Airworthiness Concern Sheet Issued For PA18 Super Cub.

[Steve Pierce]

An Airworthiness Concern Sheet was filed on June 29th by the FAA concerning cracks at the tail post. I spoke with the FAA representative Keith Noles who is the contact person on this ACS and deals with issues with Piper aircraft. The subject aircraft was a PA18-135 in the Netherlands, you can read the report below. Seems there were several cracks found during a tear down for recover. I asked about evidence of corrosion, when it was last recovered, year model and if it was stored outside. I suspect this is an L21B that was built in the 1950s. Will post when I find out ore details. Having repaired a lot of fuselages I have seen a lot of corrosion in this area and some cracks, most that corrosion were a major contributor. Will be interesting to hear other's comments on this subject.

[mike mcs repair]

cracks are very common, not due to corrosion usually... especially for off airport planes here..

another contributing thing is the bigger tail springs people run....
also the pilots spinning plane when loaded heavy... or in tundra.... some pilots will never have an issue, and others will have multiple bent and cracked tails.. just from how they treat them

[aktango58]

A reason why our northern sub-breed of birds has an exposed tail post...

[skywagon8a]

Cracks and/or corrosion are not uncommon in this area. This is definitely not a new discovery unless the mechanic has never worked on a taildragger tube and fabric airplane before. Loose tail springs at the tail and uncorrected shimmying also contribute to the cracks. Removable tail belly pans and leaving the tailpost triangle uncovered makes for easier inspection and dirt removal/accumulation prevention. Most all taildragger tube and fabric planes which I have worked on over the past 50+ years have collected dirt in this lower aft corner of the fuselage. The dirt holds moisture which promotes corrosion. The fuselage tubing can be pristine all over and this lower cluster can be rotted out with the fabric holding it all together.

This FAA notice, which seems to be research for an AD note, should be addressed to all tube and fabric tail draggers, not just PA-18s. Also the naked eye and a magnifying glass is a suitable inspection tool.

[MainlandCub]

I find these things remarkable in that they carry on like they've discovered another continent or something.

Nothing new and not restricted to PA-18's.

[Steve Pierce]

I did find out that the airplanes were L21Bs and total time is unknown as is last recover and any other history. Attached is an x ray of the cluster showing the crack. Does anyone in the US x ray tube fuselages? I have always blasted and inspected. I am told there was no corrosion at the effected area. Appreciate any comments as i am putting together a response to the FAA on this.
PA-18FuselageTailCracking.pdf

[mike mcs repair]

that crack is semi common, but not as common as the other i have seen, here is a picture of where i check and have seen many cracks.....

kinda strange after 60 or 70 years they get excited about known issues..

hmm having browser trouble trying to add pictures....

[turbofan]

Is the tubing lighter or smaller anywhere in the -135 or L21 models? I'm in the middle of a prepurchase of a L 21

[Steve Pierce]

No, just an older airframe. There are a few tubes under the battery box that may or may not be on a 135 fuselage.

[turbofan]

This L21 had the additional tubes installed at the factory.....making the future 0320 conversion easier.....owner says it has a .049 mount but I haven't verified that......it has the 0290

[turbofan]

This L21 had the additional tubes installed at the factory.....making the future 0320 conversion easier.....owner says it has a .049 mount but I haven't verified that......it has the 0290

[ndill]

I find these things remarkable in that they carry on like they've discovered another continent or something.

Nothing new and not restricted to PA-18's.

Government, first they overregulate, then they destroy the economy, now no one can afford to fly so they have to do stuff like this to justify having to many people on the payroll rather then laying people off like a private company.

[mike mcs repair]

click for bigger
cracks.jpg

[bob turner]

You know that A/D that tests for 4130 on lower longerons? There is a loophole, and some of the early Cubs that have 150 engines do not have to have the test done.

I agree with Mainline. You would think they have discovered a new continent.

Happily, the Cubs will be the last things flying in non- corporate GA. Opinion.

[MainlandCub]

I did find out that the airplanes were L21Bs and total time is unknown as is last recover and any other history. Attached is an x ray of the cluster showing the crack. Does anyone in the US x ray tube fuselages? I have always blasted and inspected. I am told there was no corrosion at the effected area. Appreciate any comments as i am putting together a response to the FAA on this.
PA-18FuselageTailCracking.pdf

Steve,

This is not a "usual" employment for x-ray. The "usual" way to check for cracks like this using other than visual methods is MPI or FPI. X-ray is usually used to inspect hidden structure.

Furthermore, x-ray images require a high degree of interpretation. My experience has been that defects tend to be over-magnified on the x-ray image and NDT engineers tend to panic a bit.

In my opinion the crack pictured would be able to be picked up visually possibly even without sandblasting and not until is was visible visually would it actually be an immediate airworthiness concern. 4130 does not have a particularly high crack propagation as what happens with the high strength steels used on airliners.

