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.