Fuselage Skin Repair

[Stelthav8r]

Hello,

Found something on a previous skin repair on a 170. I’ve heard conflicting opinions. One says they believe it’s wrong because it was cut off and doesn't wrap around to the belly. The other says it’s a matter of opinion and is ok. Anyone have any solid guidance that I can reference? Picture of both sides attached.

 

[bnt83]

Somebody took the gear box out of that side?

It sure looks weird, wonder what Textron would say about it. They often have standard repair procedures that are not published due to poor access that involves cutting things up. Unless a mechanic/inspector has seen the procedure with their own eyes before determining proper conformity at a later date could be difficult unless said procedure was located in the aircraft records.

 

[Bell206]

Anyone have any solid guidance that I can reference?

Is there any reference given in the logbook?

Regardless I agree its more on the opinion side vs an issue. Don't know whats meant by "wrap to the belly" as there are two separate skins at that junction and it appears only a tab is missing from the upper skin.

As to guidance there are only general references out there in CAM-18 or AC43-13 depending when repair was complied with.

Just a guess but more than likely the skin was pulled back for access to something behind it.

 

[Magman]

It may not be the best but I don‘t think it is structurally unsound.

The opening for the strut still has to be there.

Using Cherry- Max would not be first choice though.


Cessna has used several repair methods over the years.

ie. The Spar Repair Kit for the Aft Spar which typically buckles behind the

Fuel Tank with a wing tip strike.

This procedure is documented in early 100 Series Service Manuals.

Not later though and I may have used the last kit in existence.


It seems the Standards for documenting repairs has also changed.

Old School tended to have sketches and New School is more references.

 

[TCABM]

Without the right MLG inspection cover off, I’d be more worried about what caused the crease on the left side. That’s likely what necessitated the repair.

ETA: what kind of documentation (detailed log entry? 337?) is available regarding the repair?

 

[mtnquail]

I think it could have been done better, no doubt, but I’m not sure if I would go as far as to say its unairworthy. I am in agreement with Magman, I do not like the cherrymax rivets and would much prefer to see solid rivets in this location.

 

[Stelthav8r]

Thanks. I found a 337 on the repair so it should be ok.

 

[Bell206]

FWIW: CherryMax have their place and are legal in a number of instances to replace a solid rivet . Cherry now actually offers a true structural blind rivet.

While they could have been pulled a little better my take is it was their only option to use in this instance vs a solid.

 

[Magman]

Some things I’ve found with CherryMax.

1. They cannot be cut shorter so the shop has to stock quite a few lengths
of rivets.

2. They are not tolerant of oversized holes. Again this requires having
substantial amount of stock to include the oversized rivets.

3. Other requirements such as the simple gauge to determine proper
length are not used which results in installing incorrect rivets .
Obviously this results in a poor installation.


They are great if used properly.

 

[Bell206]

Some things I’ve found with CherryMax.

To add:
4) Proper hole prep and clamp up is everything to ensure good rivet install

5) Use of proper install tools that provide the required pulling pressures to include the serviceability of that equipment.

 

[Magman]

In re-visisting the second pic I noticed the skin is not clamped up tight.

This can be a continuing concern particularly if the inspecting Tech has not observed that quirk on this aircraft before.

Another option in cases like this is to utilize the AN 525 Washer Head Screw
which will provide a clamping action.

This is authorized by some Cessna manuals as it is a Structural Fastener.

As Bell noted; hole size and prep is critical particularly since the bolt
will not expand like a driven rivet.

 

[Dan Thomas]

The engineers don't design in stuff that isn't necessary if it involves more work and material. That missing tab is structural; look at the bottom of the tab on the left side. Two rivets in it. It is carrying some load. That airplane is semi-monocoque, after all, with the skin taking the majority of the loads.

 

[Silvaire]

That airplane is semi-monocoque, after all, with the skin taking the majority of the loads.

True but that area around the main gear box section is probably the most structurally beefed up part of the whole airplane. That little bit of skin isn't going to determine whether it comes apart or not.

