What happened
The FAA’s narrative, in sentence case. The FAA writes it in capitals and removes private details.
(.4) The Boeing model BV-107-II departed on a maintenance check flight with 1.4 hrs total flight time after conversion from a model HKP-4 per FAA project #TDO639NY-R. About 37 min later, witnesses saw the rotorcraft moving erratically & tumbling out of control. Postcrash exam of the rotorcraft's flight control system revealed a disconnect between the lower bearing end of the aft directional and lateral control pushrod & the inboard clevis of the forward mixing unit section bellcrank. A bolt, consistent with hardware for that connection, was found in the control closet area. An improper part (collective bellcrank, pn 107C2606-8) was found in place of the required lateral bellcrank (pn 107C2606-9). Clevis width of the -8 part was slightly larger than the -9 part; thus, the clevis bolt was not long enough to allow a cotter pin to be properly installed through the nut & bolt with the required washers (2 thick & 1 thin) installed. To compensate (allow for installation of the cotter pin), 2 thin washers were used in place of the 2 thick and 1 thin washers. Metallurgical exam of the bolt revealed evidence that a nut had been applied to the threaded end, but there was no evidence that a cotter pin had been inserted. No pre-accident engine malfunction or crew impairment was evident. The national transportation safety board determines the probably cause(s) of this accident as follows. Failure of maintenance personnel to install a cotter pin in a clevis bolt in the flight control system, which resulted in the aft directional and lateral control output pushrod to become disconnected from a bellcrank in the forward portion of the first stage mixing unit. A factor relating to the accident was the use of an improper bellcrank, whick was wider in the clevis area.
As the FAA filed it
(.4) THE BOEING MODEL BV-107-II DEPARTED ON A MAINTENANCE CHECK FLIGHT WITH 1.4 HRS TOTAL FLIGHT TIME AFTER CONVERSION FROM A MODEL HKP-4 PER FAA PROJECT #TDO639NY-R. ABOUT 37 MIN LATER, WITNESSES SAW THE ROTORCRAFT MOVING ERRATICALLY & TUMBLING OUT OF CONTROL. POSTCRASH EXAM OF THE ROTORCRAFT'S FLIGHT CONTROL SYSTEM REVEALED A DISCONNECT BETWEEN THE LOWER BEARING END OF THE AFT DIRECTIONAL AND LATERAL CONTROL PUSHROD & THE INBOARD CLEVIS OF THE FORWARD MIXING UNIT SECTION BELLCRANK. A BOLT, CONSISTENT WITH HARDWARE FOR THAT CONNECTION, WAS FOUND IN THE CONTROL CLOSET AREA. AN IMPROPER PART (COLLECTIVE BELLCRANK, PN 107C2606-8) WAS FOUND IN PLACE OF THE REQUIRED LATERAL BELLCRANK (PN 107C2606-9). CLEVIS WIDTH OF THE -8 PART WAS SLIGHTLY LARGER THAN THE -9 PART; THUS, THE CLEVIS BOLT WAS NOT LONG ENOUGH TO ALLOW A COTTER PIN TO BE PROPERLY INSTALLED THROUGH THE NUT & BOLT WITH THE REQUIRED WASHERS (2 THICK & 1 THIN) INSTALLED. TO COMPENSATE (ALLOW FOR INSTALLATION OF THE COTTER PIN), 2 THIN WASHERS WERE USED IN PLACE OF THE 2 THICK AND 1 THIN WASHERS. METALLURGICAL EXAM OF THE BOLT REVEALED EVIDENCE THAT A NUT HAD BEEN APPLIED TO THE THREADED END, BUT THERE WAS NO EVIDENCE THAT A COTTER PIN HAD BEEN INSERTED. NO PRE-ACCIDENT ENGINE MALFUNCTION OR CREW IMPAIRMENT WAS EVIDENT. THE NATIONAL TRANSPORTATION SAFETY BOARD DETERMINES THE PROBABLY CAUSE(S) OF THIS ACCIDENT AS FOLLOWS. FAILURE OF MAINTENANCE PERSONNEL TO INSTALL A COTTER PIN IN A CLEVIS BOLT IN THE FLIGHT CONTROL SYSTEM, WHICH RESULTED IN THE AFT DIRECTIONAL AND LATERAL CONTROL OUTPUT PUSHROD TO BECOME DISCONNECTED FROM A BELLCRANK IN THE FORWARD PORTION OF THE FIRST STAGE MIXING UNIT. A FACTOR RELATING TO THE ACCIDENT WAS THE USE OF AN IMPROPER BELLCRANK, WHICK WAS WIDER IN THE CLEVIS AREA.
NTSB investigation
The NTSB’s investigation of this event, with its probable cause and findings. Matched to this report by N-number and date.