Gear legs again?

[mit]

ANC10LA026
On March 17, 2010, about 1530 Alaska daylight time, a ski-equipped Cessna 170B airplane, N3492C, sustained substantial damage when the main landing gear collapsed during taxi for takeoff, at Lake Hood Seaplane Base, Anchorage, Alaska. The airplane was being operated by the pilot as a visual flight rules (VFR) personal local flight under Title 14, CFR Part 91, when the accident occurred. The airline transport pilot and sole passenger were not injured. Visual meteorological conditions prevailed at the time of the accident, and no flight plan was filed.

During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on March 18, the pilot said he was taxiing for takeoff on Lake Hood when the right main landing gear leg broke near where the leg exits the fuselage. He said when the leg broke the right wing impacted the ground, bending the wing from the aileron outward toward the tip. He said there were no mechanical problems with the airplane prior to the accident.

During a meeting with the NTSB IIC on March 25, the pilot presented the broken gear leg to the IIC and an FAA air safety inspector. The break appeared to be a single linear break perpendicular to the leading edge of the gear leg. The break was about 6 inches from the gear leg mounting bolt hole, at the widest portion of the gear leg. No other damage was observed. The gear leg was retained by the FAA inspector for metallurgical examination.

[blueldr]

Obviously, a classic case of "Pilot Error". He should not have been out there trying to go flying.

[GAHorn]

If this is the same accident I read two weeks ago.... the gear leg had a long-existing fracture at the wedge-blocks (where it enters the fuselage.) I predict the cause will be faulty inspection/maintenance.

[mit]

I just wish Cessna was making new gear legs, at a resonable price! What a dreamer

[Showboatsix]

Tim,

Too damn many lawyers , too damn many FAA officials to sign off of 432 times for PMA approval for that to ever happen!

So in the mean time we all fly with 60 year old gear legs that are one landing away from disaster!

[GAHorn]

...So in the mean time we all fly with 60 year old gear legs that are one landing away from disaster!

Not to start an argument, but I think that's crying "Wolf!" and unnecessarily raises the spectre of more potential regulatory oversight. There's nothing inherently wrong or dangerous about 60 year old gear legs that are inspected/maintained according to the book. They are NOT "one landing away from disaster!"

If you think your gear legs are "one landing away from disaster" then you'd best ground your airplane until they are properly inspected/replaced. Replacement gear legs are available... What I think you mean is.... you simply don't like their purchase price?

[mit]

It would be impossible for a George inspected gear leg to fail

[Showboatsix]

Just wondering, do you all remove and magnaflux your gear legs each annual? if not, how do you know that there are no new cracks in your gear leg just waiting for that next landing?

I wonder how soon the FAA will require that each year to keep the lawyers happy.

Remember..... the FAA is not happy until you are NOT happy!

[GAHorn]

It would be impossible for a George inspected gear leg to fail

Just wondering, do you all remove and magnaflux your gear legs each annual? ...

Nope.

And, Nope.

And it's certainly possible a propeller blade will separate, rip the engine out of the mounts, and kill folks on the ground...if their still alive from the meteor-strike and after the heart-attack. ("Hell,... the FALL is gonna kill you!"--Butch Cassidy)

But it's damn unlikely because I do what history and experience and the mfr and the FAA all know and advise us to do.... inspect for corrosion and cracks, and if you haven't SEEN it in the last year.... then you'd better LOOK at it, because you're overdue!

100 Series Cessna Service Manual, 1962 and Prior, page 2-16, item 8:
"Main landing gear spring struts for secuity, cracks, bends, deep scratches, dents, discooration, escessive wear, and lubrication."

(Magnafluxing is not mentioned.) There is no record of any gearleg failing because it was not "magnafluxed". The detailed visual inspections called for in the manual will catch indicators that should lead to more detailed investigation, if required.

Do you think this example of a failed gearspring had no previous evidence prior to failure? (HINT: Look at the paint/rust adjacent to the fracture) Do you think it wasn't possible to detect this incipient failure on any good preflight inspection? Do you think it would have required Magnafluxing?:

click to ENLARGE

It's been discussed here and other TIC170A and Cessna publications many times, there are areas of this relatively durable and simple gear that should receive close attention. It is so simple...as to be misleading... and many owners think it's a maintenance-free design. It is not.

The gearleg receives a lot of punishment. Every time we land it must absorb a huge change of force and it chafes against it's own supports/bolcks/wedges, and it has rocks thown up against it's undersides and the axles stress the lower ends heavily. The bolt-holes the axle mounting bolts pass thru should be inspected at least every 3-5 years for cracks emanating from their edges. The edges should be slightly radiused, especially if the lower bolt holes have been enlarged to accomodate the larger bolts of later axles. (Do YOU know about YOURS? Has it's lower holes been enlarged from the early 5/16" to the later 3/8"?) When was the last time you actually REMOVED your axles and checked for corrosion and cracks between the axle and gearleg? (Mine is checked every 500 hours/5 years, the same schedule as my main tailspring is changed out. It's arbitrary but we've discussed this before....it agrees with our collective experience regarding that spring.)

