Narrative:

After dropping the only banner for the day at the east end of the runway at the airport, I circled with a l-hand pattern for landing to the west on the east/west private strip. The windsock showed very little movement, but indicated approximately 20 degrees left of the runway centerline. I used full flaps since the winds were so favorable. As usual, I extended the final as necessary to clear the banner now lying in the runway. I flared for landing, arresting a sink rate that was higher than usual. The touchdown was in a 3-POINT attitude and more firm than I usually did. After the wheels contacted, I heard a noise and felt a sharp 'bump' in the left floor of the aircraft (the vicinity of the left main gear attach point). The aircraft ballooned up, and as it settled I used a combination of right aileron, right brake, rudder, and elevator to hold the left wing up as long as practical, and to keep the aircraft moving relatively straight. As it slowed and settled, the nose began to pitch down, stopping the engine and threatening flipping over on its nose. I added back elevator and released the right brake. The aircraft then pivoted about 45 degrees to the left as it stopped and settled. It now rested on the right main and tailwheel, with the left side being supported by the left wingtip, the left wheel, and the inboard end of the left gear leg, now buried in the soft ground. There was no evidence of fire, so I secured all electrical switches, mixture control, and fuel shutoff valve before exiting normally through the door on the right. A short while later, I was curious as to what broke, causing the problem. I dug underneath the left main attach point and saw the bolt hole at the inboard end of the gear leg. I then walked back to the touchdown point where I picked up the piece of the broken bolt, marked 'ms 20007,' assumed to be the left main inboard attach bolt. On the broken end, 1/2 of the shaft was irregular and jagged with no discoloration in the metal. The other half was mostly flat, with concentric semi circles radiating from a point on the edge of the shaft. This part was also darker, appearing to be slightly corroded from a previous crack. The other part of the bolt was later seen still frozen in place in the steel structure of the fuselage, having broken where the gear leg and fuselage structure meet. Possible contributing factors: 1) the harder than normal landing. This was obviously a contributing factor. The bolt in question required a higher than normal stress to cause it to fail. It just was not that hard. 2) the cracked bolt: this was the primary factor. Even a person unknowledgeable about metals would agree that a bolt cracked 1/2 way through would only have 1/2 the strength. But any engineer who understands the effects of cracks would confirm that this compromised part had much less than 1/2 of its designed strength. Callback conversation with reporter revealed the following information: the reporter stated the landing could not be considered a hard landing and the primary cause of this incident was the fractured left landing gear inboard bolt. The reporter said the manufacturer issued an alert bulletin on this bolt as being life limited to 500 hours. The reporter said a maintenance history search of the logbook indicates the bolt was replaced at the first 500 hours but no record of subsequent bolt replacements. The reporter stated the airplane had 1300 hours total time and had exceeded the manufacturer's advisory by 300 hours. The reporter said the FAA was contacted and ruled the event as an incident.

Google
 

Original NASA ASRS Text

Title: A CITABRIA ON LNDG HAD THE L MAIN GEAR COLLAPSE INCURRING DAMAGE TO THE PROP AND L WINGTIP. CAUSED BY A FAILED LIFE LIMITED BOLT.

