L13 Blanik Mandatory Bulletin

Let's not overreact here.

It is incorrect to say that all metallic materials have a finite
fatigue life. Metal structures have been certified with an infinite
fatigue life.

I doubt anyone here is a structures engineer with experience in, or
access to DaDT tools.

If you want to get closer to the real story, There is an EXCELLENT,
plain language discussion of the design considerations and fatigue
calculations in the Blanik repair and overhaul manuals published in
the mid 70's. The Factory went to great lengths to educate the
operators, more than I have seen in ANY light aircraft manual or
publication. Sadly, many do not RTFM, put prefer to pontificate in
forums.

Corrosion is the real intractable issue with metals, and causes much
more cumulitive damage in aircraft structures than fatigue alone. I
would inspect visually the affected area, as stress corrosion cracking
in the short tranverse direction in common in high heat treat or alloy
metals, and can appear to be fatigue at first blush.

Aerodyne

And just in case pilots of composite aircraft feel good due to the
high fatigue life of their planes and general lack of corrosion
sensitivity, be aware that composites are very susceptible to out of
plane impact damage. A stone hitting a composite can cause (almost)
invisible delamination between layers inside the composite. The effect
is that one moment you're flying along without a care and the next
moment you have a huge hole in your wing or control surface (for
example).

German manuals tell you to look for "pressure damage"; what they mean
is to look for a small nick made by a stone hitting the composite. If
you find such a nick, a simple way to determine if there is underlying
delamination is to use a 1/8" diameter drill rod and drop its blunt
end onto the composite's surface from about an inch away. If you hear
a high pitch ring, it's OK; if you hear a dull thunk sound you've got
a delamination. You can get an idea how big it is by marking out the
extent of the area where the dull thunk sound is heard. Ultrasonic
inspection is used to search for delamination damage in military and
commerical aircraft.

-John

On Jun 24, 7:48*pm, wrote:

It is incorrect to say that all metallic materials have a finite
fatigue life.

There are indeed metals that have effectively infinite fatigue lives.
Many steel and titanium alloys have that property. Though even among
those there is some evidence that the stress/cycle graph never goes
entirely asymptotic, I agree that it is fair to say that their fatigue
lives are essentially infinite.

Aluminum, however, does not have the "knee" in the stress/cycle graph
that takes it effectively parallel with the X axis. With aluminum, the
curve heads inexorably towards the X axis.

Metal structures have been certified with an infinite
fatigue life.

That is almost always true for aircraft certified under the old CAR 3
regulations which did not address fatigue. It is even true for some
aircraft certified under the more modern Part 23 and JAR22
regulations. Unfortunately, the map is not the territory: Just because
the CAA or whoever certified that it is so doesn't mean it is actually
so.

There is an EXCELLENT, plain language discussion of the design
considerations and fatigue calculations in the Blanik repair and
overhaul manuals published in the mid 70's.

That sounds like valuable material, I would definitely like to read
it. How can I get a copy of that documentation?

Corrosion is the real intractable issue with metals, and causes much
more [cumulative] damage in aircraft structures than fatigue alone.

I completely agree there. Corrosion is and should be a far more
pressing concern that fatigue alone. Very often, failures that
initially appear to have resulted from fatigue are actually more
directly caused by corrosion that reduces the effective cross-
sectional area and causes stress risers that result in local yielding
and accelerated fatigue.

Thanks, Bob K.

Inspection was done on our Blahnik this afternoon....

Our Blahnik has a low number of flight...ie. only 1200 hours with 1750
starts.
No acrobatics flight except a few rare loopings...

Always aerotowed...No winch launches...

Inspection with eyes and bright light showed nothing. We took pictures
wtih a small digital cam equipped with a flash (small Olympus one).

When we uploaded the pictures on the PC, we carefully looked at the
pictures and we saw ...longs thin cracks...
With the eyes it was impossible to detect due to the fact that the
available room for inspection is very limited....

We spoke in the previous post about fatigue....well, I am deeply
convinced that our Blahnik has never been "fatigued"....
The cracks seems to be produced because the rivet from which the
cracks are born, has been installed by a hand process during the
manufacturing. The manual process for installing these rivets is clear
on the picture I took.
I strongly suspect a poorly manual manufacturing process instead of a
fatigue problem.

