Watch those gearboxes, Folks

[mvivion]

A sharp eyed mechanic today, while doing an annual inspection on my airplane noted that the rivets on the casting that clamps the gear legs in place have no heads on them.

So, unless some mechanic managed to find the worlds only supply of headless rivets, this area of my airplane has undergone some pretty significant stresses.

The back side (the heads) of these rivets are really, really hard to see. If you watched the gear box while someone really reefed up and down on the wing, you could see the rivets working, but it was still really really hard to see.

So, I now get a very nice lower gearbox inspection panel, so that these rivits can be replaced, and so that in future, it will be a lot easier to examine this portion of the airframe more fully.

This will cost more than I'd hoped the annual would this year, but thanks to a very sharp eyed mechanic, I didn't find out about it the hard (and VERY expensive) way, out on a gravel bar somewhere.

The moral: Make certain your mechanic REALLY, REALLY looks carefully at the gearbox of your airplane at every opportunity. As in REALLY carefully. This damage was extremely hard to see.

Now, since all of my landings are nearly perfect, and almost impossible to improve upon, I'm wondering who borrowed my airplane in the middle of the night and did this dastardly damage.

Happy to have found it here, as opposed to there, and thank heavens for great mechanics,

Mike Vivion

[Metal Master]

Pat the guy on the back once for me to! Good job

[R COLLINS]

Mike,
Are you talking about the outboard gear box brackets with the slot that the gear leg goes through. If so, it has a row of rivets along the top and bottom plus six bolts in the front and back. These brackets are prone to intergranular corrosion and will swell to the point that the rivet heads pop off. It may have nothing to do with stress or fatigue from landings etc. This corrosion may not be visible since it starts internally and the zinc finish looks ok. Mine had a bubble look until removal of gear leg and then it was obvious it was delaminating from the inside out. Hopefully yours is something that will be an easy fix. If it's along the lines of what I'm talking about then call P-ponk. Their brackets are half the cost of Cessna's and milled from a solid block which is supposed to alleviate part of the problem. Good luck, Randal

[spiro]

what Randal said. Those rivets aren't breaking from stress. You musta pulled the gearleg out to get a good look at those rivets, you didn't notice any delaminating in the "slot" of the casting? Wait, don't you do 2 floats changes a year, so somebody ought to be looking each time... I'm confused, never seen these rivets break from stress unless surrounded by lots of more seriously bent metal.

ditto also on the PPonk "casting" being better/cheaper. Curious though about, "a very nice lower gearbox inspection panel". Never seen such a thing, I'd be real interested to see pictures and/or basis of approval. I've only seen these castings replaced outboard or thru the top, so maybe your mechanic has a way to do it thru the bottom?

paul

[johneeb]

Mike,
Could you post pictures or part numbers of the area you are discussing
Thanks
Johneb

[GAHorn]

piggy-backing onto Paul's comment... Be very careful installing "inspection panels" near the gearbox area. This is a highly stressed skin area and I doubt if any new inspection holes can be safely added there.

[mvivion]

George,

Uh, there is a prescribed and Cessna approved method of installing an inspection panel in the gearbox area of the belly of single engine Cessnas. I don't make this stuff up, Cessna does. It turned out that the mechanic who found the problem is double jointed enough that he could get in there without cutting a new hole, which is never a great thing, even if it is approved.

The part with the headless rivets is the angle bracket that the gear leg bolts to at the inboard end, and the rivets are on the vertical portion of that angle. The heads of the rivets are on the inboard side of it.

With ample force applied to rocking the wings, you could just barely see the two metal parts moving apart just slightly.

These are all old airplane. Mine has over 5,000 hours on it, and was used hard for many years.

I've heard a lot of gearbox horror stories this year. I think mechanics are getting more paranoid about them, as they should be, and finding problems that they might have missed a few years ago.

It is pretty hard to find these kinds of problems, as compared to the big gearbox problems.

Anyway, just a heads up, and a complement to a good mechanic.

Mike Vivion

[spiro]

mike,
not doubting you, can you get us a pointer on, "prescribed and Cessna approved method of installing an inspection panel in the gearbox area of the belly of single engine Cessnas". This might come in handy sometime.

ok, get out your parts book, page 40, Figure 21. Randal & I thought you were talking about the outer "casting", #47. Now, "The part with the headless rivets is the angle bracket that the gear leg bolts to at the inboard end, and the rivets are on the vertical portion of that angle." sounds like the inner "ledge", #58. Except that it's not riveted, only bolted to the inner "casting", #53 - which *is* riveted, but only at the top and bottom, not vertically. Help me understand...

paul

[johneeb]

Mike,

Here is parts manual Figure 21 that Spiro is refering to. Are the loose rivets in the virtical leg of the bracket labeled number 55 (cirlced in red):?:

You have certainly the attention of a lot of us with this item.

