What’s the Answer to the Blanik Issue?

Author Dafydd Llewellyn—now retired—has been an unlimited Australian structures DER for 37 years, and was the designer of the L-13A1 Blanik modification, in 1978. For more about the Blanik Life Extension Program, based on Mr. Llewellyn's modification, click here.

They CAN be made to fly again, but not by spraying them with “Crack Bane.” It’s time to leave denial and wishful thinking behind, and start to face facts:

There is nothing mysterious about the fatigue life of a metal aircraft structure; you can look it up in FAA Advisory Circular AC 23-13A.  Every stress cycle takes a little bit out of the life of the structure. It does for glass, too, but not in quite the same way as for metal.

A stress cycle is defined by the mean stress, and the alternating stress.  For each combination of mean and alternating stress, the structure will withstand a certain number of cycles before it fails. How many cycles it takes at each combination of mean and alternating stress, was the subject of intense study after WW2; 100 sets of surplus P-51 wings were tested to destruction in Australia, and this provided most of the data shown in Figure A2-1 in Appendix 2 of AC 23-13A  (known as the “Heywood Joint” data).  These data were also used by the Czech VZLU in their fatigue life calculations for the Blanik, in 1977. We are still using these data today.

The other factor is, of course, the usage to which the aircraft is subjected. The higher the stress, the shorter the life,  so when estimating the fatigue life, it is critically important to estimate how many cycles of each “G” level the aircraft will, on average, suffer each flying hour. This is called the “Load Spectrum”. The Czech VZLU used a spectrum derived from experience in the Czech usage of the Blanik. (This was later compared with a measured spectrum for an Australian Blanik, and was found to be realistic, for normal soaring and training usage).

How can an aircraft designer set the intended service life? By choosing the stress level in the critical parts of the structure, at 1G.  The 1G stress level for the L-13 is around 6000 psi.  That was suitable for the original design requirement for the L-13, of 3000 hours safe life. The Czechs announced this in 1977;  however in most parts of the World, people did not concern themselves about the fatigue lives of light aeroplanes, let alone Gliders. So in most parts of the world, including the U.S.A., nobody took any notice of this announcement. You were told, but you did not listen.

In Australia, because of the generally good flying weather, aircraft accumulate hours faster than in most of the rest of the World;  and the summer turbulence is pretty strong.  So there had been a number of fatigue failures of aircraft of various types (including a Vickers Viscount), and the Czech announcement  did not pass unheeded. There were several specimens in Australia that were well past the 3000 hour figure, when that announcement was made. So the Czechs put a Blanik in their fatigue test rig and tested it until it failed, persisting until all the critical failure locations had been identified.  As a result of these tests, the life was extended to 4000 hours (aerotow)/ 3750 hours (winch).

However, the only way to achieve a substantial increase in the life of an aircraft structure, is to reduce the 1-G stress.  That means, add more metal - and replace as much of the original metal with new material as is practical.  To take a Blanik that had flown 4000 hours, and give it a useful new lease of life, requires that the items that can be, are replaced;  and the total amount of new material reduces the 1-G stress sufficiently to extend the life.  In the critical zone at the wing root, that requires that the 1-G stress be less than half its original value.  There are other high-stress spots further outboard, and elsewhere, that must also be suitably reinforced.  In all, to extend the life to three times its original value, requires approximately 22 Lbs of additional material.  Most of this goes inside the wing;  but some must go outside.

Where does eddy-current testing of the fastener holes fit into this?

Nowhere; because in the original Czech tests, the initial failure occurred in the internal strap—and it DID NOT go through the holes:

Fig. 31 is an excerpt from the Czech Aeronautical Research & Test Institute (ZPRAVA VZLU) Report Z-31, 1977, titled Fatigue Life Analysis of the L-13 (Blanik) Glider, author Vaclav Kahanek. The report is in English.

This failure occurred at 516,000 loading cycles—and the main extruded angle failed after two more cycles, causing catastrophic wing failure.

So there is no “magic feather” that will allow Blaniks to fly again with a useful extended life; the wings have to be opened and reinforced, and other critical points in the structure must be likewise replaced or reinforced, and that it all there is to it.

Why would one go to this expense for a glider that had a market value of around $10K before the grounding? Well, before the grounding, that glider was nearing the end of its existing fatigue life, and was also about due for its 40-year inspection.

What is its value after it has gained another 8000 hours of life, and passed its 40-year inspection? Still $ 10K ? Hardly. Doing all this work costs around $40K—with a full refurbish, probably nearer to $50 K. Reckon you’d still buy it for $10K after that? Don’t be silly; the market value will increase, probably to around $60 K.

