The International Cessna 170 Association

I’ve got a fresh can of juicy worms to open, so I hope all the birdies have an appetite.

56D has stock landing gear for a 1952 model. She also has (for now) original hollow aluminum axles, and an opportunity to buy ski axles came up, so I snagged them. The airplane is in the middle of the first annual now, and I’m replacing a lot of the hardware in the airframe, so replacing the axles means new attachment hardware. As long as I own the airplane, she will never see skis and will never see anything larger than a 6.00x6 tire. I fly plenty from all surfaces save for stuff that one actually needs big rubber for, but mostly off of fair condition asphalt and unmanicured grass.

Looking in the IPC, there is an difference in bolts called out for ski planes versus wheels, AN for wheels and NAS for skiplanes. Again, while I won’t ever install skis, my inner monologue is screaming at me to use NAS hardware since the axles will be considerably stronger. I’m sure the AN hardware is plenty strong, but after an interesting read through a post a while back from Gary Hanson differentiating AN/MS/NAS hardware, I am left with a few more broad questions than I have answers. I’m sure dimensionally the clearances for the holes are the same for the hollow axles versus the ski axles and the reason for calling out the difference in the IPC is solely for the difference in tensile strength of NAS hardware versus AN.

Looking up the NAS bolts, they’re internal wrenching heads…does this seem right given that AN bolts will have a six-point head? Is there a service kit anywhere that specifies that if ski axles are to be used then one must use NAS hardware? I’m mostly looking for a concrete answer to give to my IA. I’d hate for the weak link to be the attachment hardware.

If that can of worms didn’t hit your satiety center, I’ve got another one.

I’ve had a PPonk bracket sitting on a shelf for over a decade that the gentleman to whom my father sold the airplane bought. I may be picking the fly you-know-what out of the pepper here, but I’m changing the dynamics of strength throughout the gear leg system and I’m now considering that the beef up bracket might not be a bad idea. Again, I’m never going to do anything hard on the airplane, but beefing up the axle transfers “weakness” in the system to the leg and the retention bolt. I have already decided that I can make solid arguments either way about putting the beef up bracket in, but I’d like to hear some other voices than mine discuss. I do not plan to install lady legs or 180 gear as I genuinely love the stock soft gear legs. A full 3/4 of my 36 laps around the planet have involved flying this airplane and I’ve already decided that I don’t want to change the things I like about the airplane and am used to, chiefly the wing and the gear legs. Maybe I’m turning into George in my old age, but the fact of the matter is the airplane does what I need it to do perfectly and I have several outlets for 170s with fun mods when the mood strikes. I’m keeping this old girl my “old man, law of primacy” machine, but I simply want to ensure I’m approaching repairs and upgrades with the requisite amount of caution so as to not induce another failure down the road.

Thanks for listening!

Ryan

COW#1
I do not know of a requirement for NAS hardware with solid axles (steel or aluminum) or what ever you are calling "skis' axles. IPC says NAS is optional for ski axles, or not required for ski axles as I see it. If I was worried about the hollow axle and was swapping to solid to remove the hollow, just changing the axles and using AN accomplishes that. Use NAS? If I got them for free or little more cost over new AN. If little to no cost why not use NAS and be done with it even though you doubt you will ever use the higher strength.

COW#2
In my mind to use P-Ponk or not makes no difference what axle or axle hardware you have. You should install it if you are of the mind you'd rather not have a minor tear out of the gear leg box for an excursion to the higher limit the P-Ponk affords knowing that if you exceed the limit of the P-Ponk the gear box and the fuselage is likely not salvageable. And you are at peace with the invasion of the torn gear box into the passenger leg space we have seen with wrecked P-Ponk installations vs what might not be, but still could be, a pretty bad invasion of the passenger leg space by the gear box without the P-Ponk mod.

I like you had an uninstalled P-Ponk kit. I would have installed it had I had the chance.

If you don’t plan to put it on skis… whichever bolts you use makes little difference…except in the knowledge that NAS bolts are stronger than AN bolts.

If you don’t plan to ever allow the airplane to “hop” sideways in a gusty cross-wind that might allow a gear leg to attempt to sneak underneath the fuselage then installing a P-ponk will make no difference.

If you can’t predict such things….or don’t know if you’ll ever need stronger bolts or more secure gear leg attachment….then you might consider adding a bit more leeway toward your security by using NAS bolts and adding P-ponk.

