ABRIDGED VERSION OF OFFICIAL
LETTER TO BLANIK OWNERS by Roger Harris, GNZ National Airworthiness Officer Aircraft involved. All L-13 & L-13a Blanik gliders, all serial numbers. (We are of course only interested in L-13 Blaniks in New Zealand, and we have no L-13a’s in NZ.) The Blanik has been the subject of a number of manufacturer’s Mandatory Service Bulletins and Airworthiness Directives issued by EASA. Most of these gliders are now well beyond their original safe life of three thousand hours total flight time, and have progressively had their life extended through ongoing inspection procedures. Operators are well aware of the problems presented by metal fatigue and many other problems. Engineers have been fighting the insidious advances of metal fatigue for many years. The Blanik that suffered the failed wing in Austria, had logged only 2400 hours time since new. This is lower hours than most NZ Blaniks. EASA have concluded that the inspection method described in Aircraft Industries a.s. Mandatory Bulletin No. L13/109a is not sufficient to detect potential fatigue cracks, and the usage ratios have not proved sufficient to ensure safe operation. EASA also considers that the difference in loading between solo and dual flight is significant. This EASA A/D is considered an interim action to immediately address the unsafe condition until a modification and/or a proper inspection programme has been developed. There has been some discussion in regard to a Blanik modification program referred to as “the Llewellyn Mod.” This is an extensive modification to the L-13 Blanik, carried out in Australia. There were eight done, not all of which are now airworthy. On completion of this modification the aircraft were issued with Supplemental Type Certificates (L-13 A1) and the fatigue life increased to 12,000 hours. These modifications are significant, and were quite expensive, costing some A$20,000 back in the ’80s. No Blaniks in NZ have been through this modification, although some may have had a small portion of the mod carried out. The Gliding Federation of Australia (GFA) had hoped that so modified Blaniks might be able to continue operating, however EASA has discounted this. The holder of the current Blanik Type Certificate (Aircraft Industries a.s., in Kunovice, Czech Republic) has provided the following statement: In connection with the crash of the L 13 glider, S/N 175117, … where the first findings of investigators in part resulted in suspicion of occurrence of fatigue cracks in transition of cap of the left in the joint with the wing attachment, and in part have identified considerable overloading of the glider caused by a high frequency of acrobatic flights in the value of approximately 8% of the last 400 hours of operation, (at the same time as regards the preceding nearly 2000 hours of operation, no records were found at all). The attention of glider operators and certification agencies … focused to the problem of fatigue life of the glider and conditions of its proof.
Investigation of this crash has not been yet officially concluded. Aircraft Industries a.s., as manufacturer, stress that the service life of L 13 gliders and other related types is based on the concept of safe fatigue life, which was determined on basis of fatigue tests of the glider’s structure and fatigue tests of samples, on which the cyclic loading derived from average operation published in operational documentation of gliders was applied. In the event that actual operating conditions of the glider deviate from the specified operation conditions towards a greater loading of the glider, (e.g. by not keeping the percentage of acrobatics, etc.) then the safe life of the glider expressively decreases under the specified value of service life. Following the issue of Mandatory Bulletin L13/109a, and EASA A/D 2010-0119-E, it became evident that a number of operators of the L 13 glider do not keep any records or perform only incomplete records as regard to the real operational conditions of gliders. This status does not render any possibility to evaluate what portion of the fatigue life was consumed by the actual operation, and what portion remains available, which is in contradiction with the concept of the safe fatigue life. Visual inspection of the critical place required by Bulletin L13/109a is not able to ensure reliable identification of possible cracks. The certification agencies together with Aircraft Industries a.s. share the standpoint that an advisable way towards renewal of airworthiness of gliders L 13 and L-13A is development, certification and introduction into production of the NDT method. This method will reliably identify any possible incipient cracks in critical areas of the gliders structure, together with determining the periodicity of inspections on basis of tests and analysis of the speed of crack spreading in critical locations to determine residual structural strength in critical areas. The company Aircraft Industries a.s. in cooperation with the Research and Testing Institute in Prague, is preparing a system of checks. Due to the fact that the critical place is hidden under two layers of metal sheet and under countersunk heads of rivets, elaboration of a proof method is time demanding. Aircraft Industries a.s. presumes that for renewal of airworthiness of gliders whose operational records are properly kept, execution of the NDT method with satisfactory results will be sufficient. We also expect that the method will be verified and approved at the beginning of the year 2011. For gliders with non–existent or incomplete records on operation, it seems to be necessary to introduce repeated inspections with specified periodicity. We presume that this method will be approved at the end of the first quarter of 2011. The proposed way to restore the airworthiness of gliders is subject to approval by EASA. So, …were does this leave the NZ Blaniks? Do not turn your aircraft into a kids’ playground just yet; however, you may have to wait a little while for a resolution, as I suspect that the proposed test will take some time to arrive at a satisfactory state where a consistent result can be guaranteed.
December 2010
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