So, why would you spend this amount of money? Because the cost of hull amortization per flying hour is half that of any new 2-seat training glider.

Use some common sense, please. Yes, it’s a pain to have to do this now, when an extra 20 odd pounds of metal in the original aircraft would have avoided the problem – but how was LET to know what a success the Blanik was going to be, back then?

I'm hoping that this modification will shortly become available in the U.S.A., but that depends upon factors that are outside my control.

Latest posts by Dafydd Llewellyn (see all)

  26 comments for “What’s the Answer to the Blanik Issue?

  1. P.
    September 5, 2012 at 12:03 am

    This is so true, but what about the gliders way behind 3000hrs, say 1500?

    • September 5, 2012 at 12:34 am

      There are L-13’s in the US with as little as 300 hours, some with around 800 hours, and several with less than 1800 hours. This is well below the 2200 hours of the failed airframe that resulted in the groundings. How much abuse did that airframe suffer? We don’t know because they aren’t talking. The German STC is nothing more than an AMOC to align the glider with the serial modification of the L-13 into the L-13A, which is blessed to fly a restricted envelope to 5000 hours by EASA. The US FAA was quick to follow suite in the grounding, but won’t budge on restoring them to service (if an L-13A), nor will they entertain a US AMOC. The Llewellyn modification is attractive and has been studied by the US FAA. However, the US has no bi-lateral agreement with Australia on STC’s, so it remains a non-runner unless things have changed recently. Perhaps some enterprising person(s) will buy up the US L-13 fleet and ship them to a country for rehabilitation and return to service. At the moment, the IS28-B2 fleet in the US is being grounded as they surpass the 35-year service life limit and it appears there will be no further life-extensions as the company is bankrupt.

      • September 5, 2012 at 2:55 am

        The only way to get the Llewellyn mod. into the U.A.A. at this time is for a U.S. citizen to obtain licence rights to the IP and use it to obtain an FAA STC

      • David Zapletal
        September 5, 2012 at 3:59 am

        I agree with Frank, lots of the gliders grounded are way before reaching the original lifetime. EASA questioned manufacturer fatique tests and calculations based on single incident of one particular glider, having most service history unknown and the rest most likely beyond the limits. About 3000 Blaniks have been built and milions of hours flown, how many wings broken off? This case simply sucks, as does the whole Fourth Reich aka European Union. We’re not very rich country/people here in Czech, so the result is obvious – many clubs spent money on Blaniks and now have to buy old cheap vintage stuff, to be at all able to fly training. Serious contribution to safety, flying even older doubleseaters, with characteristics and handling more than half a century behind present high performance glass ships. Wondering if it’s rather incompetence or bad intention behind the Blanik case, as the time goes I’m pretty convinced of the latter.

        • Malc
          February 13, 2014 at 7:55 am

          As an Englishman I can tell you that the European Union demonstrates that the French won the bloomin’ war ‘cos they seem to make all the rules but the Germans are making all the money

  2. Steve
    September 5, 2012 at 3:55 am

    “the market value will increase, probably to around $60 K”
    I would never ever by a Blanik for 60K, even if it would have a life cycle until the end of the universe.

  3. hpaircraft
    September 5, 2012 at 10:50 am

    The engineering is, of course, impeccable, but I take issue with the economics. The market value is most directly connected with what people will pay for something, and that is connected with what value it holds for them. In the case of the Blanik, it is hard to understand why someone would pay $60k (US or AUD) for a modest-performing two-seater designed in the 1950s and best known for flaps unlike those of any other sailplane, mediocre ergonomics, and long lead times for parts. I can see the price in serviceable units taking a bit of a jump, but I think that we will mostly see the impact of this service issue in increasing prices for ASK21 and G103 and other more modern two-seaters as various clubs and commercial operators move over.

  4. Bill Daniels
    September 5, 2012 at 5:38 pm

    I applaud the injection of some common sense into the Blanik L-13 debacle. Yes, these old metal gliders can be fixed – but not at a price many will pay. If someone does pay, they’ll get a 1950’s glider with another 1000 – 2000 hours life.

    Better, I think, to start saving for an ASK-21 or better still, invest in a domestic company designing a 21st Century training glider. Contact Windward Performance or HP Aircraft to discuss the idea.

    And too, change your organization’s finances to put long term cycles of fleet modernization into the plan. i.e. don’t get stuck holding the bag next time.

