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L13 Blanik Mandatory Bulletin



 
 
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  #11  
Old June 23rd 10, 08:32 PM posted to rec.aviation.soaring
bildan
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Posts: 646
Default L13 Blanik Mandatory Bulletin

On Jun 23, 11:45*am, Bob Kuykendall wrote:
On Jun 23, 8:17*am, Derek C wrote:

Actually the maximum bending load on the wingspar during a winch is
equivalent to about 3 g, due to the point loading on the fuselage and
the lack of g unloading on the wings, but that still shouldn't cause a
failure...


I think that 3g equivalent load is a large enough percentage of the
limit load to constitute a fatigue concern. If I were assessing
service histories, I would definitely want to know the cycle count on
activities likely to cause that kind of load.

Thanks, Bob K.


Actually, it's probably worth worrying about any old, high-time metal
glider. Hard landings and turbulence flex the wings too. Blanik
maintenance manuals expressly limit the airframe life if used with
winch launch.

AFAIK, no composite glider has exhibited a failure mode anything like
this.
  #12  
Old June 23rd 10, 09:47 PM posted to rec.aviation.soaring
Tony[_5_]
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Posts: 1,965
Default L13 Blanik Mandatory Bulletin


AFAIK, no composite glider has exhibited a failure mode anything like
this.


yet. everything wears out eventually. even my beloved wood gliders
will probably eventually wear out. but at least the parts grow on
trees.
  #13  
Old June 23rd 10, 11:51 PM posted to rec.aviation.soaring
Derek C
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Posts: 114
Default L13 Blanik Mandatory Bulletin

On Jun 23, 6:45*pm, Bob Kuykendall wrote:
On Jun 23, 8:17*am, Derek C wrote:

Actually the maximum bending load on the wingspar during a winch is
equivalent to about 3 g, due to the point loading on the fuselage and
the lack of g unloading on the wings, but that still shouldn't cause a
failure...


I think that 3g equivalent load is a large enough percentage of the
limit load to constitute a fatigue concern. If I were assessing
service histories, I would definitely want to know the cycle count on
activities likely to cause that kind of load.

Thanks, Bob K.


Most modern gliders are stressed to take at least +5.3/-2 g without
damage. A winch launch comes nowhere near this as long as the correct
weak link is fitted, which will break well before the glider does. I
believe that the glider that failed had being doing aerobatics
immediately before, which is a more likely cause of any
overstressing.

Derek C
  #14  
Old June 24th 10, 12:35 AM posted to rec.aviation.soaring
Judah Milgram
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Posts: 2
Default L13 Blanik Mandatory Bulletin

On Jun 23, 6:51*pm, Derek C wrote:
On Jun 23, 6:45*pm, Bob Kuykendall wrote:

On Jun 23, 8:17*am, Derek C wrote:


Actually the maximum bending load on the wingspar during a winch is
equivalent to about 3 g, due to the point loading on the fuselage and
the lack of g unloading on the wings, but that still shouldn't cause a
failure...


I think that 3g equivalent load is a large enough percentage of the
limit load to constitute a fatigue concern. If I were assessing
service histories, I would definitely want to know the cycle count on
activities likely to cause that kind of load.


Thanks, Bob K.


Most modern gliders are stressed to take at least +5.3/-2 g without
damage. A winch launch comes nowhere near this as long as the correct
weak link is fitted, which will break well before the glider does. I
believe that the glider that failed had being doing aerobatics
immediately before, which is a more likely cause of any
overstressing.

Derek C


If I recall correctly, the concern was with fatigue damage
accumulating at loads below the limit load. If fatigue cracks do form,
you could get a static failure below limit load - and not necessarily
during a winch launch. How serious this concern should be in the case
of the L-13 I couldn't say but given that they think it might have
been a fatigue crack, the AD seems pretty reasonable.

JM.
  #15  
Old June 24th 10, 01:27 AM posted to rec.aviation.soaring
Bob Kuykendall
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Posts: 1,345
Default L13 Blanik Mandatory Bulletin

On Jun 23, 3:51*pm, Derek C wrote:

Most modern gliders are stressed to take at least +5.3/-2 g without
damage...