Narrative: AFTER DROPPING THE ONLY BANNER FOR THE DAY AT THE E END OF THE RWY AT THE ARPT, I CIRCLED WITH A L-HAND PATTERN FOR LNDG TO THE W ON THE E/W PVT STRIP. THE WINDSOCK SHOWED VERY LITTLE MOVEMENT, BUT INDICATED APPROX 20 DEGS L OF THE RWY CTRLINE. I USED FULL FLAPS SINCE THE WINDS WERE SO FAVORABLE. AS USUAL, I EXTENDED THE FINAL AS NECESSARY TO CLR THE BANNER NOW LYING IN THE RWY. I FLARED FOR LNDG, ARRESTING A SINK RATE THAT WAS HIGHER THAN USUAL. THE TOUCHDOWN WAS IN A 3-POINT ATTITUDE AND MORE FIRM THAN I USUALLY DID. AFTER THE WHEELS CONTACTED, I HEARD A NOISE AND FELT A SHARP 'BUMP' IN THE L FLOOR OF THE ACFT (THE VICINITY OF THE L MAIN GEAR ATTACH POINT). THE ACFT BALLOONED UP, AND AS IT SETTLED I USED A COMBINATION OF R AILERON, R BRAKE, RUDDER, AND ELEVATOR TO HOLD THE L WING UP AS LONG AS PRACTICAL, AND TO KEEP THE ACFT MOVING RELATIVELY STRAIGHT. AS IT SLOWED AND SETTLED, THE NOSE BEGAN TO PITCH DOWN, STOPPING THE ENG AND THREATENING FLIPPING OVER ON ITS NOSE. I ADDED BACK ELEVATOR AND RELEASED THE R BRAKE. THE ACFT THEN PIVOTED ABOUT 45 DEGS TO THE L AS IT STOPPED AND SETTLED. IT NOW RESTED ON THE R MAIN AND TAILWHEEL, WITH THE L SIDE BEING SUPPORTED BY THE L WINGTIP, THE L WHEEL, AND THE INBOARD END OF THE L GEAR LEG, NOW BURIED IN THE SOFT GND. THERE WAS NO EVIDENCE OF FIRE, SO I SECURED ALL ELECTRICAL SWITCHES, MIXTURE CTL, AND FUEL SHUTOFF VALVE BEFORE EXITING NORMALLY THROUGH THE DOOR ON THE R. A SHORT WHILE LATER, I WAS CURIOUS AS TO WHAT BROKE, CAUSING THE PROB. I DUG UNDERNEATH THE L MAIN ATTACH POINT AND SAW THE BOLT HOLE AT THE INBOARD END OF THE GEAR LEG. I THEN WALKED BACK TO THE TOUCHDOWN POINT WHERE I PICKED UP THE PIECE OF THE BROKEN BOLT, MARKED 'MS 20007,' ASSUMED TO BE THE L MAIN INBOARD ATTACH BOLT. ON THE BROKEN END, 1/2 OF THE SHAFT WAS IRREGULAR AND JAGGED WITH NO DISCOLORATION IN THE METAL. THE OTHER HALF WAS MOSTLY FLAT, WITH CONCENTRIC SEMI CIRCLES RADIATING FROM A POINT ON THE EDGE OF THE SHAFT. THIS PART WAS ALSO DARKER, APPEARING TO BE SLIGHTLY CORRODED FROM A PREVIOUS CRACK. THE OTHER PART OF THE BOLT WAS LATER SEEN STILL FROZEN IN PLACE IN THE STEEL STRUCTURE OF THE FUSELAGE, HAVING BROKEN WHERE THE GEAR LEG AND FUSELAGE STRUCTURE MEET. POSSIBLE CONTRIBUTING FACTORS: 1) THE HARDER THAN NORMAL LNDG. THIS WAS OBVIOUSLY A CONTRIBUTING FACTOR. THE BOLT IN QUESTION REQUIRED A HIGHER THAN NORMAL STRESS TO CAUSE IT TO FAIL. IT JUST WAS NOT THAT HARD. 2) THE CRACKED BOLT: THIS WAS THE PRIMARY FACTOR. EVEN A PERSON UNKNOWLEDGEABLE ABOUT METALS WOULD AGREE THAT A BOLT CRACKED 1/2 WAY THROUGH WOULD ONLY HAVE 1/2 THE STRENGTH. BUT ANY ENGINEER WHO UNDERSTANDS THE EFFECTS OF CRACKS WOULD CONFIRM THAT THIS COMPROMISED PART HAD MUCH LESS THAN 1/2 OF ITS DESIGNED STRENGTH. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE LNDG COULD NOT BE CONSIDERED A HARD LNDG AND THE PRIMARY CAUSE OF THIS INCIDENT WAS THE FRACTURED L LNDG GEAR INBOARD BOLT. THE RPTR SAID THE MANUFACTURER ISSUED AN ALERT BULLETIN ON THIS BOLT AS BEING LIFE LIMITED TO 500 HRS. THE RPTR SAID A MAINT HISTORY SEARCH OF THE LOGBOOK INDICATES THE BOLT WAS REPLACED AT THE FIRST 500 HRS BUT NO RECORD OF SUBSEQUENT BOLT REPLACEMENTS. THE RPTR STATED THE AIRPLANE HAD 1300 HRS TOTAL TIME AND HAD EXCEEDED THE MANUFACTURER'S ADVISORY BY 300 HRS. THE RPTR SAID THE FAA WAS CONTACTED AND RULED THE EVENT AS AN INCIDENT.

Data retrieved from NASA's ASRS site as of July 2007 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.