    • September 9, 2012 at 6:09 pm

      Hmmm so much for common sense…. not so common. The Llewellyn Mod extends the life of the Blanik to around 10,000 hrs. We are a very small club in Ausrtralia with a grounded Blanik and spent the money to have our trainer returned to service with enough hours to last us another 50 years. We have no capacity and would never have the capacity to pay 4 times the amount for an ASK 21. We now have a great Ab-Initio trainer (intended use) to introduce more budding pilots to gliding without putting undue pressure on our club to pay off a massive debt (and once paid off go again to redo the glass). No I wouldn’t buy one as a private person but these aircraft are still a great asset to a gliding club as an initial trainer with flaps and retractable under carrage they cannot be beaten for cost over life.

  5. Morgan
    September 5, 2012 at 6:07 pm

    Ultimately it comes down to where you want to put your money. Do I want to spend any money on our L-13 when the manufacturer has shown they wish to abandon the airplane? If the fix was possible for a small enough amount of money, maybe. But for the dollar figures we are talking about and the risk of the next issue going untreated, it’s pretty hard to find much faith in LET.

  6. Oscar
    September 5, 2012 at 6:53 pm

    If you have a Blanik with say 3,000 hours on it, another 2000 or so hours life is what you would get from the German modification. The Llewellyn modification takes the life out to 12,000 hours (or 11,500 for mostly winch launch).

    The German modification price is EU 6,500 for the bits only; you have to add the eddy-current testing (which is of debatable value given the information contained in Dafydd Llewellyn’s article of the actual failure found by the Czech authorities), the fastenings, and the qualified labor to do the modification. A realistic cost for the German modification is around US$15k all up.

    The Llewellyn modification is quoted at Aus. $37, roll-in, roll-out of the certified repair shop.

    Now , if you were to value your existing grounded 3,000-hour time Blanik at zero worth, the German modification effectively gives you a cost amortisation of around $7.50/hour life left. The Llewellyn modification, therefore, that gives you another 9,000 hours for the same flight conditions, amortises out at a bit over $4.00/hour life left.

    Yes, the question of where do you want to put your money is critical to most clubs. However, as with any attempt to arrive at a meaningful cost/benefit analysis, there is an over-riding caveat, and that is: if the cost is unaffordable, then the benefit is meaningless. If you are operating on a budget that isn’t able to stretch beyond – let’s say – $50k or so, what is realistically available within that budget, and how does it stack up as an economic proposition?

  7. Lee Harrison
    September 6, 2012 at 6:58 pm

    Sadly the problem in the US is wider than just the individual economics of whether it might make sense to do what will surely be an expensive AD … if one is ever qualified.

    The problem is that L-13s were a very large fraction of the training fleet, and now they are all gone. What can you buy to replace one? The ASK-21 is the only “obvious” trainer in current production. Price one … and think about it.

    And then there’s the fact that the operational cost of fiberglass sailplanes, particularly trainers, is considerably higher, compared to metal. They must either be hangared, or trailered. The assembly time and difficulty from trailering is non-trivial, and ensures that the glider will be flown much more rarely … “not worth assembling” unless conditions are good and there are multiple students.

    In the US anyway, I fear for the future of our sport if we don’t have a decent metal trainer in continuous production, with manufacturer support.

    • Bill Daniels
      September 7, 2012 at 9:32 pm

      Responding to Lee’s post with two points, ASK-21 and US trainers:

      Yes the ASK-21 seems expensive but it’s priced about the same as the old trainers in deflated dollars. You can check that with one of the many on-line “Dollar Deflator” calculators. It’s also a FAR better trainer than those old metal gliders.

      I’m not sure I agree the ’21 is comparatively more expensive to maintain either. The cost of metal and fabric work has skyrocketed in the last few years. I routinely assemble two ’21’s every time we fly the CAP gliders – usually without experienced help. Most would agree they are about as hard to assemble as a 15M single place. They are kept in their trailers when not flying which keeps maintenance at a minimum.

      I very much hope a new US made trainer is in the offing. That is the real solution to an affordable trainer. If you agree, read the HP-24 project article above then contact Bob Kuykendall. His next project is called “Aurora” – a training glider.

      • September 10, 2012 at 7:20 pm

        You know, the cost of achieving Type certification for a new training glider is something a lot of wishful thinkers overlook. I’ve been heavily involved in the certification of several light and very light aeroplanes, and the cost can run into seven figures. It’s a fundamental error to assume that an aircraft built under the U.S. Experimental category is necessarily but a small step from being a certificated product.

        I agree that the Blanik is not very attractive to a private individual, but it’s one of the best basic trainers ever built – and if the gliding movement is to survive, clubs must have trainers that are affordable and available for attracting new people into gliding. The fundamental asset of a trainer is NOT high L/D; it’s the ability to stay up in weak conditions, and the Blanik is pretty good at that..