I hope that helps.

Andrew.

[articfox]

When I was more of a kid, I remember my Dads ag cubs and everyone elses cubs getting those cracks. A lot were from ski flying and landing in the fields and from corrosion/chemical that started in the cluster. When it was cheap to fix, the tails just got fixed and away they went. So, not much new has changed.

[RCharles]

Had to replace my fuselage L21B many years ago because of this problem . Crack in tail group right where Mikes diagram shows one. The airframe and struts on my plane had been passing the punch , awl test , for 6 years yet there were significant problems in several tube clusters . It is very hard to see these cracks with the fabric on . One should be very suspicious anywhere they see what looks like brown stains on any tubes . I removed the struts that had been passing the punch test for six yrs , cut them in two and poured handfuls of rust out . So, new struts ...... 1954 cub , Italian heritage , with 2700 hrs on it at the time .

[porcuypine]

Steve,
I think the key point we need to keep in front of the FAA is how successful the current inspection process has been.

What we need to avoid is having to incorporate a "level two" inspector when all that is needed is a good set of eyes at every annual and for those of you big rocking it - a thorough pre-flight.

That being said I sent two cubs to Dans this summer for a tail R&R. One I have been annualing for a few years (light use weekend warrior) and the other a pre-buy/annual (T-bird freight dog).

R

Rocket

[Longwinglover]

Not PA-18 but J3...

John Scott

[behindpropellers]

This "problem" is not really a problem if a person who is rebuilding or recovering one of these does a proper paint removal via sand or bead blast and then inspects.

I bet that most of the ones with cracks were just recovered.....not blasted and inspected.

[cubscout]

And frankly, I've found these conditions with a decent pre-flight inspection, on both Bellanca 8GCBC (two different ones), and on PA-25 Pawnee. This is just normal stuff. Although it has enhanced my reputation of being able to break an aeroplane just by looking at it...

Thanks. cubscout

[Steve Pierce]

Not PA-18 but J3...

John Scott

Early J3 probably 1938 or 39. That should have been updated a long time ago, structurally inadequate. Send those pictures to Clyde and see what he thinks.

[Longwinglover]

Actually a '41 J3L. Clyde is very familiar with the airplane. The pictures were taken in 1995.

John Scott

[Steve Pierce]

I had one here recently and discussed with him. Seems there were some issues with that tailwheel spring attachment. Did he recommend replacing it with the later style?

[Steve Pierce]

So did it have cracks like the concern sheet?

[mike mcs repair]

well used... not sure what was still holding it on....

[8GCBC]

Mike:

What was Calendar Time and Total Time?

[Longwinglover]

So did it have cracks like the concern sheet?

Steve,

Sorry, don't know how I missed your two follow-up queries. The rear was replaced with a Rocky Mountain Airframes "short" rear section.

John Scott

[mike mcs repair]

Mike:

What was Calendar Time and Total Time?

just a couple years old.... but that plane is used hard always... not surprising at all....

[8GCBC]

just a couple years old.... but that plane is used hard always... not surprising at all....

Welding issue maybe? Corrosion issue? Hard landings. 4130 alloyed wrong. All of the above? Makes me a little uneasy looking at the photographs.

(nice pictures thank you, my internet is too slow to watch the video, will try later)

[Steve Pierce]

So this was a brand new fuselage from Univair or Airframes?

[mike mcs repair]

So this was a brand new fuselage from Univair or Airframes?

well..... was a new tail section (1-2? years old?) on a new fuselage(3,4? years old), not gonna name brands, not their fault, this plane gets used very hard.... but gets the job done... tail sections are just like gas and oil.. consumables....

[8GCBC]

I am going to wheel land from now on.

[mike mcs repair]

I am going to wheel land from now on.

these guys land & taxi where its hard to even walk...

[aktango58]

The one looks like a loaded plane taxi and turn issue...

Is this bird from east and a bit north of you?

Looks like things that should be discovered on the preflight...

[8GCBC]

Just a thought ... Maybe use white paint (powdercoat) next time. Easier to see crack(s) early? Black was/is used to hide flaws easily sometimes.

[mike mcs repair]

..
Is this bird from east and a bit north of you?
....

*grin*
but then again I still don't know my left from my right....

[mike mcs repair]

..
Looks like things that should be discovered on the preflight...

oh it's been out of commission for weeks.... notice the snow machine trailer its sitting on.... the rust might be after that....

[Steve Pierce]

The Italian Cub that started all this had cracks above the diagonal tube and that had the FAA excited because they were concerned that it would crack all the way around and effect the rudder.

[Steve Pierce]

The FAA opted to issue a Special Airworthiness Information Bulletin instead of an Airworthiness Directive on this issue. I applaud that decision. I now need an ex-ray machine though.