L13 Blanik Mandatory Bulletin

[Ernst]

After a fatal accident in Austria LET issued a Mandatory Bulletin:
http://www.let.cz/files/bulletines/MB_L13_109a_english.pdf

EASA issued an Emergency Airworthiness Directive:
http://www.caa.co.uk/docs/33/easa_ad_2010_0119_E.pdf

[alautert]

On Jun 22, 7:50*pm, Ernst > wrote:
> After a fatal accident in Austria LET issued a Mandatory Bulletin:http://www.let.cz/files/bulletines/MB_L13_109a_english.pdf
>
> EASA issued an Emergency Airworthiness Directive:http://www.caa.co.uk/docs/33/easa_ad_2010_0119_E.pdf

Does anyone know if the L23 could have similar problem?

[BT[_3_]]

Reading the MB, It appears that based on the "flight data" of the aircraft,
and high number of launches per flight hour, the concern is overstressing
from repeated winch launches. LET is requesting all users submit flight
statistics for further evaluation plus a one time inspection of the load
bearing aft spar cap in the fuselage carry through structure.

I do not know if the L-23 and the L-13 have the same construction for the
aft spar cap.

"Ernst" > wrote in message
...
> After a fatal accident in Austria LET issued a Mandatory Bulletin:
> http://www.let.cz/files/bulletines/MB_L13_109a_english.pdf
>
> EASA issued an Emergency Airworthiness Directive:
> http://www.caa.co.uk/docs/33/easa_ad_2010_0119_E.pdf

[Derek C]

Provided that the correct weak link is always used, it should not be
easily possible to damage a glider by winch launching it. What
aerobatics had this glider done?

Derek C


On Jun 23, 3:23*am, "BT" > wrote:
> Reading the MB, It appears that based on the "flight data" of the aircraft,
> and high number of launches per flight hour, the concern is overstressing
> from repeated winch launches. LET is requesting all users submit flight
> statistics for further evaluation plus a one time inspection of the load
> bearing aft spar cap in the fuselage carry through structure.
>
> I do not know if the L-23 and the L-13 have the same construction for the
> aft spar cap.
>
> "Ernst" > wrote in message
>
> ...
>
>
>
> > After a fatal accident in Austria LET issued a Mandatory Bulletin:
> >http://www.let.cz/files/bulletines/MB_L13_109a_english.pdf
>
> > EASA issued an Emergency Airworthiness Directive:
> >http://www.caa.co.uk/docs/33/easa_ad_2010_0119_E.pdf- Hide quoted text -
>
> - Show quoted text -

[John Smith]

Am 23.06.10 04:23, schrieb BT:
> Reading the MB, It appears that based on the "flight data" of the
> aircraft, and high number of launches per flight hour, the concern is
> overstressing from repeated winch launches. LET is requesting all users

Sigh. The urban legend lives.

[Bob Kuykendall]

On Jun 23, 6:58*am, John Smith > wrote:
> Am 23.06.10 04:23, schrieb BT:
>
> > Reading the MB, It appears that based on the "flight data" of the
> > aircraft, and high number of launches per flight hour, the concern is
> > overstressing from repeated winch launches. LET is requesting all users
>
> Sigh. The urban legend lives.

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
fatigue and to make a measurable difference in long-term service
life.

When fatigue has weakened a structure, and then you apply a load that
exceeds the diminished capacity of that structure, it will fail. You
can say that it was overstressed, or you can say that it wasn't
overstressed and that it just exhibited a fatigue failure. Whatever
you call it, you still have a broken structure.

Thanks, Bob K.

[John Smith]

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.

[Derek C]

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

[John Smith]

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.

[Bob Kuykendall]

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.

[bildan]

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.

[Tony[_5_]]

>
> 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.

[Derek C]

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

[Judah Milgram]

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.

[Bob Kuykendall]

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.

[harold]

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.

[Judah Milgram]

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

[Bob Kuykendall]

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.

[ZZ]

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

[bildan]

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.

[[email protected]]

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

[jcarlyle]

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

[Bob Kuykendall]

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.

[nimbus]

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

[Sandy Stevenson]

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.

[Bob Kuykendall]

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.

[bildan]

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.

[bildan]

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.

