CONDOR

I just found out that there exists a Modification for the Wing Main Spar of Blaník L-13. The type designation of such a altered Blaník is „Blaník L-13 A-1“ and this Type of Blaník is not affected by the current EASA Emergency Airworthiness Directive which means they are still allowed to fly on Blaník, and also doing Aerobatics. This could be a lifesaver for our future with Blaník !
It´s up to us now – we have to show the Manufracter, Let Aircraft in Kunovice that we are interested in such a Modification and even show them that we are interested in continuing the operation of Blaník. We have to stick together and do something because it would be to bad for such a unique plane to let it laying around anywhere in the hangar without use. Blaník was built for flying not for staying in the Hangars!
Let´s stick together - we have to find out all about this this modification –copy it or at least find a similar solution.
Please inform all Blaník Owners and Pilots you know around the world and spark the interest of Let Aircraft Industries in doing such a Modification on our Gliders, too! Please ask all Pilots you know about that Modification, we have to find out all technical data and Information!
I wrote a email to Let Aircraft yesterday, asking them about Information on such a Modification and showing them the Interest of our Aeroclub and Blanix Mirrorflight Team to continue the operation of Blaník.
We want to provide with all the opportunities to continue Instruction of Gliding Students and of course - display performance of Blanix Mirrorflight Team in the future.
Maybe your aeroclubs and Owners of Blaník can also write a Email or a brief letter to Let Kunovice to show them that we want to have a Future for our Blaník Gliders!
Let´s stick together now - Otherwise we have to fear that we will never ever do aerobatics or even normal Flights on Blanik again.

PLEASE help me in saving the future of Blaník! Happy Landings, Elisa

Hello Elisa,

aerobatics may be limited to certain amount of flight hours or even forbidden, but Blaniks will fly. Maybe not every Blanik, but they will. As far as I know (judging from the situation in our club and info from other clubs in the Czech Republic) this EASA directive is just a minor problem if all the manufacturer bulletins are carried out and the plane is maintained properly. Similar (and maybe more thorough) checks are performed everytime a Blanik reaches certain amount of flight hours and its operator asks for service life prolongation. Unfortunately those prolongation bulletins are only valid and required for Blaniks operated in the Czech Republic, so they probably are not performed abroad. But thanks to this it took only 3 days to sort things out in our club and we can fly again.

As for the L13 A-1. Are you sure about this? There is nothing about this version in the monograph about Blaniks. The only website I found referring to L-13A1 is The Gliding Federation of Australia and it seems that L-13A1 on that site is just another name for L-13A (which is affected by the EASA directive).

I'm sure that people in the Let factory will try to help you, but it may be quite expensive. Maybe you should consider buying L-13ACs for aerobatic display and use L-13s just for gliding...

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EASA has just published a new AD: EASA AD 2010-0185-E.

It grounds all Blaniks with immediate effect, including L-13, L-13A and
all aircraft that passed earlier AD inspections. Reason is that the failed
wing spar on the Austrian accident glider failed before reaching its
theoretical lifetime, and it failed due to fatigue.

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Chris Wedgwood,
Condor Team

Safety first... but EASA is going too far imho, basically they crippled soaring in the Czech Republic and Slovakia for this season.... Just by applying the principle of collective guilt they decided to ground 2500+ planes all around the world without considering their actual condition. We have a Blanik with brand new wings and overhauled fuselage (= new aircraft with 0 hours) since this spring. It flew 50 hours tops this year, no aerobatics and it's grounded too. EASA didn't even listen to Czech Civil Aviation Authority which expressed doubts about the austrian Blanik's condition and maintenance.

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Sorry JBr, but a wing fell off. This is a serious problem, and as it is identified as a fatigue failure of the main spar, you go on flying these aircraft at your peril. There was a similar problem, probably before you were born, with a glue called 'beetle' which had extensive use in some British gliders. It became brittle with time and eventually let go. All the aircraft which had it were grounded, inspected, and in the case of my Olympia had the tailplane completely rebuilt. You can't just ignore a safety directive because it doesn't suit you. How are you going to feel when the next wing comes off and two more pilots are killed?

