in the yokes from the stabilator is unwise.  I don't really know how to phrase that more strongly and still be polite?

Regards,

William

Hi,

As William mentioned, you do not have to remove the stabilator in order to balance it.  For Comanches, it is balanced on the aircraft which is unlike other brands.  Section 4 of the Twin Comanche Service Manual goes into detail on how to check and balance the stabilator.  If your shop follows the directions exactly, they should have no problem doing it.  A "Master weight" is needed to balance the Twinco's stabilator.  And Piper gives you the specs on the weight.

I would ask one question, though.  How do you know that the stabilator is causing the vibration?  Let me assume that it is because you feel it in the control yoke.  Here's another possibility. Many years ago when I first got into working on Comanches, I had a local twinco that was shaking a bit.  The yokes shook a bit, the visors shook, etc.  We went through "everything" on that stabilator.  Checked balance, travels, tensions, and didn't find much out of spec.  But we still had the vibration in the yokes. And you could look back and see the stabilator shaking a bit.  This was back in the day when Maurice Taylor was alive.  He was the ultimate Comanche guru!  I called him and told him what was going on and what we did.  There was a pause on the line and then he asked "How old are the engine Lord mounts?" I wasn't prepared for that question and told him that I didn't know.   Long story short, he recommended changing Lord mounts every 5 years.  The mounts on our twinco were over 10 years old.  We installed new mounts, and the vibration disappeared.  We have done that on a couple of other twincos since. Same positive results. Now, I do not change the mounts every 5 years on all the Comanches we maintain, but if there is a vibration, and the mounts are old, then we change them.  

Hope that helps, Cliff

Heritage Aero, Inc.

Thank you both for the replies.  I have sent both to my mechanic, it is currently sitting in his shop.  I am having him check the mounts and will continue with balance check as well to ensure safety etc.  At the time he told me about removal, he didn’t have the manual so we should be good to go now.

I do remember looking back and seeing the stabilizer shaking during a flight.