Frame/Wing Fatigue/Life

[dodd]

I was reading an article in Aviation Safety Magazine last week.
Apparently some aircraft have life limits.
Wing life limits, etc. The Cessna 170 was not mentioned.
My aircraft is a 52B/180 hp.
Any and all comments appreciated.

Dennis Dodd N8018A
"Finally a tail wheel guy"

[Bruce Fenstermacher]

Yes this is true. I believe the Gruman Yankee and Tiger series airplanes have a wing life of 50,000 hours or some rediculious number.

[GAHorn]

Depending upon which regulation applies to a particular category aircraft, it may be designed with "life limited" parts, or it may be designed with "on condition" , or "fail-safe" part designs.
Large and transport category aircraft usually have either "fail safe" (meaning a failure in any particular part will not result in catastrophe because alternate load-paths are incorporated into the design) or they may be "life limited" (meaning after a specified number of hours or cycles (flights), certain parts must be replaced.
Our Cessna 170's were manufactured under an "on condition" basis, meaning there are no life-limited parts, but that the entire airplane must be inspected annually (unless on a progressive inspection program) and any parts not meeting their airworthiness inspection must be repaired and/or replaced.

The recent Alert letter sent out by the FAA about the spring gear was FAA's attempt to determine the public feedback about a possible "life limit" being placed on the landing gear legs. This is why I solicited feedback from all members, and why I forwarded all the responses to the FAA Airworthiness office in the hope of preventing a "life limit" being placed on our landing gears. So far, the effort appears to have had the desired result.

[1SeventyZ]

Nice work, George.

[c170b53]

Design progression; On condition, Condition monitored, Fail safe then the MSG process which resulted in life limits to structures. The maintenace / inspection program is based on the design. Modern design ensures the structure can withstand failure from fatique, corrosion and accidental damage up to the life limit. So if you airframe exceeds the design life hours (and modern day operators do) then you come up with a supplemental inspection program geared at finding those problems and addressing possible fatigue failures.
There's true beauty in designing something simple and effective like our planes.
Corrosion will be the most likely culprit affecting the structual soundness of a 170.

[Indopilot]

I would agree that corrossion is going to be our biggest challange maintenence wise in the future. So far ALL of the wings for 170's and early 172's that I have worked on have had intergranular corrossion issues in the main spar. On our 171 I had to replace both main spar main angles. The stiffners( flat with a bulb formed on one edge) that rivet onto the main angles are also extruded aluminum and so are susceptable to corrosion as well.
Going back together I like to etch/alodyne and chromate epoxy prime the parts before assembly. Then doing the paperwork I make certain to document what I did for corrossion in case there ever is an AD addressing this issue. Brian

[1SeventyZ]

So far ALL of the wings for 170's and early 172's that I have worked on have had intergranular corrossion issues in the main spar.

Brian-

Isn't intragranular corrosion the type that begins internally in the material, and doesn't present itself until it radically distorts the material, like a water-logged piece of particle board? Photos I've seen look like a crystalline explosion that starts within the member. The aluminum will begin flaking away like chalk.

From what I've read this is much different than the more common aluminum oxide scale that is common in older Cessnas, and actually proceeds inward into the material without much visible external sign until it's too late to salvage the member with an acid etch process.

How many older Cessnas have you seen with this problem? Kind of scary.

[Indopilot]

Almost had to take my shoes off but the count stopped at nine fingers so I was ok.
A/C that have spent time at a coastal location seem to be more prone to this. Some of the wing shops would probably have a some good input if you asked them.
If it is not obvious by swelling or bulging, it shows up quickly when cleaning the parts by glass beading to remove light surface corrosion. What looks like a small spot starts to bulge and then flake away.Brian

[c170b53]

An extrusion, formed by forcing semi molten aluminum through a die, tends to have a longitudinal alloy grain structure. The corrosion attacks along the grain paths of those elements of the alloy that cannot withstand the corrosive environment. For sheet metal mechanics its like following a black gold vein through the aluminum. By the time its visually located on the surface, it often means part replacement. The corrosion can start at the end of the piece or at a rivet hole or from the surface if there's no surface protection.