Watch those gearboxes, Folks

[zero.one.victor]

The short & dirty about the Ponk mod:
It will prevent the gear leg from being ripped out of the gearbox. But it will not prevent the gearbox from being ripped out of the airplane instead (infinitely worse). Think forward door post damage-- and the forward & aft doorpost/spar carrythru/bulkhead box is what holds our airplanes together.
That said, mine has the Ponk mod (installed by previous owner)-- I wouldn't put one in myself, but I'm not gonna bother to remove it either.

Eric

[mrpibb]

Last annual my buddy and I installed the Pponk kit, took about 6 hours for the one side, then 2.5 hours for the other. I installed them for several reasons. I figured I as a new pilot about to get his tailwheel endorsement in a plane I just bought I'll need all the structural integrity I can afford. also I have seen Cessna's with folded gears taking out door posts, i figure with my luck my leg would find its way in between. And if I side load it to the point of ripping the gear box out well then I'll just pick up the phone and call AIG. But this won't happen right? because we 170 drivers can keep it straight. But most of all I put it in cause I wanted to....aahh the joys of being a aircraft owner/A&P technician.
By the way, thinking of picking up a Rearwin sportser with a buddy of mine, to compliment the armada.

[GAHorn]

Yeah, I like Mike's flavorful response to this. If you can tell us what kind of accident scenario you intend to have, we can tell you if you should have the P-ponk or not.
If you intend to smash your airplane into the ground hard enough to ruin the doorposts and gearbox anyway, it's doubtful if the money you spent on a P-ponk was helpful. (I don't subscribe to the theory that you can predictably rip out a gearleg without causing sufficient damage requiring doorpost/gearbox replacement anyway. But I do believe you can prevent a simple botched landing from ripping out a gearleg with a P-ponk kit.)
The P-ponk website does a fairly convincing job of describing what they believe their kit does. http://www.pponk.com/
Especially examine this link:
http://www.pponk.com/HTML%20PAGES/gldr. ... chor147619

My airplane had it installed by it's restorer Bill Goebel. If I owned an airplane that didn't have it, and had no record of having it's main gearlegs removed for inspection recently, I'd probably have it installed at the next annual. It costs less than my deductible.

[Metal Master]

[/quote]

Gear Box Story and Thanks for this view Johneb

I just finished putting the gearbox together on my 170A a couple of weeks ago. There has been much written both pro & con about the P-Ponk gear mod. I have rebuilt a few tail dragging Cessna’s after ground loops that tore the gear legs from the gearbox with varying degrees of damage.
I had always been apposed to the P-Ponk mod on the basis that it would move the damage in a ground loop into more substantial structure. I have changed that view. The following is why. But first a couple of notes. The above gearbox is similar to the gearbox in a 170B. It is different from a 170 and a 170A in that the steel fittings item 74 & 83 at the most left and close in the picture is shorter by three holes at the inboard end. This is true of all four steel fittings at the outboard end of each bulkhead. The 170 & 170A did not originally have the approximately three-inch diameter hole in the web doubled by the ring item # 72 & 81 just above the end of the steel fittings mentioned earlier. The 170 & 170As do not have the flap pulley and bracket at the center of the triangular lightening hole at the center of the bulkhead not shown in this view. My 170A did not have the P-Ponk mod (STC). This view is also similar to a 180 gearbox
In the ground loop that my aircraft experienced the aircraft was sliding side ways at the very end of the landing roll at according to the previous owner slow speed, the gear attach bolt did not break. In stead the leverage on the gear as it tripped over the left gear leg ripped both the forward and aft gear bulkheads in half at a location about the center of both gear bulkheads and in the middle of the triangular cut out in the center of the rear bulkhead. The remaining portion of the gear box attached to the gear leg was ripped completely from the aircraft after having been torn by being pivoted around the fulcrum of the wing strut attach bolt at the outer end of the landing gear rear bulkhead. It sheared the rivets that attach the forward gear bulkhead to the forward doorpost former. And ripped the steel fastners in the same location out of the top of the forward gearbox bulkhead.
I find in thinking about it that the forces involved must have been astounding. Just the rear bulkhead is .050 thick and is doubled with a .063 thick aluminum piece (item# 80) that covers the entire web from the inboard gear fitting all the way up to and including the attachment to the doorpost. The steel fitting (item#80) mentioned earlier doubles this. These are in turn doubled on the forward side by two .063 aluminum doublers (item#84) that pick up fasteners above and below the wing strut attach bolt. These are further doubled by two additional .050 aluminum doublers (item #85) on the aft side of the rear gearbox bulk head that pick up two fasteners below the strut attach bolt and the strut attach bolt itself. Theses last four doublers act to increase the thickness of the gear bulkhead and the bearing for the ½ inch bolt that attaches the strut. The bottom line is that all of these doublers were ripped in half at what was reported to be a fairly mild ground loop pivoting around the wing strut attach bolt. In my opinion the P-Ponk mod (STC) would not have prevented this from happening. In this event the P-Ponk mod does not hurt anything by being in there. What the mod does provide however is of value in that it traps the inboard end of the gear leg at the attach hole between two heavy pieces of aluminum so that a failure of the gear attach bolt will not allow the gear leg to flip out or fall out. Additionally it makes the gear bolt less likely to break because the tension load is reduced on the bolt.

