Warbird Information Exchange

Recently checked out in a PT-26 with a tail wheel shimmy issue, on paved runways.
Any ideas/improvement comments will be appreciated.

Check:
Inflation pressure
Condition of the tire itself
Bearing and race condition, lubrication

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Live to fly, Fly to live.....

I'm not familiar with a PT-26 tail wheel system but some thing's I've seen on other aircraft.....

- Bad dampner
- Wheel or tire out of round (flat spot or worn?)
- Loose or bad bearings
- Loose mounts

Before you tear the aft end of the airplane to pieces, check the axle nut on the tailwheel and see if it's tightened correctly. Start with the easy, simple stuff then work your way up to overhauling the engine. Too many times in commercial aviation I've watched techs tear a system apart because 'the light didn't come on when I flipped the switch' only to find out sometimes hours later, the lightbulb was burned out! :15 second bulb check vs. 3 hour disassembly to discover bulb is bad, which scenario would make you look like a hero or a boob?

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Don't make me go get my flying monkeys-

The Fairchild Club is probably the best source of info regarding shimmy. It's quite common. The general idea is to take apart the tailwheel post and drag link and reduce every possible source of play. They have several articles about it.

And as was said, an out-of-round tire should be replaced, and work from simple to complex. Jack up the airplane, shake the tail wheel from side to side, and see what's flexing. Make sure nothing is broken.

However there is a simple fix, though it won't last forever. The tailwheel post is held (encircled) at the bottom by a collar, which is a brass casting, made in 2 halves and bolted together. The brass isn't as hard as the steel post, and thus wears quicker. The hole gets bigger, the fit gets sloppier, and it's a source of play.

You can jack up the fuselage, remove the casting (2 bolts) and clean off all the grease and oil. Then lay the 2 mating surfaces of the casting on a piece of 120 grit sandpaper on a flat surface, and rub off an educated thou or 2.

When you put them back together the hole they encircle is now very slightly smaller. It will grip the tailwheel post more tightly. This makes it a bit more difficult to turn the aircraft while taxiing (the post doesn't swivel as easy), but the shimmy is gone.

An alternative is a shim, but it's hard to keep a shim in the right place, and hard to get the thickness right.

All this requires a certified AME if you're flying a certified aircraft. The bolts have to be lockwired when reassembled.

Would be LOL funny if it is was not unfortunatly all too common to see these days.

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Expert:
Ex: Former.
Spurt: A drip under pressure.

Hi MJR

I have sent you a PM

Mark D in Pa.

That's because no one these days wants to ask simple questions of the pilot or crew like' When did this start?, has it done anything like this before, even a little bit?. what were your power settings/configuration,?' simple, direct questions that, if you are even a marginally bright individual, could put you on the path to being a hero and not a putz. No one even tries to troubleshoot any more or eliminate non applicable theories of whats not right (and there's a big difference between what's wrong and what's not right), just open the parts counter window and chuck parts @ the subject, it's automotive mechanic thought leaking over into aviation, but when people decide they want to be a mechanic and have zero background in keeping their $1500,00 beater alive, let alone an airplane, and that goes back to education since all our schools want to teach nothing but how to beat on a keyboard and learn how to become a 'zekutive-might have some bearing on why we can't build anything here anymore-


See, I did put my soapbox away

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Don't make me go get my flying monkeys-

Last summer I was on a gate call for the FBO I worked at when a long time friend of the family called me about a shimmy in her champ tail wheel. I finished the call and stopped by her hangar to check it out. It looked ok to me but she insisted that she wanted us to take it apart. She told me that it was ok yesterday but today when she took a guy flying that she noticed it. I told her to let me finish my airline paperwork and she would take me around the pattern so I could feel it for myself. When I got back to the hangar I got to thinking about her problem and called her back. I told her to go around the pattern by herself and she asked why? I then asked, was the passenger heavy? She said that he was a big guy so I told her to go without me. I got a call 20 min later and she said "it's fixed!".