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Old wooden ship pilot
Reg No. 464 (LLC 377)
CN: AW

I understand that, it is a serious thing. BUT the wing fell of a plane that no one proved to be properly maintained, flown within limits of its envelope and still had acro hours left. There hasn't been a single problem like this with Blaniks for the past 50 years, otherwise there would already be a special factory bulletin and something like that would hardly happen after that. Czech and Slovak Blaniks undergo periodic checks at Let factory because most of them have 4000+ hours and the checks are required for CAA's permission to extend limits to 4500, 5000 hrs (after reaching 5000 hrs the check is after every 100 hrs iirc). Prolongation is allowed at most cases. From time to time some Blanik's life is not extended, mainly because excessive amount of acro hours in the past.

It seems that some foreign operators are not required to undergo these checks or don't care about them. Guys from the Let factory even talked about a Blanik flying in Australia that has more than 10 000 hours (which would be impossible here) - probably local regulations are not as strict?

Believe me, judging from the response to this AD here in the CzechRep I am not the only one feeling bitter taste in my mouth, because the AD puts properly maintained and periodically checked planes on the same level as the "shady" ones. And we are afraid that the only result will be that we'll pay fortune for a check that will show nothing else than the previous (not imposed by some AD) did.

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I would agree with JBr. It really makes difference when gliders a properly maintained including professional checks on the factory where they were produced and somehow-in-the-field-self-maintained gliders.

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I agree it is unjust that the well maintained gliders are lumped in with those whose history is not well established. Fatigue however, is not a phenomenon which lends itself to exact prediction. Material specification can vary - machining and surface finish has a big influence - stress concentration round holes (remember the Comet?) - there are too many variables. I'm an engineering designer (trained by Rolls-Royce) and I have encountered many instances where a device failed well below the calculated life, particularly in situations where overload was possible. Even a small overload can reduce fatigue life dramatically. The correct solution for the Blanik (and I know this is expensive) is to re-inforce the main spars of all Blaniks. Even this re-inforcement is not an easy thing to design. Just thickening it up locally where it broke last often just moves the weakness to another stress point. Believe me this is a minefield. I had several cases where a complete re-design was required to cure the problem.

Let me put it like this: I would not now fly a Blanik until this weakness in the main spar has been eliminated by re-design, then overload, and cyclic testing of the proposed solution. I would suggest that a campaign by owners to get LET to tackle this problem vigorously and promptly is your best hope. Meanwhile your beloved Blaniks are grounded. It's horrible I know, but lives are at stake here. Gliding has a good reputation for keeping its house in order. Let's not throw that away.

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Old wooden ship pilot
Reg No. 464 (LLC 377)
CN: AW

From the AD,

A fatal accident occurred to a L-13 BLANÍK sailplane, in which the main spar of
the right wing failed near the root due to positive load. The right wing detached
from the aircraft and the pilots lost control of the sailplane.
The preliminary investigation has revealed that the fracture may have been
due to fatigue.
The AD 2010-0119-E required immediate inspection of the main spar at the
root of the wing to detect fatigue cracking and the accomplishment of the
relevant corrective actions as necessary. In addition, this AD 2010-0119-E
imposed operational limitations. AD 2010-0122-E retained the requirements of
AD 2010-0119-E, which is superseded, and extended the applicability to L-13
A BLANÍK sailplanes.
The requirements of AD 2010-0122-E were considered as interim action to
immediately address the unsafe condition. After issuance of AD 2010-0122-E,
based on further information provided by the Austrian Accident Investigation
Board (AIB), EASA re-assessed the inspection method as described in Aircraft
Industries a.s. Mandatory Bulletin No. L13/109a and concluded that the
inspection method might not be sufficient for detecting a crack similar to those
observed on the accident sailplane.
EASA AD No.: 2010-0185-E
EASA Form 111 Page 2/2
AD 2010-0160E retained as an interim measure the requirements of AD 2010-
0122E, and required determination of and compliance with specific usage.
However since issuance of AD 2010-0160-E, further analysis indicates that the
June 2010 L-13 BLANÍK accident occurred before the sailplane wing main spar
reached its theoretical estimated fatigue safe life limit.
Consequently, the AD 2010-0160-E action requiring to determine and to
comply with specific usage ratios might not be sufficient to ensure safe
operation. Indeed, there is high variability of the load levels and the frequency
of load application during aerobatics flight hours, and therefore a high
variability of the induced fatigue damage. This concern is accentuated when a
doubt exists on the completeness or accuracy of aerobatic flight hours records.
For the reasons stated in the above paragraph and knowing that the inspection
method as described in Aircraft Industries a.s. Mandatory Bulletin No.
L13/109a is likely not sufficient to detect the potential fatigue cracks, as a
conservative measure, this AD is prohibiting any operations of L-13 and
L-13 A BLANÍK sailplanes.
This AD is considered to be an interim measure, until a modification and/or a
proper inspection programme has been developed, EASA-approved and made
available by the approval holder of the design change for in-service application.