Johneb

[GAHorn]

George,

Uh, there is a prescribed and Cessna approved method of installing an inspection panel in the gearbox area of the belly of single engine Cessnas. I don't make this stuff up, Cessna does.

Mike Vivion

Regarding Structural Repairs in the Landing Gearbox area:
Section 19 of the Cessna Service Manual, para's 19-52, "Landing Gear Bulkhead Assemblies" states: "Standard sheet metal rapair practices may be used to repair damage to landing gear bulkhead assemblies. In making such repairs, however, good workmanship is of particular importance since the landing gear bulkheads may be heavily loaded in a hard landing. Elongated holes, double holes, and insufficient edge distances must be avoided. Nicks and scratches should be dressed out, and the dressed area primed with zinc chromate primer to prevent corrosion. Cracked or damaged forgings must be replaced."

Para. 19-53 "Replacement of Portions of Skin Panels" goes on to state: "In case metal skin is damaged extensively, repairs should be made by replacing an entire sheet panel from one structural member to the next. (italics mine) The repair seams should be made to lie along stiffenging members, bulkheads, stringers, and the like. Each seam should be made exactly the same in regard to rivet size, spacing, and rivet pattern as the parallel manufactured seams at the edges of the original sheet."

If there is a "prescribed and Cessna approved" method of installing inspection holes/panels in the gearbox area, I too would like a copy of that document for TIC170A files.

[mvivion]

Number 53 to number 54. Six rivets. All heads on the inner side. Not visible from inspection panel inside aircraft.

All replaced by mechanic with double joints, and who will undoubtedly sue me forty years from now for causing his arthritis.

As to the inspection panel, I was told by a knowledgeable IA who has repaired many a gearbox, that there is a legal installation for an inspection panel in the center of the belly of these aircraft, approved for seaplanes. I'll ask him for a reference. They didn't have to use it in this case.

It did involve a round inspection cover, with multiple rows of rivets (as in any cover in a structural area), in the center of the belly. The mechanics comment was that it would be a whole lot easier to install a PPonk kit after the inspection cover was installed.

I have never been a fan of the PPonk.

Mike Vivion

[spiro]

now I get it. Not an easy place to replace rivets.

I'm not a big fan of the PPonk "beef-up" kit either. Besides the STC'd beef-up kit, which mounts on the inner casting, Pponk also sells a PMA'd replacement outer casting, which is what Randal & I were referring to earlier.

paul

[jrenwick]

Paul and Mike,

What exactly bothers you about the PPONK beef-up kit? I have one I'm planning to install, but if there's a problem with it, I'd like to know what it is.

Thanks,

John

[mvivion]

The Ponk kit is one of those things that if something bad happens, the kit will probably save you a lot of damage.

If, on the other hand, something a little badder happens, the presence of the kit will likely cause substantially more damage than would have been done with a stock gearbox.

That's what I get from a bunch of mechanics, at least. The Ponk kit does significantly strengthen the gear attachment area. So, if you apply a little more stress than the airplane was designed to handle to the gear, the Ponk kit will keep the gear from coming off the plane, and substantial damage from occuring.

On the other hand, if you apply a little more force than that, the whole gearbox will literally be ripped out of the plane, because the Ponk kit ties it all together.

So, the answer as to whether its a good thing really can't be answered till you decide how bad you are going to screw up.

Also, I'm told by my mechanics that it requires somewhat more labor time to install and remove floats with a Ponk kit installed. That's really the reason I'm not installing one. And, of course the fact that when I screw up, I generally do it with gusto.

I think if I was strictly on wheels, I'd install one. I don't think they are a bad idea, but the float installation issues kind of turned me off.

Mike

[jrenwick]

Thanks, Mike! That's the same analysis I went through when I looked at PPONK and decided I liked it. I didn't think about floats, but that won't be a factor for me (I've got a J3 for that!). I thought it would be a Really Good Idea to install PPONK before installing skis, because I believe the side loads imposed by skis are likely to be higher than those from wheels, and side loads are what the PPONK beef-up is designed to protect against.

Best Regards,

John