On June 20, 1998 Brian Porter was flying Millennium #000 in competition at the Nationals in Dinosaur Colorado. Brian was at 1000' near goal in relatively smooth air when he felt the glider shake as though one of the rudders was fluttering, something that we've seen many times in earlier flight testing. He looked at the rudder and noticed that the entire winglet and metal tip rib were moving up and down, something he had never seen before. He was flying at around 50 mph when this happened, so he slowed down and the entire winglet tore off from the D-tube. It was still attached to the wing by the rear rudder bellcrank and began to flap violently, damaging the elevon. the glider entered an uncontrollable turn and Brian decided to deploy. The deployment went very well and the glider was desending nose-up until Brian shifted his weight forward and it tilted nose-down. He landed in a trailer park with no injuries, but a moderately damaged Millennium.
Brian was flying Millennium #000 which has several significant structural differences from the production gliders, especially in the way the tip rib mounts to the spar. Based on an initial inspection of the wreckage we believe that the D-tube of #000 was damaged in Brain's blown launch at Slide Mountain one week earlier. An aluminum spar inside the winglet was bent forward on the winglet that failed in Dinosaur, indicating damage that occured during his severe crash at Slide mountain. Based on calculations, the load required to bend the spar is 3 times greater than the largest estimated in-flight load. We believe the spar was damaged at Slide and since Brian was in such a rush to leave for the Nats, he repaired the glider himself and did not notice that the D-tube was also damaged. The rudder then broke during his next flight at Dinosaur.
On Monday June 22,1998 we load tested a production Millennium D-tube wing tip to failure to see what the load capacity and failure mode is. The joint withstood 124 ft-lb of torque before failure, which is more than three times the value of the largest flight loads it can experience. We feel that this is safe for people to fly, but we want pilots to inspect the rear of the D-tube at the wingtip after any hard landings. To inspect the wing, look inside the sail at the tip (with the winglet removed) and focus on the back of the D-tube. While applying a load (50 lbs is the max value) to the steel tip rib (BOTH UP AND DOWN LOAD!) look to see if the layers of composite in the edges of the spar caps show any relative motion. If the material has delaminated due to a high bonk-load you will see layers in the spar cap(s) moving relaive to each other. If this is the case, the glider must be brought back to the factory for immediate repairs. Otherwise, the tip is not damaged and is safe to fly.
Bright Star will add extra carbon to this area on future Millenniums to further reinforce it against crash loads and if any Millennium owners would like us to do this to their gliders we will be glad to do so for free if you can bring the glider to our shop. It took a fairly severe impact to damage the tip joint and just to give you an idea of how hard a bonk is required to damage the wing tip, Brian's crash at Slide mtn. bent the 3" cage tube 30 degrees, damaged most of the remaining steel tubes in the cage, and tore the skin on one of the D-tubes. This damage is indicative of a crash and not what we would refer to as a 'bonk'. Even a hard bonk that damages the front nose assembly will probably not damage the spar at the wing tip, but this area must be inspected after every hard landing by performing the load test described above. If you have any questions concerning this accident or the Millennium's structural integrity please contact:
Steve Morris Bright Star Gliders, Inc. (650) 424-9552 mlbco@sirius.com----------
Fred again:
While Steve's report stands on its own I just want to make a couple of
comments. The structural failure of the Millennium prototype that Steve
describes occurred a week after a blown launch severe enough that it
bent every tube in the (steel) hang cage. The subsequent load test
showed that the bending of the spar that led to the in-flight failure
occurred as a result of a force corresponding to an aerodynamic load of
roughly 40g - more than would ever occur in an actual flying situation,
but possible in the instantaneous loading of a crash like the one on
launch at Slide.
It's kind of creepy whenever we hear of a structural failure of some internal element that isn't inspected during preflight. However, this was not a case of a failure following typical or even moderately rough use. After a crash that bent every tube in my control triangle I would open up my hang glider and inspect every structural component. I gather that the blown launch at Slide was a crash of this severity yet a similarly thorough tear-down inspection was not performed. While any structural failure of a new design is cause for concern I believe Steve's explanation (and suggestion for inspection methods following suspected damage) addresses all the issues and allays the worries I might have about a Millennium coming apart in the air.
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Fred VachssFor more good stuff check out the factory website at: www.sirius.com/~mlbco/Millennium.html (case sensitive).