A LEAR 55 ON TOUCHDOWN EXPERIENCED A VIOLENT SHUDDERING AND VIBRATIONS WITH DIFFICULT DIRECTIONAL CTL; CAUSED BY FAILED R MAIN GEAR SCISSOR.

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ACFT WAS SCHEDULED FOR A STALL TEST FLT TO BE FLOWN IN ORDER TO RETURN THE ACFT TO SVC AFTER COMPLETING THE 12 YR AND 12000 HR INSPECTIONS. THE FLT CREW FOR THIS FLT WAS THE COPLT AND MYSELF. WE DEPARTED VFR; AT APPROX XA45 AM. THE TKOF WAS UNEVENTFUL. A STALL FLT WAS COMPLETED USING LEARJET ENGINEERING SPECIFICATION LES-FT-1679B. AFTER COMPLETING THE STALL TESTING; WE RETURNED TO LAND. THE CONDITIONS AT THE TIME OF LNDG WERE OVERCAST SKIES; VERY LIGHT RAIN; AND THE WIND WAS APPROX 200 AT 15. THE FLT WAS ROUTINE UNTIL THE ACFT TOUCHED DOWN ON THE RWY AT APPROX XB47 PM. AS THE ACFT TOUCHED DOWN AND THE BRAKES WERE APPLIED; IT BECAME EVIDENT THAT SOMETHING WAS WRONG. THE ACFT SHUDDERED AS IF THE ANTI-SKID WAS CYCLING VIOLENTLY AND A TIRE HAD BLOWN. AS THE ACFT SLOWED; DIRECTIONAL CTL BECAME INCREASINGLY MORE DIFFICULT. DIRECTIONAL CTL WAS MAINTAINED USING L RUDDER AND THE L BRAKE. THE ACFT STOPPED ON THE RWY; SLIGHTLY TO THE R OF THE RWY CTRLINE. TWR NOTIFIED US THAT IT APPEARED THE MAIN GEAR BRAKES HAD LOCKED UP; BLOWN A TIRE; AND THAT THEY WERE SENDING THE ARPT AUTHORITIES OUT TO THE AIRPLANE. TWR CALLED AGAIN AND NOTIFIED US THAT THE R MAIN LNDG GEAR WAS BROKEN AND WE SHOULD NOT TRY AND MOVE THE ACFT. THE ACFT WAS SECURED AND WE EXITED THE ACFT. FURTHER INSPECTION OF THE ACFT REVEALED THAT THE SCISSOR LINK ON THE R-HAND MAIN LNDG GEAR HAD CRACKED AND THE R-HAND MAIN TIRES WERE POSITIONED PERPENDICULAR TO THE ACFT CTRLINE. THE RWY WAS CLOSED FOR ABOUT TWO HOURS WHILE THE ACFT WAS JACKED; A DOLLY PLACED UNDER THE R MAIN GEAR; AND REMOVED FROM THE RWY. ALTHOUGH THERE WAS NO WAY TO DETERMINE WHAT THE PROBLEM WAS FROM THE COCKPIT; REFLECTING ON THE SEQUENCE OF EVENTS DOES REVEAL MORE DETAILS. AT TOUCHDOWN THE SPOILERS WERE SLOW TO DEPLOY; THIS WAS PROBABLY DUE TO THE R SQUAT SWITCH REMAINING IN AIR MODE. BOTH SQUAT SWITCHES MUST BE IN GND MODE FOR THE SPOILERS TO DEPLOY. IF THIS WERE THE CASE THE ONLY BRAKE SURFACE AVAILABLE TO STOP THE ACFT WOULD HAVE BEEN THE L OUTBOARD BRAKE; UNTIL THE RIGHT SQUAT SWITCH WENT INTO GND MODE. WITH THE R SQUAT SWITCH IN GND MODE (L ALREADY IN GND MODE) THE SPOILERS EXTEND AND THE L INBOARD BRAKE BECOMES AVAILABLE. THE ACFT HAD 9606 LNDGS ON THE LNDG GEAR. LEARJET HAS NOT YET DETERMINED WHETHER THE GEAR WAS DESIGNED FOR 9000 LNDGS OR 12000 LNDGS. ENGINEERING BELIEVES THAT THE LOWER SCISSOR ARM BROKE DUE TO FATIGUE; POSSIBLY CAUSED BY CORROSION AND AN EVENTUAL CRACK. THE PARTS HAVE YET TO BE EXAMINED BY THE FACTORY.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT ON TOUCHDOWN FOR A FEW SECONDS THE ACFT WAS UNCONTROLLABLE WITH THE R MAIN GEAR STRUT PISTON ROTATED 90 DEGS DUE TO FAILURE OF THE SCISSORS LINK. THE MAIN LNDG GEAR AND COMPONENTS WERE LIFE LIMITED FOR 12000 CYCLES AND THIS AIRPLANE HAD 9606 CYCLES. THE ONLY MAINT ON THE SCISSORS LINK WAS GREASING AND CHKING THE WEAR ON THE BUSHINGS AND REPLACING THE BUSHINGS AS REQUIRED.