Here's a pic of what a stress-riser from abrasion of an adjacent structure looks like:

During gearleg inspection,... JACK that airplane off the ground and give those dangling legs a serious SHAKE! If any looseness is found....investigate! That upper-end is usually held by only one small bolt and when the airplane is at rest on the ground, the weight of the airplane tends to hide any looseness. But each/every time you fly that gear must go from supporting to hanging and back again. CHECK it.
Take an actual LOOK at where the legs pass thru the blocks. Open up the floor, remove the fairings, clean the area and use a bright light. Check the wedges,...AND.... check for wear against the gearlegs FROM those wedges.

Remember, any nick, scratch, cut, ding-from-a-rock, abrasion, dent, ...... should be treated as seriously as a similar injury to your propeller.

Do NOT use chemical strippers on your gearlegs when stripping for paint unless you KNOW the specific stipper will not embrittle the steel. (Some strippers cause hydrogen embrittlement. I've talked about this before....Google it.)
If you buy used gearlegs from Joe... then have them completely NDT'd/documented before you install them. Don't buy gearlegs from fleabay, strip and paint them to match and install them and then wonder why they fail like the accident airplane.

You don't have to remove gearlegs to check their undersides for rock-damage, paint damage, rust, corrosion, and ... the holes drilled thru them without approval for steps! (I've seen steps WELDED on gearlegs!)

Do not "assume" your Inspector did all this simply because your 100-hr/annual inspection is signed off. SPECIFY that it be done. (There is a sly thing that creeps into our mental attitudes about inspections.... for our pocketbook's sake, We HOPE nothing is found wrong. Then, slyly, we disregard casual inspection practices because we subconsciously know that less will be found wrong. Keep in mind that we WANT to find things wrong BEFORE they fail....otherwise we truly ARE damaging our pocketbooks. We wasted money not catching the failure before it got expensive..... and we wasted money performing a NON-inspection. You might as well find a pencil-pusher to whip your mx records.)

[hilltop170]

It's not that hard to remove the gear legs for inspection and is the only way to do a good inspection.

George mentioned to not use paint stripper without knowing whether it causes hydrogen embrittlement (which is bad).

Also, do not sand blast the gear legs. The underside of the legs are shot peened for fatigue resistance. Sand blasting will compromise the shot peening effectiveness and can cause stress concentrations leading to failure.

Likewise, power sanding/grinding should not be done. Some blending of surface defects is allowed but the service recommendations need to be followed to insure you don't do more harm than good.

To do a proper inspection, the gear needs to be removed from the plane to be able to inspect the area where the wedges are jambed up against the leg. Wear in that area will be a stress concentration and is where fatigue failure can start. Fatigue failures are not usually immediate but progress over time with each stress reversal of the part, ie, each landing, take-off, and bounce.

Signs of fatigue failure can be visible cracks or reddish stains which comes from fretting of a crack. If you see either, remove the gear and inspect before another flight.

[jlwild]

Mit, thanks for calling this maintenance issue to everyone's attention. It is very timely as many of us will be starting on our "spring" Annual Inspection soon .

George, Your pictures, of what to look for, are better than a 1000 words. Everyone should appreciate those pictures. I am printing out a copy of this article so my IA will be aware of the problem and what to look for. My plane has never had the stress of landings and takeoffs with skis during the 24 years I have owned the plane. But, the landing gear has also never been removed to inspect for stress wear resulting from the wedges used to hold the gear legs in place.

Point taken Richard and George, I will have all these areas carefully inspected during my May annual.

" I sure get my money's worth out of being a 170 Association member "

[lowNslow]

Also, do not sand blast the gear legs. The underside of the legs are shot peened for fatigue resistance. Sand blasting will compromise the shot peening effectiveness and can cause stress concentrations leading to failure.

If you are literally talking about "sand" blasting you are correct, but "media" blasting (i.e. plastic or other shot) should not effect shot peening.

[minton]

Given there are so many posts as time marchs on that repeat posts are a good thing.

Few of us take the time to do searchs or go back into the archives.

I think you are talking about a sharp and hard blast medium. There are many of them out there. Soda, plastic, walnut shell and glass bead are acceptable.

There is a guy out there that is FAA certified to inspect, shot peen and recertify gear legs. North Sound Aviation, Jim Hayton, 360-661-3302. I know he has come up before and he is as slow as Mole-asses, but if you are serious about this use him.

Would,nt be a bad idea to have a stand-bye set of gear legs.

[mit]

Would,nt be a bad idea to have a stand-bye set of gear legs.

Or a set of wings, or a set of control surfaces, or a set of tail feathers, or a set of mags, or a carb, or a engine, or a generator, or a prop, or a stand-by plane.....

I still wish you could buy a new set of gear legs.....

[hilltop170]

I still wish you could buy a new set of gear legs.....

Tailwheel Tom at XP mods was the last place I know of that sold new legs (besides Cessna) and he is now out of business. His legs were for a C-180 or C-185 anyway. Early C-180 gear works well on the 170 if your not mostly an originality nut.