I think that explains the issue quite clearly, unfortunate for Blanik owners but doesn't look like the end.

Where does the idea that the Blanik in question was not properly maintained come from? In aircraft accident investigation maintenance is something that is looked at very closely.

I don't see how maintenance could have made any difference at all to a fatigue crack.

Maybe inspection could have seen it sooner, but if it's difficult to see, then EASA have taken the correct action.

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Chris Wedgwood,
Condor Team

Hi,

Shear wind is the most important factor of fatigue stress for any glider.

See the excel sheet here under (remade in five minutes).



So aerobatics don't induce accelerations so important than shear wind. At low altitude the probability of shear wind is more important than at high altitude and, as you can understand with this Excel sheet, the calculations are made with a security factor of 10 for any glider.

In 1945 at the end of WWII, the NACA made an real experiment with 1000 wings, comming from the factory, of the Fighter P51 Mustang, and publish a fatigue breakage cycles curve for aluminium wings as the blanik's wing (this curve is known as the Mustang Curve).

Faulty design, bad maintenance and inspection, and corrosion (between to sheet of aluminium and rivets), could be factors that accelerate the fatigue breakage phenomena.

As said Olympia, be careful, and I will add: there is no good pilot, there is only old pilot.....

My 2 pence

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*****- Xavier - (XDL * F-DELX) -*****

I congratulate you on a fine piece of analysis. As a mechanical engineer with an aeronautical engineering major I agree with your assumptions which, if anything, are a little conservative. It shows just how near to disaster a bit of careless flying will bring you.

I also agree with OXO that fatigue is only established by inspection, and even if you can't actually see anything you need to do ultrasonic or MRI testing to be sure. The only real answer is to take a Blanik wing from production and set it up in a rig and test it to destruction. This is expensive, so maybe a complete redesign using a much higher factor of safety would be cheaper (and quicker). The wings will gain weight and the L/D will get worse Meanwhile a large cloud hangs over the Blanik and it's not the kind we usually like to see.

I think the saying is: There are old pilots, and bold pilots, but NO old bold pilots. A formula I observed rigorously in my own flying days.

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Old wooden ship pilot
Reg No. 464 (LLC 377)
CN: AW

Also demonstrated when a Vega pilot some years ago decided to final glide at VNE in very rough conditions... the result was a wing failure, this final glide was simulated with a new wing which suffered progressive damage leading to failure. The accident investigation got a little awkward at one point as stress analysis on the Vega was originally done at Airbus ... except Airbus didn't know about that (not officially)

Back on Blanik topic, corrosion will accelerate fatigue but its not mentioned in the AD so I doubt its a factor. The way I read it is that inspection would probably not have prevented this failure so its possible that the crack started in an area which cannot be accessed in normal maintenance.

