ACFT DAMAGED AS A CHALLENGER II ULTRALIGHT PVT PLT EXPERIENCES A NOSE OVER EVENT DURING A HIGH SPD TAXI ON A LAKE AS THE PROBLEMATIC NOSE GEAR COMES UNSTUCK NEAR S59; MT.

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PLT WAS UNDERGOING TRAINING AND FAMILIARIZATION FLTS ON 2 SEATER CHALLENGER 2 ULTRALIGHT ON LAKE KOOCANUSA IN MONTANA. THE ACFT IS AMPHIBIOUS; EQUIPPED WITH BOTH WHEELS AND FLOATS SUPPLIED BY COMPANY IN CANADA. AFTER SEVERAL SUCCESSFUL LNDGS ON WATER WITH WHEELS RETRACTED; THE PLT TRIED TO LOWER THE WHEELS FOR A GND LNDG BUT FOUND THE FRONT GEAR TO BE JAMMED IN THE RAISED POS. THE PLT THEN RETRACTED THE REAR WHEELS AND LANDED ON WATER IN ORDER TO ASSESS THE CAUSE OF THE FRONT WHEEL HAVING JAMMED IN THE 'UP' POS. IT IS ALSO POSSIBLE THAT WHILST LOOKING FOR SUITABLE LNDG SITES; THE PLT MAY HAVE INADVERTENTLY FLOWN WITHIN 500 FT OF PEOPLE OR STRUCTURES. DURING A HIGH SPD TAXI TOWARDS LAND; THE FRONT GEAR WHICH IS SPRING LOADED SUDDENLY BECAME UNJAMMED AND BEING SPRING LOADED; IMMEDIATELY DEPLOYED ITSELF TO ITS FULL EXTENT INTO THE WATER. THE RESULTING DRAG CAUSED THE ACFT TO NOSE OVER; RESULTING IN SOME MINOR COSMETIC DAMAGE AND BENDING OF MINOR SPARS AND STRUTS. THE PLT EXITED THE ACFT AND ARRANGED TO HAVE IT TOWED TO SHORE WHERE IT WAS RIGHTED AND REMOVED FROM THE SCENE. NO INJURIES RESULTED. UPON LATER INSPECTION; IT WAS DETERMINED THAT THE TWIN FRONT WHEELS WHEN BEING RETRACTED ON SOME OCCASIONS ARE FORCED EITHER SIDE OF THE 3 INCH DOWN TUBE HOLDING THE TELESCOPICALLY MOUNTED TUBE TO WHICH THE WHEELS ARE ATTACHED. THIS RESULTED IN THE INABILITY TO MANUALLY LOWER THE FRONT WHEEL. A MODIFICATION IS BEING MADE IN THE FORM OF A STOP PLATE TO STOP THE FRONT WHEEL FROM BEING RETRACTED BEYOND A CERTAIN DISTANCE. IT IS MY BELIEF THAT ALL ACFT FITTED WITH SIMILAR FLOATS AND WHEELS; SHOULD HAVE A SIMILAR STOP PLATE FITTED TO PREVENT SIMILAR INCIDENTS FROM OCCURRING.