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Long Live the N3N-3 "The Last US Military Bi-Plane" 1940-1959
Badmouthing Stearmans on WIX since 2005

Inspector, you need to lighten up a bit on us A&P's. I'm not an old fart yet, but it's coming. You'd be surprised at some of the young talent out there. I work with 2 younger than me A&P's and they're doing well. Must be the part of the country you live in !!
David
WHF Maint

We had a shimmy on the SNJ-5 that got so bad the whole tail shook violently on roll out.
No one could figure it out. I was talking to a friend who flew SNJs in flight
training in the 50s and he had a idea what it was. I went to the hanger and
suggest to the assembled brain trust that the do-hicky (stay with me you non-
mechanically inclined) the locking pin goes into was enlarged from 35+ years
of use letting the pin vibrate. Guess what...that's exactly what it was
I got the front seat on the next flight has a reward for my forward thinking
But guess what when we landed I couldn't unlock the tailwheel cause it was to tight now
and we were stopped on the runway unable to turn with the very nice lady in tower screaming
at us to clear the runway I couldn't get a word in over her tongue lashing. Ed got so annoyed
with her yelling he got on the r/t and politely told her to shut up we had a little problem.

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Don't touch my junk!!

Sabremech;
Perhaps I should, but years of working in a large MRO where the folks on the floor went from steady full time employees to hired gypsies who will hack up work and then blow town during lunch beacause some other MRO in Mule Nostril is paying $1.75 more an hour leaving a huge, mixed up pile of FOD that was supposed to be the in process repair, oh yeah. and absolutely zero updating on the paperwork.

The average new hire putting together the brand new BSB (big silver bird) up the street has no investment in the end product because to them it's 'just a j o b man, and nothing more'

You tend to become like the folks in the Emergency Rooms, you realize theres no use in reacting so you just carry on and try as hard as you can to put good work out on the ramp because you may be the ONLY person on that project who gives even a passing thought to the milions of people you'll never know, who ride on your work and subconsciously depend on you doing things right.

Jack and Jeff; that's good detective work on both of your parts and something that wouldn't enter the minds of 9 out of 10 'mechanics' today.

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Don't make me go get my flying monkeys-

Guilty. Rewired an ignition switch last week to solve a radio noise problem. Turns out it was just a dud filter.

Can you influence the shimmy at all? For example, does it only shimmy when landing on pavement and you have the stick sucked all the way back? Sometimes just unloading the tailwheel slightly will kill a shimmy. Once in a blue moon my Stearman will do that (I usually have to "excite" it and get it shimmying by screwing up the landing first) but just relaxing the stick slightly will make it quit pretty much instantly. What's the tire look like? Streamlined, original type tailwheel tires seem to be a little more prone to it than the flat bottomed ones, but usually they're the only ones that will fit properly. If, for whatever reason the tailwheel steering cables get tightened up, make absolutely certain that after the tailwheel unlocks and swivels freely that it will lock again when the airplane rolls straight and the tailwheel straightens out. That's been learned the hard way once or twice...

Dan

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Defending Stearmans on WIX since Jeff started badmouthing them back in 2005.

You do make a good point. IMO....It starts with system knowledge. I've seen alot of mechs make mistakes. I've seen a share of QA Inspectors make mistakes too. I've done some of both in my years of working in aviation. No one is immune to it. Aside from knowing your systems, I think the sign of a good mech and inspector is being patient and know your limitations. Saying "I don't know" is an acceptable response. The follow up should be, "but I know where to find the answer".

Absolutely one of the best tools for big airplane mechanics (working Douglas jets) was the LAMM Manual. ATA Chapter by ATA Chapter (except UAL who has their own 'system' of ATA codes) were pages and pages of simplified semi cartoons that let you see and visualize how a system operated, where the valves and so forth were in the system and it worked great as a visual aid to technicians. When Boeing bought out McD, about the second thing they eliminated were the LAMMS because 'we didn't think of it, it can't be worth anything'. Anyone who ever worked on F-4's in the NAVY is familiar with the concept as it was also sort of, in the NATOPS manuals too.

i haven't touched a McD airframe in over ten years but intend to buried with all my LAMMs DC-9/80, DC-10, MD-11 Another great tool was the FEFI/TAFI manuals for the DC-10. FEFI (Flight Environment Fault Isolation) was carried on the airplane and used by the F.E. to narrow down an issue prior to landing so the mechanic/tech could go to the TAFI (Turn Around Fault Isolation Manual) which had a 'trouble tree' for each system. And the first item usually was 'Check lightbulb function' then went from there with 'yes' 'no' prompts going deeper and deeper into a system until you've eliminated the issue. Boeing tossed that out too. If the suspected problem was oulined in black, that was an LRU and Mr. Electricity could get in but wouldn't come out, don't mess around, replace the box and see if that made Mr. Electricity happier, we have folks who fix the boxes where its always 72 degrees and the radio is on, the lighting is excellent and the coffee is always hot.

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Don't make me go get my flying monkeys-