Inspection within wing of 1948 Cessna 170

[CBogle]

Dear fellow member(s):

I have a 1948 Cessna 170 and the wings were re-covered with ceconite a couple of years ago. All of the places where there is supposed to be an inspection hole on the bottom of the wings have re-inforcement rings in-place, but, not all of the holes have been cut. At the end of each wing there is a retangular inspection plate that allows access to the strobes, but, the next hole going inward towards the fuselodge has not been cut. At Sun-n-Fun a fellow member said that we needed to cut into that inspection hole to do a proper inspection of the bracing wires near the outboard portion of the wing. The plane is so nicely done that I don't want to open up any more of the inspection holes than is necessary at this next annual. The last two annuals were very extensive and done by very reputable shops and neither one of them felt it necessary to cut any more inspection holes in the wings.

My question is, does anyone know which holes must be open to perform an adequate inspection and specifically, do the outermost holes on the underside of each wing really need to be opened to inspect the outbouard bracing wires?

Thanks,

Curt

[auxtank]

Curt,

IMHO, only your IA can tell you whether he or she believes that the outermost pair of bracing wires needs to be examined during any given annual inspection. Of course, one of the things that makes airplane ownership interesting is the wide variety of knowledgeable opinions: One year your IA might tell you to cut out the fabric from the middle of the unopened inspection rings––so those outboard bracing wires can be checked––and the next year a different inspector might tell you he or she doesn’t need to look in there to satisfy himself or herself that the aircraft is airworthy. That said, if YOU are uncomfortable not knowing the present condition of that area, you should paint some inspection-hole plates in a matching color, open up the fabric, and have a look.

Some people believe that you can tell if there is a broken bracing wire by standing under the end of the wing, holding your hands just below the wing tip with your palms up, and striking the underside of the wing sharply with your palms: A broken flying wire will announce itself by rattling against the fabric on the bottom of the wing or the ribs it passes through. Because I was taught this technique as part of a pre-flight inspection when I purchased my 1948 C170, I have used it periodically. No, I have never found a broken wire.

Given that your wings were recovered a couple of years ago, I would think that the bracing wires and their attachments were given a good inspection and received any required maintenance at that time. Seven years ago, when I prepared my wings for recovering, I found a bit of rust on the steel fittings that attach the bracing wires to the wing spars. Rather than take everything apart, I masked off the spars (to protect them from iron in the sand) and then sandblasted the fittings and wire ends. I also used a scotch-brite pad to clean up corrosion and some kind of chemical treatment that was recommended. The fittings really didn’t look too bad to begin with. But I am expecting the fabric job to last at least 20 years, and I don’t want the wings to need any internal maintenance between covering jobs.

Similarly to what was done on your plane, I installed inspection rings that have not yet been cut out. In fact, I installed more inspection rings than were in the old fabric job; after consulting with a couple of mechanics I agreed with them that there were areas that might need access sometime in the future. It is a lot easier to cut out the center of a ring and install a cover plate than to patch a hole––and gluing on a ring and covering it with a layer of fabric while you are in the process of covering is certainly cheap and easy. My IA has not yet asked me to open up any of these extra inspection holes, including the ones that are in the same area that you are asking about. However, when he, I, or some other IA gets curious about what is going on in there, it will be a breeze to take an exacto knife to the wing and make a new hole.

I'm not convinced that Cessna had an inspection hole in the outboard section of their factory covering jobs. Of course, in those days they were using cotton rather than the more durable polyester in use today. My understanding is that the wing was opened up for recovering every seven to twelve years.

Bottom line, if you are not worried and the reputable shops that are doing your annuals are not worried, why open it up. But, if you or someone you trust feels that a proper inspection cannot be done without looking in there then, by all means, open up those holes.

Gordon Sandy
N4002V

[zero.one.victor]

If I recall correctly, the 1948 IPC shows the (approximate) location of the factory inspection holes/covers.

Eric

[Dave Clark]

And for the metal wing guys a gentle bang on the bottom of the wing will sometimes detect a lost tool.....
Not that I've ever lost one.

[mrpibb]

For what it's worth here is a pic of my wing.

http://sandhillaviation.com/IMG_0039.JPG

[CBogle]

Dear fellow members:

Thanks for the very helpful responses on the question I had regarding the necessity of opening up the inspection holes. An A&I I know also said that he used the "palm up banging method" and did not see any need to open the inspection hole. When the wings were re-covered, the wing metal structure was completely re-finished and all of the bracing wires were replaced with new wires. Given that the aircraft is hangared and it has only been a few years since new wires and recovering, I'm leaning towards not opening the inspection hole.

Thanks again, everyone.

Best regards,

Curt

[GAHorn]

Question: What's the difference between a wire that's broken and laying on the bottom fabric and one that isn't (but is just about to break or pull loose?) Answer: The second one will do it in flight, perhaps the next one.

Question: When will the inspection hole previously thought might be "convenient" some day....be necessary?
Answer: During the next airworthiness (annual) inspection in order to properly inspect the airplane structure.

In my opinion, if the airplane structure hasn't been looked at .....well....then it hasn't been inspected. There's a reason for inspection holes. It's so they can be used.

If a wing is newly recovered,...certainly it doesn't need new holes. ....for about a year. The reason they weren't already cut is because the re-cover-er didn't want to spend the time. The holes don't harm the airplane's performance so go ahead and use them. Otherwise .... you're only guessing as to what's going on in there. The "thump the bottom of the wing" technique will catch a failure that's already occured.
I'd prefer to catch it before that occurance.