        Re the cost of operating glass trainers – don’t forget to factor in the odd wheel-up landing; the Blanik doesn’t mind – but a retractable glass ship sure does..

        And BTW, the ARMCOM mod costs AUD 37K, not $60K – and that can be reduced to $34,500 if the aircraft is delivered and accepted after the mod, with the flaps and control surfaces already removed.

        As far as spares go, why not acquire a spare airframe for just that purpose? They’re going cheap right now.

        Dafydd Llewellyn

        • Bill Daniels
          September 11, 2012 at 11:06 am

          In the US, the Light Sport Aircraft (LSA) regulations are perfect for glider trainers. These regulations specifically allow LSA’s to be used for advanced pilot training. The only significant limitation is the 1320 Lb GW limit but that matches most existing training gliders. There are still rules with which one must comply but they are nothing like those for Standard Airworthiness Certificates. There is a 120 Knot “cruise speed” limit but that can’t be applied to gliders. The Vne could still be the same as JAR-22 gliders.

          (Airspace and altitude limits apply to Light Sport Pilots, not LSA airframes.)

          Even a Standard Airworthiness Certificate is not impossible to get but the Production Certificate is where the big financial costs come in – you have to certify an entire factory not just an airframe.

          One could choose to get a Standard Airworthiness Certificate for the design but produce the glider as an LSA. That way buyers would have some assurance of the design’s quality.

        • Brian Hollington
          July 4, 2013 at 7:56 pm

          Excellent points. I did a lot of instructing on 13s and regard it as the best teaching glider I have flown. The L/D is unimportant. What the instructor needs is a glider which will respond predictably. In Canada we teach spins and spirals. Those barn door dive brakes are a great comfort.

  8. Hans Trautenberg
    September 7, 2012 at 10:35 am
  9. Paul Kram
    September 15, 2012 at 6:03 pm

    Wouldn’t it be great if we could buy a brand new SGS 2-33 for $15,000? /sarcasm

    To speculate that the next 30 years of soaring will resemble the last 30 years fails to take into account 1)the rapid progress and dropping cost of flight simulation training and remotely piloted aircraft http://en.wikipedia.org/wiki/Unmanned_aerial_vehicle 2)the rising price of AV GAS and changing attitudes wrt burning fossil fuels for recreational purposes. Frank Paynter and Scott Manley have demonstrated the favorable economy and efficacy of using Condor for training, and that is just the beginning. Once they master Condor, students will don a pair of very affordable VR goggles and fly ‘radio controlled’ gliders. Once they get their drone rating, we will put them into a real glider.

    One thing on the horizon that may bring down the cost of real gliders is the new wave of robotics http://www.nytimes.com/2012/08/19/business/new-wave-of-adept-robots-is-changing-global-industry.html?hp The fact that we have a solid (digital) model of the new design gets us half way there. Tesla Motors and the like will probably figure out the robotics side of making composite material structures. 3D printing will also be part of the picture.

    Rebuilding L-13 with highly skilled human labor may be cost-effective for the moment, but I expect that there will be new and more cost-effective technologies just down the road. Will there be anyone still living with the skills to repair an aluminum glider 15 years from now (and what will be their hourly rate)?

  10. dave bowman
    September 27, 2012 at 2:12 pm

    50,000 Euro for a brand new Pipistrel Taurus pure glider with side by side seating which looks pretty slick with tons of room and a 40 to 1. I think we will see more of these over here. Steve Dees in Memphis has one (motorized) for his flight school now I think these guys have the right idea. He’s on pipistrel-usa.com under training.

  11. dave bowman
    September 27, 2012 at 2:22 pm

    59,000 not 50,000 sorry. I asked if the pure glider version could be changed over later to the motorized version and was told yes if I order the reinforced option for motor mounts.