[quote

Back on Blanik topic, corrosion will accelerate fatigue but its not mentioned in the AD so I doubt its a factor. The way I read it is that inspection would probably not have prevented this failure so its possible that the crack started in an area which cannot be accessed in normal maintenance.[/quote]

I think you may be conflating corrosion and fatigue here. There is such a thing as 'corrosion fatigue' but this phenomenon occurs only when cyclic stress is applied to a metal part in a corrosive environment such as a corrosive gas or sea water for instance. The Blanik problem is just good old fashioned fatigue - the property of a material to fail if sufficient cycles of stress are applied above the stress level at which fatigue will not occur. The curve showing cycles against stress for metals shows a gentle downward slope as a rule and if you keep either the number of cycles versus applied load below it, the material will not fail.

If it is indeed impossible to inspect the Blanik wing spar at the relevant point in situ, then I'm afraid the wings will need to be detached and partially dismantled to carry out the inspection. Since no accurate record of the loading of the wing joint has been kept for any aircraft, again I'm afraid, speaking as an engineer, all wings are suspect, even ones that have only flown for a short time as you have no way of knowing what g loads may have been seen by that wing and therefore how much fatigue life is left.

In a way it is a mercy that this defect has now been detected, albeit by a catastrophic failure with two regrettable fatalities, rather than it lurking in wait for the next pair of pilots who are not watching their g meter when flying aerobatics.

We are fortunate that structural failure in flight is quite rare in gliders, maybe because aerobatics are not all that popular with most pilots, but we must be vigilant in keeping it that way.

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Old wooden ship pilot
Reg No. 464 (LLC 377)
CN: AW

Already done in 1970s.

As for the influence of maintenance mentioned above - prolongation checks and overhauls include defectoscopy of all the core parts, including wing spar. If the result is bad, Blanik's service life is not prolonged and it either needs to be repaired and all those parts replaced or it is deemed not airworthy. These checks cost fortune and are mandatory for all Blaniks operated in the Czech Republic and Slovakia.

Freebird: The idea of bad maintenance or bad use is based on incomplete flight log of the Blanik in question. Let me explain - in 1970s (when fatigue tests were done) the factory published bulletin about service life prolongation based on the results of the test. It introduces a measure called "spektrum" in Czech (spectrum in English). Spectrum defines a ratios between solo flight, duo flight and aerobatics. It also takes account of number of aerotow and winch starts. After reaching certain amount of flight hours (3000 hrs was the service life calculated in 1950s when first Blaniks were produced), the plane had to be inspected, spectrum values calculated from the flight logs and if they were within allowed range then the Blanik's life could be prolonged. Based on the results of fatigue tests the service life was extended to 3750 hrs with the condition that the spectrum values are within limits. Unfortunately as Czechoslovakia was in the "bad" part of the Europe at that time, different rules applied there. Mandatory bulletins published by the factory had the same effect there as ADs had in the "western world", but not vice versa. This info hasn't been published as AD in the west, so the owners of western Blaniks were not required to keep detailed flight logs from which the spectrum could be calculated, despite the fact that the factory documentation told otherwise.

Back to the case... The Austrian Blanik had +- 2600 flight hours. But only 20% of that time was logged in detail and could be used for spectrum calculation (8% of the hours logged in detail were aerobatic). That gives 80% of hours with no history and during that time anything could happen. Let's speculate a bit... 20% of 2600 hrs is 520 hrs. 8% of 520 is 41,6. So 41 and half hours of aerobatics... For Blanik to be prolonged to 3750 hours, it needs to have max 5:1 ratio between winch and aerotow, not more than 70% of duo flights and last, but not least - only 2%! of all flight hours can be aerobatic. 2% from 3000 hrs (original lifespan) is 60 hours. So the Austrian Blanik had 2/3 of allowed aerobatic hours used in only 1/5 of the total. And no one knows what happened during the time they didn't keep records... L-13 is not a Sukhoi 31 or Fox. Even L-13AC which is specifically designed for aerobatics has only 300 aerobatic hours allowed in its whole lifespan. You do the math...


BTW, today 31 Czech Blaniks with less than 2000 hours and no acro hours were temporarily excluded from the EASA AD by the Czech CAA for 2 months.

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