On Jun 24, 9:40*am, Bob Kuykendall wrote:
On Jun 23, 9:58*pm, "Morgans" wrote:

If that is indeed the case, the glider in question is improperly rated.

An aircraft should be rated to take the stated load, of say 6G positive, to
4G negative, _PLUS_ a safety factor, usually from 125% to 150% *of the
maximum load. (and here is the big kicker in this case) *_WITHOUT_ doing
permanent damage at the rated load. *NEVER doing damage. *Not even fatigue
damage after "X" number of cycles.

If it is being flown at or under the rated loads and fatigue cracking or
other damage is happening, it is not designed to the load that was stated,
IMHO. *It should be de-rated or strengthened.

Well, that pretty much rules out every aircraft with aluminum
structure, no matter how stout.

As I wrote previously, with aluminum there is absolutely no level of
stress that is so low that it does not cause fatigue. An infinitesimal
deflection repeated an infinite number of times will cause fatigue
failure.

Please don't misconstrue, I am not saying that aluminum structures are
inherently unsafe. They can be designed and engineered so that they
provide very good safety over perfectly reasonable service lives.
However, they cannot be made immune from fatigue. That is why they
have established (or at least estimated) service lives.

As a practical matter, the more structural margin an aircraft has, the
greater its resistance to fatigue. However, extra margin means extra
weight and decreased performance and utility. As with everything in
aircraft design and engineering, it is a compromise and a balancing
act.

As a regulatory matter, if I recall correctly, service life limits
were not specifically required under the old CAR 3 regulations under
which vast numbers of aircraft, including many sailplanes and training
gliders still in service, were certificated up through the 1950s and
perhaps 1960s. However, the more modern 14CFR Part 23, and very
similar JAR22 do require an evaluation of the structure to determine
that fatigue damage will not occur within the operational life of the
aircraft.

One example I know of where service life is an issue in general
aviation airplanes is the center and outboard main wing spars of the
AA1 and AA5 series of small airplanes built by Grumman, Gulfstream,
and others. The aircraft type certificate specifically limits the
service lives of these components to 12000 and 12500 hours
respectively, and many examples of the type have come up against these
limits.

Thanks, Bob K.

Bob, You've stated this about as clearly as it can be stated and
you're perfectly correct. Bend metal and it fatigues - keep bending
it and it will eventually fail. The damage starts at zero hours and
gets worse from there. Stated or not, there's always a life limit for
metal aircraft. There's just no way around it.

Any old, high-time metal glider is suspect. If they've spent years
out in the weather where corrosion can start in those tiny fatigue
cracks, they're even more suspect. Something to think about when you
hear the boink! boink! of "oil-canning" aluminum skins.

The way it usually works is sometime, somewhere, a wing falls off and
the aviation authorities order every owner/operator of that type to
take a really good look inside their wings. If the results are ugly,
there will be an AD ordering replacement of the fatigued metal. The
owner/operator gets to decide whether its economic to do so.
Sometimes it isn't and the aircraft gets scrapped. We're all a little
bit of a test pilot.