Oil canning forming in the wing from the sun

[Dferr34]

These dimples, or oil canning were all along the top of the leading edge, pretty much between each wing rib on both wings, the ones seen in the picture were the the biggest ones. last night at the hangar while looking things over it all feels tight and looks uniform. even the one pictured is gone and I can't tell where it was.

[GAHorn]

Do BOTH wings do this…? or only the one?

On related matters: Many times owners don’t realize the importance of the Rear Spar in the wing….and how the wing is stressed in=flight.

For instance, in a wingtip strike (hangar rash, ground loop, etc) the distance of the wingtip from the fuselage attach points can multiply the forces experienced at the wingtip…. to the attach-points at the Root… Particularly to the REAR spar attach….which is smaller and less obvious to casual observation.

But the forces imposed on the Rear Spar, especially at the attach point can be tremendous, even when insignificant damage occurs at that wingtip.


These dents are remindful that the wings of A and B models undergo stresses in a different way than does the “straight/ragwing” 170 does.

The A and B models do not have a rear-spar wing-strut…. A/B models rely upon the “box construction” to defy torsional / twisting of the wing, for example in a hard Pull-Up…or sudden Gust… one might ordinarily think the wing would suffer damage in the UP - direction.

But an A or B model wing Leading-Edge will respond in a DOWN-ward twisting direction…. as the wing is forced to twist (in a positive-G gust) with the trailing/rear-spar area UpWard… rotating about the single spar attach-point… to position the leading edge Down.
(One of the clues observed by accident-investigators in examining wreckage involving flight-into-storms and structural failure is the leading edge damaged in a DownWard manner… indicating extreme wind-gusts and wing overloading.)

Consider what that also does to the relatively unsupported Rear Spar Attach Point …which has no strut to assist the rear.
In an extreme case, the twisting might also twist the smaller, individual “boxes” which make up the leading edge skins between the D-ribs forward of the fwd spar.

This thread brings these forces to mind.

[Dferr34]

Both wings.

[n2582d]

Not withstanding what George says about torsional loads on the main spars, I wonder if the spars have been overstressed. Or is it possible the wings have been rebuilt/repaired without being jigged properly? On the C-170B the top of the main spar is flat from the root to the tip. Anybody know if the 170A is the same? I assume it is. If so, stretch a fishing line along the top spar rivet line. Use 1/4" drill bits or some other matching shims at the root end and the tip of the spar and stretch the line above these two points. A laser level would also work. Then check that the spar is parallel with the line at various points midspan.

Another “free” check might be to loosen all the fuel tank cover screws on the cover’s front edge where they screw into the spar. Do this in the sun with the leading edge in the wrinkled state and see if that makes any difference in the canning.

Cessna's Structural Repair Manual refers to wrinkles and canning in the chapter about the fuselage. But this isn't covered when they write about wing repairs. Here's the applicable section about the leading edge:

8. Wing Leading Edge Damage Criteria
A. Negligible damage: Any smooth dents in the wing leading edge skin that are not more than 0.030 inch (0.76 mm) below contour and circumscribable with not more than a 1.5 inch (38 mm) diameter circle that has no evidence of skin tears, cracks, or skin penetrations - which are not stress wrinkles and do not interfere with internal structure - constitute negligible damage. However, because of the critical nature of the wing leading edge, this cosmetic repair should be completed.
B. Repairable damage: Dents or dings deeper and/or larger than specified above must be repaired. Skin tears, cracks or penetrations must be repaired. Dings that include ribs must be repaired by reforming or removal and replacement of the rib. Reevaluation of the skin after the repair of the understructure will determine if the skin damage is negligible, repairable or requires replacement.
C. Damage Necessitating Replacement Of Parts: Where extreme damage has occurred, complete leading edge skin panels should be replaced.

If you were to replace the leading edge skin the Structural Repair Manual allows for the installation of up to five access holes per wing before you need to call Cessna Support to add more.

If there is nothing structurally wrong with the wings I wonder if one could add a STOL kit to cover this. I would think it would be cheaper than replacing the leading edges. The Sportsman kit is not STCed for the 170A but the Horton kit is under STC SA989CE.

If it were me, I'd call or write Cessna for advice. Cessna Structures: 316-517-6061, structures@txtav.com Cessna Piston Hotline: 316-517-1700, piston_support@txtav.com.

[TCU76109]

If you do go ahead with a STOL kit, I just received a field approval for a Sportsman STOL on a 170A N5429C. I’ll be providing the paperwork to the association for their records.

There’s a bit to investigate before determining to go that route, but I wanted to put that info out there in the thread for reference.