L13 Blanik Mandatory Bulletin

On Jun 23, 3:58*pm, John Smith wrote:
Bob Kuykendall wrote:
Can you please clarify that remark? I don't see any urban legendry
here.

Winch launching inherently load the wings in bending more than
aerotow. Maybe not enough to actually exceed the design load of the
wings. But certainly enough to contribute a substantial amount of

Provided the correct weak link is used, the wing load during a winch
launch never exceeds 2g. Each turbulence stresses the structure more
than that. But as I've already said: Urban legends are here to stay.

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 have looked at the newspaper reports on this accident, but
they don't state which phase of flight the glider was in when it broke
up. They say that it crashed into a forest, which sounds as though it
was in free flight, rather on a winch launch when it would have been
over the airfield. Does anyone have any better information?

Derek C

Derek C wrote:
failure. I have looked at the newspaper reports on this accident, but
they don't state which phase of flight the glider was in when it broke

To my knowledge, the Blanik was aerotowed to about 1200m AGL, from where
an aerobatics program was flown. The wing broke only after the
aerobatics had been finished. It was too low for the two pilots to bail out.

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.