This aircraft as far as records demonstrate was operated for 53 years without major ground loop damage. Which either attests to good pilot performance and good training or an inherently safe aircraft to operate? I cannot tell you the number of repeat gear up landings and nose gear failures I have seen on retractable gear Cessna’s. In my opinion poor training and a failure prone system and or operation of and in the RG’s. This point being that I want to limit my gear issues to pilot failure and remove any equipment failure potential. The cost of the P-Ponk kit is less than my deductible.

My plans are to install the P-Ponk gear STC.

Jim

[mvivion]

Jim,

Excellent discussion of "real" ground loop damage, and good logic for the installation of the kit.

George,

I promised I'd find out the basis for the access cover in the center of the belly skins for seaplane access. Here it is, as I understand it:

The early Cessna 180's had a different steering system than the later ones, requiring access into the inner belly area (center of the gearbox) to access the steering when switching to floats. So, the early 180's had an access cover in the center of the belly skin, in the gearbox area.

What the local folks are doing, if they need to get into that area, is "converting" other Cessna tailwheel airplanes (late 180's, 185's and 170's) to this configuration, following the Cessna drawings for the early 180. The local FSDO has been willing to accept this, without further ado, since it is "approved data", so the feds have accepted it for all models. And bear in mind, this wasn't my idea, I'm just reading back what the mechanics told me.

Since, as you noted, this is stressed skin, the modification requires serious doublers, and the cover is secured with 8 nut plates. In other words, this is not an "inspection cover" as most of us use the term, but rather an "access point", if that's any clearer term. But the point is, Cessna has approved drawings and specifications for this installation.

I can find out more if it would be useful, and I may not have presented this very precisely, but that's how they get access to the back side of the gearbox if they need to.

Mike Vivion

[GAHorn]

Thanks, Mike, and I think you 'splained it pretty well. What I'd like is a copy of any 337's (especially with Block 3 filled out) on any 170's your mechanics can provide, but even more importantly, a copy of the Cessna-approved engineering document used for the basis. Cessna is no longer as "helpful" as they once were, and any engineering approved docs are really helpful to owners should any need such help in the future. TIC170A maintains a library of this sort of thing just for that purpose. If you need my mailing address, PM me. Thanks.
George

[N1478D]

78D has the PPonk mod, and so do most other 170's that I know here in this area. Wonder if over 50% of the fleet has had it done?

[n3833v]

When I had all my work done, I considered it. The rebuilder told me that PPonk is good for side load and for what I do that he wouldn't recommend this mod. He said that it doesn't make the rest of the plane any stronger. I guess Cessna does their research and there are some things we can improve on.

[dacker]

Here is something else to consider. If you do tear out a gear box i.e. ground loop, the airplane will probably be totalled by the insurance company as opposed to being repaired as in years past. Part of the decision with the Pponk is whether you would keep (buy) the wreck to rebuild yourself or give it up to the insurance company. In other words the Pponk might prevent that decision from ever being made.
My two cents worth.
David

[AR Dave]

The pilots that need Pponks, don't have insurance! When those gear legs and tires go tearing through your flaps and horizontal stabilzer, you'll wish you'd tried at least one of them. Without insurance the only chance you got is Pponks, because if you're going to total the plane with them, you're for dang sure going to total it without them.

[3480C]

I had the same problem as Mike about 5 years ago.The mechanic that did the repair.Also did all the maintence on half the fleet of c185s that california highway patrol had at the time. He recomended installing the inboard potion of the p_ponk kit only.Beause that is were the cessna gear is the weakest.He explain that is where the gear is going to fail first when you put a side load on it.Breaks the inboard mount and comes up threw the floor.So thats what we did.As for sheared rivets on the outside mount they where repaired by removing the floor skin. Don,t know why they broke but there was six of them with sheared of heads.And the only way you can see them.Is with a mirror and a flashlight. Threw the floor inspection hole.
John

[mvivion]

John,

Exactly what we saw, and as you note, really hard to find. Interesting observation by your mechanic on the Ponk kit "modification".

As I said, probably the only thing keeping me from installing one is the float issue. That goat rope costs me enough each year as it is. Not wanting to jack the gear change rate up even more.

I'm thinking this is the year to blow the gear and axles apart when the plane goes to floats, and have everything magnafluxed/tested, etc.

Don't really know if it makes any difference, but maybe it gives you a warm fuzzy.

Mike

[AR Dave]

Breaks the inboard mount and comes up threw the floor.

Man that sure would be some sideload! Had he seen this on a 170?

[3480C]

Dave
What i ment by saying inboard mount is.There is one bolt going down threw the top of the gear on the inboard mount.This is the week point.The p-ponk has a saddle over it with two or four bolts cant remember.

I dont know if he ever seen this happen to a170 ? but he has worked for the same cessna dealer for the last 33 years.
John