[bildan]

> 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

[JC]

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

[Frank Whiteley]

On Jun 25, 3:42*pm, JC > wrote:
> 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

http://aviation-safety.net/wikibase/wiki.php?id=74748

Link to an image of the glider in hangar on page. Not a Red Bull
Blanik.

Frank Whiteley

[Walter[_2_]]

Blanik owners and pilots might want to read this call for help from the home
of the Red Bull Blanix team:
http://www.streckenflug.at/popup.php?xi=news/blanik_hilfe.pdf&xy=J

and this question from the author of the above PDF:
>>
Hello,
Does anyone know Detail Information on Blaník which have been modified to
L-13 Type A1, the so called "Llewellyn Modification"?
Even information on the Aeroclub that does this Modification or contact
data, also the name of the Aeroclub would be very helpful for me !

Happy landings and Best Regards!
<<

Walter

[T8]

On Jun 25, 11:41*am, nimbus > wrote:
> 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

Would you consider posting those pics somewhere?

- Evan Ludeman / T8

[Ronald Locke]

After a fatal accident in Austria LET issued a Mandatory Bulletin:
http://www.let.cz/files/bulletines/MB_L13_109a_english.pdf

EASA issued an Emergency Airworthiness Directive:
http://www.caa.co.uk/docs/33/easa_ad_2010_0119_E.pdf

Could anyone advise the current situation in USA, Canada and Europe regarding the grounding of L13 Blaniks ?

Have many been put back into service as a result of satisfactory wing spar inspections, or do most remain grounded due to the fact that the required average usage statistics required by LET cannot be met, and with respect seem most unreasonable.

For instance – what two seater training glider (or for that matter any two seater glider) would have an average of 35% dual and 65% solo usage! More likely, greater than 80% dual would be a more realistic figure I would have thought.

[Tony[_5_]]

On Jul 12, 9:39*pm, Ronald Locke <Ronald.Locke.
> wrote:
> Ernst;732355 Wrote:
>
> > After a fatal accident in Austria LET issued a Mandatory Bulletin:
> >http://www.let.cz/files/bulletines/MB_L13_109a_english.pdf
>
> > EASA issued an Emergency Airworthiness Directive:
> >http://www.caa.co.uk/docs/33/easa_ad_2010_0119_E.pdf
>
> Could anyone advise the current situation in USA, Canada and Europe
> regarding the grounding of L13 Blaniks ?
>
> Have many been put back into service as a result of satisfactory wing
> spar inspections, or do most remain grounded due to the fact that the
> required average usage statistics required by LET cannot be met, and
> with respect seem most unreasonable.
>
> For instance – what two seater training glider (or for that matter any
> two seater glider) would have an average of 35% dual and 65% solo usage!
> More likely, greater than 80% dual *would be a more realistic figure I
> would have thought.
>
> --
> Ronald Locke

I haven't heard that the FAA has issued an airworthiness directive
yet, so no L-13's in the USA should be grounded.

[John Scott[_3_]]

Ours was inspected per the bulletinby 2 IAs (concurrently), deemed
airworthy, and signed off. We're flying it.
It had just been down for an annual when the bulletin was released.

John

[Bob McKellar]

"John Scott" > wrote in message
.. .
> Ours was inspected per the bulletinby 2 IAs (concurrently), deemed
> airworthy, and signed off. We're flying it.
> It had just been down for an annual when the bulletin was released.
>
> John
Same story for my club, almost.

We pulled the wings and inspected everything and it went fine. Ours is
heading into annual next month, but we didn't want to wait.

Bob McKellar

[[email protected]]

> 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.

I see that for some Al alloys and with unnotched coupons, but most all
aircraft metallic structure are notched because of fastener holes. I
am looking at one notched curve now that goes parallel at 10^7
cycles. No glider is going to see the 60-80000 hr life that large
aircraft have proven by service and test. For example, the rewinged
C-5b has an expected service life of 100,000 hrs, validated by fatigue
test. After the test they cut one panel, notched another, and were
unable to get a failure.
>
> > 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.

I was referring to among other things, rotor blades cetified in the
70's, that had fatigue, DADT anaylysis and full scale test tests done
with simulated damage.


>
> > 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?

I believe the manuals that came with our blanik are available on the
Blanik America site. I think you will find them of great interest,
esp since the EASA AD now is also a US AD as of this Friday.

aerodyne