Oil Canning

[Botrerta]

The fleet of aircraft I'm involved with have been experiencing oil canning on flight control surfaces (mechanically fastened - generally blind rivets, aluminium, thin skin). To date I have been unable to find much info relating to the causes of oil canning and the associated airworthiness risks. Any info that anyone could provide would be greatly appreciated.

 

[WKTaylor]

Botrerta ...

Before anyone can respond a lot more info is necessary, such as:

What aircraft [make, model] are You talking about?
Are ALL the control surfaces "canning"; or is "canning" limited to specific surfaces or aircraft???
Are they within a serial numbered sequence block? IE: built one-after-the-other... or spaced years apart in date-of-manufacture? Do all aircraft look alike?
Are Your aircraft flown in a severe or abusive manner, WX or climates? IE: pipeline service, mountain flying, severe weather [WX], student pilot training, etc?
What is the average total time for Your fleet?
Are You CERTAIN that this condition did not exist when they were NEW?
Is there any other damage associated with this "canning", such as damaged fasteners, cracking, wrinkling, etc?
Do the controls operate freely, smoothly and silently... or do they operate roughly and with "popping or cracking noises" comming from the canned surfaces?
Have You contacted the manufacturer to discuss this condition ["canning"] with a tech-rep?

NOTE: many other questions pop into my mind, also. ANY/ALL info You could provide would make OUR [participants] observations and comments more precise.

Be aware that what may seem "bad" could have been "built in at the factory and is on every acft built"... especialy light aircraft. On-the-other-hand, higher performance aircraft by definition require greater rigidity [stiffness] to prevent flutter... and are usually built very rigidly... so that any "softness" could indicate real problems.

MORE INPUT! WE NEED MORE INPUT! Regards, Wil Taylor

 

[Botrerta]

Sorry, you are quire right - much more info required.

The oil canning we are experiencing is primarily on elevators that have undergone reskinning and is being detected during installation of the items (ie: this problem is not related to the operation of the aircraft). During the reskinning process solid rivets used to attach the skin to the ribs and spar caps are replaced with cherrymax fasteners (poor access precludes using solids). For info, the affected rib and skin thickenesses are 16 thou. This problem is not experieinced in factory new items. While we are using the same jigs to ensure correct alignment, the reskinning process is different to that used during manufacture (during reskinning the skin is attached to a complete spar/rib structure - in the factory the skin is attached to the ribs and then the skin/rib assemmbly is attached to the spar).

Our current thinking is that the installation of cherrymax rivets in lieu of solid rivets may somehow be distorting the structure. We are fairly confident that the alignment in the jigs is correct (that is, we are drilling the holes in the right places on the skin and ribs). Having said that we are uncertain as to the signficance of any misalignment - ie: what amount of misalignment will result in obvious/significant distortion?

Hopefully this provides a bit more useful information to go on!

 

[Kenneth]

This is a little off subject, but not much. I don't know if you've read the thread linked below or not, but anyone replacing solid rivets with blind rivets really owes it to themselves to read the entire thread located at thread16-15349. If the info in that thread is known to you, my aplogies for telling you something you already know...

 

[WKTaylor]

Botrerta...

Rivet type, hole sizes and sequence and of riveting will make big differences in thin skinned structure repair.

a few facts...

Driven rivets "swell" into the holes generating compression force that will cause thin sheet metal to strain slightly. Roughly speaking, sheet metal thickness less than 1/3 of driven rivet Dia will cause noticible distortion around the rivet head.

Blind-rivets replacing driven rivets [assuming that BR's are structurally adequate] will require holes to be oversized and the appropriate OS BR installed. When this occurs, conventional "temp fasteners" may not expand and grip the oversized holes properly ...allowing sheet-metal layers to "slip". Even slight [un-noticed] movement of the sheet metal will cause hole centers to misalign: subsequent sheet-metal difficulties will occur when [blind] rivets force the hole-centers back into alignment. If BRs are substituted for conventional rivets, and the structure must be drilled before-hand [IE: deburring], then recommend using expanding nose "CLECOS" [spring-loaded or wing-nut driven]: they will swell into the OS holes and insure holes remain aligned through-out the drilling and riveting process. Note: thin sheet metal requires using a LOT of CLECOS to minimize distortion during drilling and riveting.

Riveting sequence is also critical. I have seen thin sheet metal structure distort due to riveting strain, when rivets are driven one-after-the-other in a convenient "line-pattern". There are two "good" riveting sequences for sheet-metal, depending on the ecomplexity of the structure, as follows.

NOTE: I say again... use LOTS of tight fitting CLECOS [MINIMUM of (1) CLECO every (5) holes] to rigidly hold the structure together during the riveting process. If any parts HAVE TO be removed for accesibilty, then do so ONLY after the entire structure has been firmly CLECOED into alignment. The more CLECOS, the better.

(A) Complex structure: install rivets using a typical crossing-pattern [criss-cross] sequence. Remove CLECOS and finish the riveting job. NOTE: after the first "round" of rivets is installed, CLECO any/all removed parts in-position, to insure alignment has not changed. IF OK, then remove these parts and continue "shooting rivets". If NOT OK, then figure-out what slipped, why, and take corrective actions. Finally, install the "removed" parts using the same techniques as the basic structure [CLECOS and cross-pattern riveting].

(B) Simple structure. Install rivets adjacent to every CLECO to "tack" the structure into alignment. Next, install all rivets in small adjacent patterns progressing from the center outwards... AND/OR fore-aft. Remove all clecos and finish riveting work. Reinstall [CLECO] the removed parts last, using these same methods.

I think You can see some potential deficiencies in Your process based on my comments [above]. Try the methods listed for improved workmanship. Regards, Wil Taylor

 

Oil canning is usually caused by compressing the metal around a periphery during fastener instalation. (Shooting rivets too hard).
It can also be caused by temperature changes. Assembly of a thinskinned flight control surface in an airconditioned 70 deg. shop, prior to installation on a Tulsa. Okla. 120 deg. ramp used to do us in at American on every occasion.
Repair in summer. and install next winter might help.
Detriments of oil canning are hard to predict. On unpressurized skins those might be minimal. Boeing has several AD notes about fatigue on Sect 43 slabsided skins on the subject.