(And I don’t buy the “if you wreck it with P-ponk you must throw the airframe away” scenario. In any example you can dream up…airplanes can be rebuilt with the parts remaining after an accident. And if you can predict how hard you plan to wreck the airplane I’ll tell you if P-ponk…or seat belts…or shoulder harnesses … or helmets or any other improvement invented since 1948 will help you.)

Also, BTW, solid aluminum axles are absolutely better than hollow ones which definitely should be replaced…. and hollow STEEL axles are better than any of those.

I tried to say what George did but he did it better (this time )

Thanks for the replies George and Bruce.

My interpretation of the IPC was that if ski axles are fitted, then NAS hardware is spec’d, regardless of if ski fittings and skis are installed. And the axles I’m referring to as ski axles are solid aluminum.

27 years of flying this airplane has led me to believe that I can do a reasonably good job of keeping the old girl straight down the runway. Should the unthinkable happen, Del will have another project.

I listed the bracket assembly for sale and got no bites, so I may just put the thing in and call it a day.

If you DO install the P-ponk…. take a peek at your airframe “level” condition before the work… and it’ll be good afterwards.

We have one Member whose P-ponk was installed by someone else…that someone had failed to take corrective action regarding lateral/level prior to accomplishing the mod…and now is faced with a much complicated corrective action which may require machining the new brackets. (How do you obtain approval to modify STC/PMA’d parts..Hmmnn…)

Lateral/Level specs are with regard to wingtip height above a level hangar floor, the recommended spec is a total tolerance of 3”. (I translate that to mean a total “difference” of 3”.) This difference is adjusted by using AN960 washers at the inboard end/beneath the gear leg. Get this right BEFORE installing the P-ponk. Use the upper door frames to determine the level-condition both laterally and longitudinally. Remember, few hangar floors are actually level…same thing for tire inflation/size, etc.

Reversing the main gear side-to-side (when symmetrical early gear are installed) can sometimes improve a wing-low condition without shimming.

hilltop170 wrote:Reversing the main gear side-to-side (when symmetrical early gear are installed) can sometimes improve a wing-low condition without shimming.

That’s a good Tip, Richard.

But that would require disassembling wheels/brakes/brake-lines…Twice… to accomplish a swap…then put it all back together….. as opposed to simply installing a couple washers beneath one bolt.

The check can be performed simply by pulling the one upper gear bolt and disconnecting the brake line. No need to disassemble the wheel/brake/tire assembly. Then swap the gear legs and check wing height. If no change or it gets worse, swap the gear back the way it was. If the change improves the wing height difference, then change the wheel/brake/tire assembly.

Another thought, if the main gear is to ever be changed/upgraded say to lady legs or 180 gear, the gear change should be done before the P.Ponk beef-up kit is installed because the number of shims will most likely change.

And since if the gear changes for any reason, the number of shims will likely change, logic says it might be advisable to add a few shims to each side when the P.Ponk kit is first installed. Don't know what the STC holder says about that but it might be interesting to find out.

Several years ago, we began installing the maximum allowable amount of shims, before installing the P.Ponk block, whether needed or not. We place the shims between top of gear leg and the block. If you need to raise a wing later, just move shims from top to bottom as needed.

wingnut wrote:Several years ago, we began installing the maximum allowable amount of shims, before installing the P.Ponk block, whether needed or not. We place the shims between top of gear leg and the block. If you need to raise a wing later, just move shims from top to bottom as needed.

This is why you get paid the big bucks, Del!

hilltop170 wrote:The check can be performed simply by pulling the one upper gear bolt and disconnecting the brake line. No need to disassemble the wheel/brake/tire assembly. Then swap the gear legs and check wing height. If no change or it gets worse, swap the gear back the way it was. If the change improves the wing height difference, then change the wheel/brake/tire assembly.

Another thought, if the main gear is to ever be changed/upgraded say to lady legs or 180 gear, the gear change should be done before the P.Ponk beef-up kit is installed because the number of shims will most likely change.

And since if the gear changes for any reason, the number of shims will likely change, logic says it might be advisable to add a few shims to each side when the P.Ponk kit is first installed. Don't know what the STC holder says about that but it might be interesting to find out.

I can attest to this. Ask Richard and Del both how I know. With used but straight original gear installed one way, I had significant level problems (beyond shimming). After swapping the gear around, everything is within a 1/4 inch of level. I know the early gear are supposed to be symmetrical, but it can definitely make a difference.