LET Blanik L-13 OE-0935 History Details?

Bart,

Thank you for being sensitive to our friends in Austria. Our problems with
the L-13’s are nothing compared to the issues the families of the pilots
have.

You asked about the effect of exceeding limits. Your logic is correct.
Ductile metals, such as aluminum, have known performances. Stress-Strain
data is what engineers use to design structural components. It will tell
them the Elastic Limits, Elongation Limits, Ultimate Tensile Strength, and
when a Fracture will occur.

Your comment of a “Theoretical” scenario of constantly flying 20% above
the limit might result in a reduced life of the spare.

My “Theoretical” scenario would be one flight where the pilot got into
trouble and accidentally exceeded 50%. This would take the spare past it
Elastic Limits and significantly reduce it’s life. To add to my scenario,
if the limits were exceeded 10-20 years ago, the oxidization build up on
the aluminum might mask that damage. A fresh fracture would be shiny
because of the absence of oxidation.

Again, this is a theory that has almost no data backing it. However,
there are approximately 3000 Blaniks in the world that have been flying
for 2-3 decades. That is a solid record that speaks to the design life.
The L-13 in question is low time and had been over hauled. It does not
appear to be the norm.

Since this is a fatality, the investigators will get to the root cause for
the accident. They will probably review the design calculations. They
might also do a metallurgic test to determine the pedigree of the aluminum
in the spare. Their investigation will take time. We probably won’t see a
report for a year. (Again, my guess)

Andrew




At 22:21 30 August 2010, Bart wrote:
If a glider is routinely overstressed by, say, 20%...

I would like to add that the scenario above is purely theoretical and
I am not trying to suggest that the accident in Austria was caused by
anyone who has flown the glider in question in the past.

B.

Andrew, there would be no masking due to oxidation. Anyone who has
ever looked under a microscope at a fatigue crack scenario leading to
overload failure would agree with Vaughn above, who said "accident
investigators can fairly positively tell the difference between a
structure that has failed from overstressing and one that has failed
from fatigue." Believe me, if this failure had fatigue as its initial
cause, the evidence will be very clear - the fatigue "bench marks"
won't disappear. An overload due to high Gs that blunted the fatigue
crack temporarily will also be evident.

-John


On Aug 31, 10:12 am, Andrew Corrigan
wrote:
if the limits were exceeded 10-20 years ago, the oxidization build up on
the aluminum might mask that damage. A fresh fracture would be shiny
because of the absence of oxidation.

John,

My intent is to keep an open mind on the root cause of the failure.
Prematurely stating this is fatigue issue is a mistake.

The investigation will have to look at many aspects. How was the glider
used or abused? It might include metallurgic analysis to ensure the
aluminum spare was made from the correct type of alloy, tempered properly,
and has no inclusions, etc.

As a person that has looked through a microscope at granular structure of
metal I can tell you this. This is not a lab experiment where all the
variables are controlled. There will be a lot of unknowns because of the
age of the glider, how it was flown, and how it was cared for. As such,
this is not a text book case where engineers can pull out a chart and look
up data to determine a resultant.

As an aerobatic pilot, I can tell you this. We make mistakes. If trained
properly a pilot will be able to recover from any situation without
exceeding the glider’s limits. However, inverted recoveries are not
intuitive. A simple mistake while inverted can quickly lead to the glider
going over Vne or exceeding G limits. It’s easy to do. Something happens;
the pilot gets caught off guard, takes a few seconds to figure things out,
mean while the glider is accelerating towards the ground. If inverted,
pulling on the stick will cause the glider to accelerate. The pilot may
never have known the ASI was above Vne. I’m not saying this happened to
the glider in question.

What I am saying is this “Don’t label the root cause as fatigue until the
investigators have done their due diligence and reached a conclusion based
on ALL the data!”. Labelling the issue as fatigue without data is not just
when there are thousands of these gliders with tens of thousand hours
without failures.

Andrew



At 14:44 31 August 2010, jcarlyle wrote:
Andrew, there would be no masking due to oxidation. Anyone who has
ever looked under a microscope at a fatigue crack scenario leading to
overload failure would agree with Vaughn above, who said "accident
investigators can fairly positively tell the difference between a
structure that has failed from overstressing and one that has failed
from fatigue." Believe me, if this failure had fatigue as its initial
cause, the evidence will be very clear - the fatigue "bench marks"
won't disappear. An overload due to high Gs that blunted the fatigue
crack temporarily will also be evident.

-John


On Aug 31, 10:12 am, Andrew Corrigan
wrote:
if the limits were exceeded 10-20 years ago, the oxidization build up
on
the aluminum might mask that damage. A fresh fracture would be shiny
because of the absence of oxidation.

Update on Blanik ADs and Manfacturer Bulletins
8-31-2010

The continuing airworthiness issues, manufacturer’s (LET / Aircraft
Industries) Mandatory Bulletins, European Aviation Safety Administration
(EASA) Airworthiness Directives and the Federal Aviation Administration
(FAA) ADs concern SSA as well as the owners and operators of the affected
L-13 and L-13A Blaniks. It is likely that most of the Blanik fleet in the
United States and around the world will be affected and/or grounded before
the manufacturer and the European agencies issue appropriate methods to
inspect and modify the aircraft to prevent recurrence of the problems that
have led to structural failure and death. The SSA has worked with FAA
since the beginning of the current spar problems.

At this point the manufacturer and EASA have not developed or approved an
effective inspection or modification program to ensure safe continued
flight of the affected aircraft.

However, the FAA’s latest AD states that the flight-hour record-keeping
requirements of the manufacturer do not apply to American operators and
that the FAA will review Alternative Methods of Compliance (AMOC’s)
presented to them. These statements are encouraging for American
operators.

SSA does not have the expertise to engineer solutions, but some of our
members may. SSA encourages qualified members who work on aircraft
inspection and repair methods to contact Greg Davison, Aerospace Engineer
at FAA’s Small Airplane Directorate in Kansas City at (816) 329-4130.
Also please send copies of correspondence with the FAA to SSA and we will
summarize and publish information about them on this site on a regular
basis.

SSA will also use this page to publish other written reports of
developments or offers of assistance. In particular, FAA is working on an
informational statement that it hopes to forward to SSA for publication
in the next few days. At the moment SSA believes that acting as an
information clearinghouse is its most appropriate course of action while
it encourages its members and the owners/operators of Blaniks to work
together to develop technical solutions.

Finally, SSA also believes that owners and operators of affected Blaniks
should make the manufacturer (LET / Aircraft Industries) and its American
representative (Blanik America) aware of their concerns and the need for
their help and timely solutions. Meanwhile, owners with valid questions
and suggestions continue to be invited to contact the FAA as stated in
the AD’s.

SSA Aircraft Certification Committee

Posted: 8/31/2010 By: Government Liaison Committee

Many years (and brain cells) ago I recall looking at an old L-13
spar end cap and it had a very large number of bolts going through
it. It seemed like it was very close to what I remember as a hole
to edge and hole to hole distance requirement. My calibrated
thumb nail showed it to be within what I remember as a 2D
distance but I remember that from rivet holes to edge on wing
skins. I wonder if some of this is caused by hole to hole to edge or
radius cracks and if so that would be a very serious concern. Also,
I believe that the 2D used to be 1.5D and there had been questions
if that was hole edge to edge or hole center to edge.