L13 Blanik Mandatory Bulletin

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.

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.

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.

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

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.

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.

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

On Jun 23, 9:23*pm, Judah Milgram wrote:
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
- Hide quoted text -

- Show quoted text -

You may be right for the U.S., but I don't believe you are correct for
Canada.
I haven't had a chance to check the precise regulation, but a
mandatory bulletin from a
manufacturer or EASA becomes mandatory in Canada due to cooperation
agreements.
Canadian L-13's are therefore grounded until the AD is complied with.

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.