|37000 Feet||Browse and search NASA's
Aviation Safety Reporting System
|Local Time Of Day||0601 To 1200|
|Locale Reference||airport : dro.airport|
|Altitude||agl single value : 0|
|Operator||general aviation : instructional|
|Make Model Name||M-20 B/C Ranger|
|Operating Under FAR Part||Part 91|
|Flight Phase||landing : roll|
|Function||instruction : trainee|
|Qualification||pilot : instrument|
pilot : commercial
pilot : multi engine
|Experience||flight time last 90 days : 4|
flight time total : 920
flight time type : 378
|Anomaly||aircraft equipment problem : critical|
ground encounters : gear up landing
|Independent Detector||other flight crewa|
|Resolutory Action||none taken : unable|
|Problem Areas||Flight Crew Human Performance|
|Primary Problem||Flight Crew Human Performance|
This was a training flight with my instructor to refine pattern skills. On the sixth landing; the gear lock-down mechanism failed. Damage to the airframe was minor. Damage to the propeller was serious; and the engine needs a teardown inspection to assess possible internal damage. The retract mechanism on this aircraft is manual only; with no back-up system. My instructor saw/felt the johnson bar move from the lock-down position to the retract position on touchdown. My attention was focused on attitude cues and airspeed; and I was unaware that the johnson bar moved. My procedure for landing setup includes a gumps check when the attitude and airspeed are stabilized on 1) downwind; 2) base; and 3) final legs of the traffic pattern. Undercarriage check includes review of johnson bar position and green light that indicates gear down and locked. I cannot recall 18 positive checks (3 checks for each of 6 lndgs); so I cannot testify to gear-safe status 3 times on the last landing. 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 position; the grip is in a fully extended or 'up' position and is trapped in a fitting at the bottom of the instrument 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 system; 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 system on touchdown can back-drive the system and force the whole system including the johnson bar into the retract position. This failure mode seems to be well known in the insurance community; but is not well known in the pilot community. A switch sensing the grip position 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.
Original NASA ASRS Text
Title: 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.
Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.