 

[Brad W]

I know nothing about the particular situation here....but one observation I have is that it looks like a repair that has been in place a long time, & I don't see anything obvious pointing to failure of that joint...no loose or "smoking" screws, etc... So at least for the normal loading it has seen during it's long life it hasn't failed yet....

 

[Dan Thomas]

True but that area around the main gear box section is probably the most structurally beefed up part of the whole airplane. That little bit of skin isn't going to determine whether it comes apart or not.

This is the breakdown of that area:



The gearbox bulkheads are assembled, then riveted into the airframe as it is assemble. It transmits the loads into the front doorpost, and the whole affair is riveted to the skins. All of it. That missing bit of skin on the right side weakens the area, particularly with regard to the twisting loads on the bulkhead as the brakes are applied, forcing that forward bulkhead downward.

I do know that if a Transport Canada inspector saw that, he'd ground the airplane.

 

[Bell206]

What we don't see is what is behind the skin that was repaired. Could very well have been strapped on the backside of the skin if needed. Which is perfectly legit. Given the repair has a 337 its been reviewed as needed. Just another example of the subjective vs objective side of aviation.

 

[Magman]

When was this done?

It should have been done with the tab but wasn’t.

The 337 was probably done by the person making the repair using the Cessna Manual as a reference

so it would not be a Field Approval and likely was not reviewed but only filed.


The question is where to go from here?

It’s common for a Tech to find repairs that they would not do in that fashion.



If during an Annual; it could be just writing it up on the Discrepancy List and let the Owner find

someone to sign it off. Of course, this could recur every year!


What may be needed is some type of U shaped patch that may be ugly.

Or it could be done inside per Bell.

If outside it could be something like a Structural version of the removeable fairing.

 

[Bell206]

The 337 was probably done by the person making the repair using the Cessna Manual as a reference

so it would not be a Field Approval and likely was not reviewed but only filed.

A quick reminder the Cessna manual is not approved data. Regardless looking at those outer skins the edges at the gear opening are not riveted across and the rivet spacing is wider at same area. Be interesting to see what exactly was repaired on the 337.

 

[Magman]

That’s why I asked “ when”.

Going back Approved Data was not in the vocabulary.

Many 337’s done with statements like “ I/a/w Chap xx. “.

Even with the GADO/ FSDO reviewing these it was rare to have

one returned.

Probably the best way forward is to pull the gear leg

and do the reinforcement from the inside.Should not be too difficult.

Sometimes it is best to ignore what the Records say and

just address the issue. The owner can take it elsewhere if he/she disagrees.


A failure is this area would probably result in a Prop Strike and Wing Damage.

Most Insurance Co. would consider it a total.

Many would not cover repairs to the Landing Gear area if that failed

with this condition.

 

[Bell206]

Going back Approved Data was not in the vocabulary. Many 337’s done with statements like “ I/a/w Chap xx. “. Even with the GADO/ FSDO reviewing these it was rare to have one returned.

Interesting. Definitely a different world in your part of the country. In my world, prior to the late 90s most approved data for structural repairs came from field approvals using acceptable data from AC43.13-1A and OEM structural manuals. And up until around the same time frame, the local GADO/FSDOs I dealt with kicked back a percentage of 337s for any one reason to include if the work did not meet the requirements for a major repair/alteration. Sometimes I'd hand carry the 337 to the FSDO and waited until it was signed even before starting the work.

However, once 43.13-1B was released allowing it to be used as approved repair data in certain cases and IAs became a dime a dozen with the new requirements things changed especially when the process changed from sending 337s to the FSDO to OK City around 2005. Its unbelievable what some people send in for the aircraft permanent record. No clue when this repair was done but if prior to 1995 my bet it has a field approval for the work performed.

 

[Magman]

The FAA has always been amazing for the differences in interpretation of policies.

The first 20 years as an IA there was never any response to 337‘s filed.

I‘ve often wondered if they were actually reviewed and sent to OKC.

Many items would require a FA by todays interpretation.

Reading 337’s in files can also be enlightening.

Repairs and Alterations are noted but no reference to any data of any

sort . No drawings either.

Maybe we’ll discuss repairs by a CRS and “Grand father 337’s “ ( my term) sometime .