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Are there any guidelines or rules of thumb about when airframes become affected by metal fatigue?

More specifically, is something like a 1950s vintage Cessna still safe, or should I be worried about metal fatigue? I'm thinking of buying and completely overhauling a C172, but how can I determine if I should be concerned about metal fatigue or not? The planes are so old, and it doesn't seem possible to me that their structure has not been compromised because of their age.

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  • $\begingroup$ Yes, provided the aircraft has been maintained to even half-way decent standards. Corrosion will gnaw away at aluminium over time but a plane built in the '50s and kept in good condition is safe. Keep an eye out for planes with a long history in dry climates (brown areas on GoogleEarth, generally). You can see old planes in like-new condition and newish planes that are absolute rattletraps. Just get a good prebuy and keep up on maintenance. My next plane will probably be an early model Bonanza (pre '57) and I have no worries about the strength of the structure. Old planes were built well. $\endgroup$ – acpilot Jun 4 '16 at 7:11
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    $\begingroup$ You need to add more details. Cessna 1, built in 1955, was flown once then stored in a mothballed condition until last week. Cessna 2 was built in 1985 and has flown 20,000 hours mostly in training, has been involved in several hard landings and has numerous airframe and structural repairs. Which one is better? $\endgroup$ – Simon Jun 4 '16 at 10:53
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    $\begingroup$ Take the aircraft to your A&P and have it inspected. Look through the logbooks, etc. There is no way to answer this from a paragraph on the internet. $\endgroup$ – Ron Beyer Jun 4 '16 at 11:25
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    $\begingroup$ @Simon: almost certainly the 20,000hr plane is the better choice. Planes that sit rot. Planes that fly (especially planes that make money when they fly) have eyes on them at least once a year, often every 100hrs. Given the two planes you offer for consideration, I'd not discount Cessna 2 simply because it's high time. $\endgroup$ – acpilot Jun 5 '16 at 2:00
  • $\begingroup$ @acpilot That's why I said mothballed - i.e. in such a way as to preserve it. Regardless, my point to is that the age of the airframe is essentially meaningless. $\endgroup$ – Simon Jun 5 '16 at 4:54
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The short answer is no, age or airframe hours alone are not something to be concerned about on these light Cessnas.

A 1975 C172M I flew for work had about 20,000 hours, or more, on the airframe (can't remember exactly), largely in low level flight at or below 500 ft AGL, often in high winds, moderate turbulence, low level wind shear, etc., and flight including plenty of relatively high G maneuvering, and lots of crosswind landings far exceeding the demonstrated crosswind component. Years ago, before my employer bought it, someone managed to flip it over on it's back while taxiing. The thing has also had several bird strikes.

As an A&P mechanic, I have thoroughly inspected this airframe, to the level of an annual/100 hr inspection. Keeping in mind the airframe's age and history, my inspections included paying special attention to items of structural interest such as spar and strut attach points, control surface attach points, firewall, spars, etc. I have no concerns about the structure of this particular airframe, and I've trusted my life to it over 840 flight hours.

I have quite a bit of experience maintaining a number of single engine Cessna models, including quite a number of older, well worn individuals. I cannot recall encountering any structural issues that were due to metal fatigue alone. Most of the structural instances I can think of would be due to one time events such as a hard landing causing a firewall to wrinkle or crack. Some, such as cracked gear strut castings—in a U206G for example—are relatively common on such airframes that are used for rough, backcountry flying. I suppose that could be chalked up to a type of metal fatigue, but I don't think it is what you are concerned about with this question. Such items are a known issue and won't cause you to fall out of the sky.

The real age-related concerns regarding structural issues will be corrosion. Obviously, corrosion can occur over a short time frame, but give an airframe an extended stay in a harsh, corrosion conducive environment and it may fare more poorly than in a "good" environment. That being said, there are plenty of great airframes from the '50-'70s that have been hangered or sitting in barns in humid climates that are in excellent shape. The same will probably not be true of the engines if they sat unused, but the airframes may be in great shape.

Every airframe is different, and a thorough pre-buy inspection will be necessary to ensure that yours does not suffer from some major structural issue. But age and hours alone should not be of real concern.

I would not hesitate to buy a high-time, vintage Cessna airframe merely on account of those factors.

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    $\begingroup$ How do you inspect for metal fatigue on a Cessna? Do you just visually inspect for cracks or other signs or do you get out special analysis tools (ultrasound or electrical resistance, etc.)? $\endgroup$ – TomMcW Jun 5 '16 at 0:48
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    $\begingroup$ You can get fancy with x-rays (Beech 18), dye pen (Be 35, 36, 55, 58, etc), or other NDT methods but there is nothing wrong with the old Mk. I Eyeball for most of the light singles out there. Think of it this way: both hypothetical planes expierence 1G just sitting in the hangar. In level unaccelerated flight they are at 1G. They might, for a tiny, tiny fraction of their lives, see 2G (60° bank). Trainers may spend more time "pulling Gs" but we're talking <2G in the vast majority of cases. If the 20k hour trainers haven't produced any ADs I'm reasonably confident that old 172s are fine. $\endgroup$ – acpilot Jun 5 '16 at 5:11

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