I am really upset to discover this problem now especially that there
is no repair procedure nor a replacement procedure.

The AD just states that the Blanik is grounded. This is unacceptable

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 25, 8:41*am, nimbus wrote:

When we uploaded the pictures on the PC, we carefully looked at the
pictures and we saw ...longs thin cracks...

I would be really skeptical about things that show up in a digital
photo that don't appear under direct observation. It could be that
what looks like cracks are artifacts of the digital photography or
JPEG compression. I would recommend you get a second opinion before
taking any action.

Also, it could be that what looks like cracks are actually just normal
striations from the extrusion process by which the spar caps are
formed. They could also be local disruptions in the surface anodizing,
if the parts are anodized. We've seen that around rivets on the
anodized spars of RV-series homebuilt airplanes, and it has not been a
structural concern.

Thanks, Bob K.

On Jun 24, 8:48*pm, wrote:
Let's not overreact here.

It is incorrect to say that all metallic materials have a finite
fatigue life. * Metal structures have been certified with an infinite
fatigue life.

I doubt anyone here is a structures engineer with experience in, or
access to DaDT tools.

If you want to get closer to the real story, There is an EXCELLENT,
plain language discussion of the design considerations and fatigue
calculations in the Blanik repair and overhaul manuals published in
the mid 70's. *The Factory went to great lengths to educate the
operators, more than I have seen in ANY light aircraft manual or
publication. *Sadly, many do not RTFM, put prefer to pontificate in
forums.

Corrosion is the real intractable issue with metals, and causes much
more cumulitive damage in aircraft structures than fatigue alone. I
would inspect visually the affected area, as stress corrosion cracking
in the short tranverse direction in common in high heat treat or alloy
metals, and can appear to be fatigue at first blush.

Aerodyne

I know of a couple of well trained structural guys in this discussion.

The Blanik maintenance manuals are good and they say about the same
thing Bob K is saying. Remember, it was a Blanik that started this
thread.

We're talking aluminum gliders here, not titanium spaceships. Every
aluminum glider ever made is subject to fatigue failures.

On Jun 25, 7:34*am, jcarlyle wrote:
And just in case pilots of composite aircraft feel good due to the
high fatigue life of their planes and general lack of corrosion
sensitivity, be aware that composites are very susceptible to out of
plane impact damage. A stone hitting a composite can cause (almost)
invisible delamination between layers inside the composite. The effect
is that one moment you're flying along without a care and the next
moment you have a huge hole in your wing *or control surface (for
example).

Of course, you have dozens of examples of huge holes in wings and
control surfaces resulting from stone strikes - right?.


German manuals tell you to look for "pressure damage"; what they mean
is to look for a small nick made by a stone hitting the composite. If
you find such a nick, a simple way to determine if there is underlying
delamination is to use a 1/8" diameter drill rod and drop its blunt
end onto the composite's surface from about an inch away. If you hear
a high pitch ring, it's OK; if you hear a dull thunk sound you've got
a delamination. You can get an idea how big it is by marking out the
extent of the area where the dull thunk sound is heard. Ultrasonic
inspection is used to search for delamination damage in military and
commerical aircraft.

-John

A coin works as well as a drill and yes, everybody who owns a
composite glider probably knows the trick. Delaminated areas are easy
to fix.

As for aging gracefully, try parking a mid sixties Labelle next to any
aluminum glider. I'll take composite structures any day. However,
I'll stay away from 40 year old metal gliders.

I completely agree there. Corrosion is and should be a far more
pressing concern that fatigue alone. Very often, failures that
initially appear to have resulted from fatigue are actually more
directly caused by corrosion that reduces the effective cross-
sectional area and causes stress risers that result in local yielding
and accelerated fatigue.

Thanks, Bob K.

Corrosion and fatigue work hand in hand. Corrosion begets fatigue and
fatigue begets corrosion. Together, they can cause mayhem.

Bill D

It would be a coincidence but could this be one of the Red Bull
Blaniks? They have Austrian registration and they get flown pretty
hard.
Is there a news article on the accident?

Regards,

Juan Carlos