Horizontal stab oil canning

[redacted]

Doing the annual. Mechanic checking the tail and when you grab the RH horizontal and try and move it the top skin under the vertical oil cans. Borescoped the whole tail, nothing loose, no cracks, no smoking rivets. I wonder if it’s an artifact of how that skin was riveted on slightly loose that you can move it. But we went deep and found no problems with the stab.

Thoughts? Thanks everyone. No surprised on the annual this year. I installed new window hinges to pax window and new mags are being installed. Good compressions for a 1400 hour O-360 and new oil sample sent for analysis.

[Joe Moilanen]

Hi Nathaniel,
I noticed the RH horizontal doing the exact same thing on my '53 when we did the first annual on it. Scrutinized it about the same as you have yours. This May I'll be doing my 35th annual on the plane. After 3,000 hrs and 35 years I've decided not to loose sleep over it!

Joe
4518C

[redacted]

Good stuff, thanks!

[GAHorn]

It’s a “monocoque” construction…and the thin skins may “oil can”…. if stressed sufficiently to flex that skin…. The problem being “subjective”.

What do you mean by “grab…and move it”…??? How much “force” is being applied..? (See what I mean… even a healthy airplane can be forced to “oil can” the skins. )

BUT….THAT DOES NOT GUARANTEE your airplane is without damage unless you carefully and fully inspect it according to Acceptable Methods.

“Boroscoped” does not adequately describe any process fully. You should open all available access panels/inspection plates and using a bright light look for internal damage to all supporting structure…. before anyone can say that YOUR airplane is airworthy. Just sayin’….

An example: A few years ago I bought a 172C to train a family member. The Annual Inspection I used for purposes of “prebuy”…. found an AD note applicable to early 172s involving a “beef up” of the main horiz-stab main spar at the attach point to the aft fuselage bulkhead.
There was no reference specifically to that AD note…. but the records did state “All ADs Complied”. (Horrible phrase)
<edit> This comment was made in error…it has been pointed out that this was a Mandatory Service Bulletin not an A.D. Thx, Gary.>

SO… We gained access to that main spar and confirmed that a re-inforcing doubler HAD been added to the spar exactly as the repair scheme required (to avoid potential cracking/replacement).

Yes….the skin can be made to slightly “oil can”…just like many Cessnas and other airplanes exhibit when vigorously given a “shake’..(like we were all taught to do as student pilots during pre-flight inspections.) Is “giving it a shake” and observing “oil-canning” and being told “on line” that that is “normal” and “OK”….. a valid inspection of suspected irregularity?

[voorheesh]

I think the fuselage of a Cessna 170 is actually semi monocoque with substructure providing reinforcement. The skin provides strength as a stressed shell so its integrity and condition are important. Is this observation of oil canning new? Not previously detected?

As George advises, careful inspection of anything associated with attachment and support of the horizontal, is needed to ensure that the symptoms on the skin are not resulting from a more serious problem. I would hope a competent IA could accomplish that and determine its airworthiness or any corrective action.

Oil canning of aluminum skin is more serious if there is evidence that it does not return to its original shape. As opposed to minor distortion. So If the skin exhibits deformation after displacement, I would be more concerned. The area where the horizontal and vertical join is probably subject to repeated stress and effects of aging. Its also difficult to access. So my thought is to proceed carefully. Best luck and hope this is not a serious problem.

[n2582d]

… An example: A few years ago I bought a 172C to train a family member. The Annual Inspection I used for purposes of “prebuy”…. found an AD note applicable to early 172s involving a “beef up” of the main horiz-stab main spar at the attach point to the aft fuselage bulkhead.
There was no reference specifically to that AD note…. but the records did state “All ADs Complied”. (Horrible phrase)

SO… We gained access to that main spar and confirmed that a re-inforcing doubler HAD been added to the spar exactly as the repair scheme in the AD note required (to avoid potential cracking/replacement).

George,
You’re scaring me! I had begun to think I missed this AD when signing off an annual on a friend’s C-172. It’s a Mandatory Service Bulletin — SEB94-8, not an AD.

SEB94-8.pdf

(375.12 KiB) Downloaded 39 times

. The IPC’S for the C-170 and the C-172 don’t illustrate a lightening hole at the centerline of the horizontal stabilizer.

Click to Enlarge

The difference is that the C-170 doesn’t have this lightening hole while the C-172 does. It’s interesting too that the C-172 uses two less ribs than the C-170 in the stabilizer. I think the main take away from this Service Bulletin is this paragraph:

NOTE: Improper ground handling can cause cracking and deformation of the horizontal stabilizer structure. It is recommended that a tow/steering bar be used whenever the airplane is to be manually, positioned on the ground. Do not steer the airplane by pushing down on the horizontal stabilizer.

A good reason to add BAS pull handles or use a tailwheel tow bar.

[GAHorn]

My posts are intended to be interesting…not factual!,…Haven’t you read the disclaimers on this website?)

(bran-pharrt)

Seriously, the point being that a complete and thorough inspection of the area in accordance with Accepted Practices/Procedures may be called for….. oil canning on one airplane doesn’t mean it can be ignored on others….and I’m certain that’s not what was intended/implied.