It is true that operation within the limit load should not cause any
damage in the sense of bent, stretched, or torn structure that
precludes continued operation. However, at the same time every single
load cycle causes the accumulation of fatigue that, given a long
enough service life, will eventually cause failure.

Aluminum structures require careful consideration of fatigue and
service life, since there is no level of stress below which fatigue
does not accumulate. If you take an aluminum wing designed to a 5.3g
limit load and load cycle it from 0g to 1g and back a relatively large
number of times, it will eventually break without ever having been
stressed over 1g. That is one of the reasons that there is some margin
(usually 50%) between limit load and ultimate load.

The key, question, of course, is how many cycles does it take? The
extreme example I cite above will probably take many, many times the
number of cycles equivalent to the flight hours in the planned service
life. Given greater loads, the number of cycles to failure is reduced.
And of course, given lower loads, the number of cycles is increased.
But the important thing is that, for aluminum at least, there is no
level of loading at which the cycle count goes to infinity. An
infinitesimal loading, repeated enough times, will result in eventual
failure.

Thanks, Bob K.

  #16  
Old June 24th 10, 02:31 AM posted to rec.aviation.soaring
harold
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Posts: 11
Default L13 Blanik Mandatory Bulletin

On Wed, 23 Jun 2010 16:35:56 -0700 (PDT), Judah Milgram
wrote:

snip

Most modern gliders are stressed to take at least +5.3/-2 g without
damage. A winch launch comes nowhere near this as long as the correct
weak link is fitted, which will break well before the glider does. I
believe that the glider that failed had being doing aerobatics
immediately before, which is a more likely cause of any
overstressing.

Derek C


If I recall correctly, the concern was with fatigue damage
accumulating at loads below the limit load. If fatigue cracks do form,
you could get a static failure below limit load - and not necessarily
during a winch launch. How serious this concern should be in the case
of the L-13 I couldn't say but given that they think it might have
been a fatigue crack, the AD seems pretty reasonable.

JM.


Actually and AD has not been issued by the FAA. A mandatory bulletin
from the manufacture has been issued. There is a huge difference. An
AD is mandatory in the US. A mandatory bulletin by the manufacture is
optional.

  #17  
Old June 24th 10, 04:23 AM posted to rec.aviation.soaring
Judah Milgram
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Posts: 2
Default L13 Blanik Mandatory Bulletin

On Jun 23, 9:31*pm, harold wrote:
On Wed, 23 Jun 2010 16:35:56 -0700 (PDT), Judah Milgram

wrote:

snip





Most modern gliders are stressed to take at least +5.3/-2 g without
damage. A winch launch comes nowhere near this as long as the correct
weak link is fitted, which will break well before the glider does. I
believe that the glider that failed had being doing aerobatics
immediately before, which is a more likely cause of any
overstressing.


Derek C


If I recall correctly, the concern was with fatigue damage
accumulating at loads below the limit load. If fatigue cracks do form,
you could get a static failure below limit load - and not necessarily
during a winch launch. How serious this concern should be in the case
of the L-13 I couldn't say but given that they think it might have
been a fatigue crack, the AD seems pretty reasonable.


JM.


Actually and AD has not been issued by the FAA. *A mandatory bulletin
from the manufacture has been issued. *There is a huge difference. *An
AD is mandatory in the US. *A mandatory bulletin by the manufacture is
optional.


You're right, the subject AD was issued by EASA, not FAA. But given
that a wing just failed due to a possible fatigue crack, most US
owners will probably want to comply anyway (just guessing here).

Judah Milgram

  #18  
Old June 24th 10, 04:40 PM posted to rec.aviation.soaring
Bob Kuykendall
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Posts: 1,345
Default L13 Blanik Mandatory Bulletin

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.
  #19  
Old June 24th 10, 07:01 PM posted to rec.aviation.soaring
ZZ
external usenet poster
 
Posts: 68
Default L13 Blanik Mandatory Bulletin

On 6/23/2010 9:58 PM, Morgans wrote:
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.


Isn't this true only until the maximum service life has been reached?

Paul
ZZ
  #20  
Old June 25th 10, 12:18 AM posted to rec.aviation.soaring
bildan
external usenet poster
 
Posts: 646
Default L13 Blanik Mandatory Bulletin

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.
 




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