  12. Norman Harding
    June 20, 2013 at 8:49 pm

    Hi, I have trolled through all the related comments, and cannot find any evidence of serious attention to the unanswered issues. The original EASA accident report ststed that the records of the flight times both total and aerobatic could not be confirmed. In my 40 years as an Australian L.A.M.E. with 10 of those years in aerial agriculture and having dealings with a few gliding clubs I no stranger to the practice of losing the ballpoint pen. Understating the flying hours is not an uncommon practice and I can prove at least one example. the proof also contains correspodence from the then regulatory authority that they were not interested! Turning to the wing structure, I bought two written off blaniks from the Gliding Club of W.A. and have now s/n 026636 in flyable order with 1450 hours on it. the other one, s/n 173918 i poened up the wing stucture in the area of the failure, the FAA comment was that the filure was through the three outboard 6mm rivet hole, that is, a vee shaped failure. I haven’t seen photos of the fracture. Has anyone? the critical rivet holes in the LH wing of 173918 were oversize out of round, one being 8mmX6.5mm , the holes in both wings were drlled rough and not square to the surface some as much as 7 dgrees off . the pattern is also irregular. I had previously had to work in this area of another l-13 of the 02XXXx group nad found the workmanship better. If you look at the AD&C promotion for their mod you can see evidence of bad holes in another 17XXXX wing. Open up all the dmaged ones you can find and see if there is a trend. I have a copy of the Czech fatigue report, Dafydd Llwellyn’s discounting of the eddy current test as a proof may be based on the faiure of the inner strap in a place other than where the real failure happened and several other examples pictured in the report failed I think overlooks (as I read from the report) the inadvertant overloading of the subject wing, and that wing was the one where cracks in the dimple in the spar had been allowed to propogate before repair. The edge of the repair can be seen in the photo. I dunno if anyoune can calculate the influence of those factors, I certainly can’t. The poor workmanship (was that the initiator of the failure?) is a matter of srtict liability.

  13. July 4, 2013 at 9:54 pm

    Yes, it seems that the root of the problem (pardon me) is the unreliability of the log books, especially in regard to the aerobatic usage. I agree with Norm Harding that the original drilling is pretty rough – the bore finish of the holes is far too rough for reliable eddy current testing of the rivet holes. The Llewellyn mod. deals with all that by (a) regarding the original spar cap angle as being worthless as a tension member, and relying on it only for the shear connection between the web and the spar caps; (b) replacing everything else in that region with new material; (c) reaming all the root fitting holes to take 5/16 inch Hy-Lok fasteners as precision fit; (d) Increasing the width of the root fittings themselves and the internal straps that replace the original internal strap, to accommodate the increased fastener size. The external straps are much wider, so fastener edge distance is not an issue with them. I supplied my mod. IP to a U.S. citizen last year, and I understand he’s submitted it to the FAA (Fort Worth office) but he’s been out of contact – probably on contract work in Iran – so i do not know what’s going on there.

    The very recent LET SB calling for conductivity measurement on the edge of the spar cap in the critical failure region may possible help to get the low-time aircraft back into the air – though LET are being somewhat close-mouthed about its true purpose.

    • October 31, 2013 at 9:22 pm

      The LET SB for the conductivity check has now been mandated by an EASA AD. Now, conductivity alone is insufficient to verify the heat treatment condition of aluminium alloy; it needs both conductivity and hardness data to assess that. So I read this SB as NOT being aimed at identifying wrongly heat-treated extrusions, but (especially given the location specified) to look for fatigue micro-cracking (i.e. the stage before other NDT means can find anything). If so, the collated results may in time provide a basis for returning low-time Blaniks to service.

      This SB has no relevance for Blaniks modified to L13A1 status or incorporating Australian STC SVA-542, because those modifications assume that the spar has no remaining tensile strength in this area, and rely on it merely to connect the spar cap to the web. All other material in this area of the lower spar cap is replaced with considerably more new material. I have therefore applied for an Australian AMOC in regard of this EASA AD for the existing Australian L13A1s and aircraft modified under STC SVA-542

      • November 6, 2013 at 11:29 pm

        CASA has issued an exclusion to EASA AD 2013-0252, for Australian L13-A1 gliders and L13 gliders modified IAW Australian STC SVA-542

  14. Jason Wentworth
    August 25, 2013 at 9:09 am

    Regarding Lee Harrison’s comment about the need for a metal two-seat trainer sailplane in the USA, could K & L soaring http://www.klsoaring.com (who have set up facilities to rebuild existing Schweizer 1-26, 1-34, and 2-33 sailplanes–they retain their type certificates) possibly resume production of new 2-33 or 2-32 two-seat trainers? The all-metal 2-32 can even seat *three* people (with two in its rear bench seat) if they don’t weigh over 150 pounds each).

  15. Norman Harding
    November 7, 2013 at 12:48 am

    I feel there is not going to be much uptake on the MBL13/116a without the owners havoing some assurance that a good reading will return their blanik to flying. I think the test should also be done on the spar 100 mm outboard of the specified test site where there is 25-30% more metal in the assembly as this will more likely give an indication of a change of state in the same peice of metal. I have read Dafydd’s comments and agree that a hardness test of some sort ,perhaps on the rear edge of the angle extrusion where the stress levels are highest next to the last row of 6mm rivets. this position is too small for a conductivity test with the specified probe size but a Leeb type rebound tester might work.

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