M20 LANDING GEAR COLLAPSES ON LNDG WHEN GEAR DOWN LOCK NOT PROPERLY ENGAGED IN COCKPIT RETAINER MECHANISM.

Narrative

THIS WAS A TRAINING FLT WITH MY INSTRUCTOR TO REFINE PATTERN SKILLS. ON THE SIXTH LNDG; THE GEAR LOCK-DOWN MECHANISM FAILED. DAMAGE TO THE AIRFRAME WAS MINOR. DAMAGE TO THE PROP WAS SERIOUS; AND THE ENG NEEDS A TEARDOWN INSPECTION TO ASSESS POSSIBLE INTERNAL DAMAGE. THE RETRACT MECHANISM ON THIS ACFT IS MANUAL ONLY; WITH NO BACK-UP SYS. MY INSTRUCTOR SAW/FELT THE JOHNSON BAR MOVE FROM THE LOCK-DOWN POS TO THE RETRACT POS ON TOUCHDOWN. MY ATTN WAS FOCUSED ON ATTITUDE CUES AND AIRSPD; AND I WAS UNAWARE THAT THE JOHNSON BAR MOVED. MY PROC FOR LNDG SETUP INCLUDES A GUMPS CHK WHEN THE ATTITUDE AND AIRSPD ARE STABILIZED ON 1) DOWNWIND; 2) BASE; AND 3) FINAL LEGS OF THE TFC PATTERN. UNDERCARRIAGE CHK INCLUDES REVIEW OF JOHNSON BAR POS AND GREEN LIGHT THAT INDICATES GEAR DOWN AND LOCKED. I CANNOT RECALL 18 POSITIVE CHKS (3 CHKS FOR EACH OF 6 LNDGS); SO I CANNOT TESTIFY TO GEAR-SAFE STATUS 3 TIMES ON THE LAST LNDG. MOTION OF THE JOHNSON BAR ON TOUCHDOWN INDICATES THAT THE LATCHING PROCESS WAS NOT EFFECTIVE. THE LATCHING MECHANISM INCLUDES A 'SLIDING GRIP' AT THE TOP OF THE JOHNSON BAR. IN THE GEAR-DOWN-AND-LOCKED POS; THE GRIP IS IN A FULLY EXTENDED OR 'UP' POS AND IS TRAPPED IN A FITTING AT THE BOTTOM OF THE INST PANEL. WHEN THE GEAR ARE DOWN-AND-LOCKED; THE GRIP IS FULLY ENGAGED IN THE RETAINER FITTING AND LOCKED IN PLACE BY A SPRING-LOADED CROSS BOLT THAT ENGAGES A CIRCUMFERENTIAL GROOVE IN THE TOP OF THE GRIP. NOMINALLY; THIS IS A SIMPLE AND RELIABLE SYS; HOWEVER; IT CAN FAIL IF THE GRIP IS NOT FULLY 'UP' AND ENGAGED BY THE CROSS BOLT. IN SUCH A CASE; THE DRAG LOAD ON THE GEAR SYS ON TOUCHDOWN CAN BACK-DRIVE THE SYS AND FORCE THE WHOLE SYS INCLUDING THE JOHNSON BAR INTO THE RETRACT POS. THIS FAILURE MODE SEEMS TO BE WELL KNOWN IN THE INSURANCE COMMUNITY; BUT IS NOT WELL KNOWN IN THE PLT COMMUNITY. A SWITCH SENSING THE GRIP POS ACTIVATES THE GREEN LIGHT THAT INDICATES GEAR DOWN-AND-LOCKED. A MINOR DISCREPANCY IN ADJUSTMENT OF THE SENSING SWITCH CAN ALLOW AN ERRONEOUS 'DOWN-AND-LOCKED' INDICATION.