Occurrence Summary
On a day in 2006, a McDonnell Douglas Helicopters Inc. (MDHI) MD600N helicopter, registration C-FTAZ, serial number RN011, was conducting heli-slinging operations 25 nautical miles northwest of Zama Lake, Alberta (CFT9). The pilot, the sole occupant, had picked up the fifth and final bag of seismic equipment and was returning to the staging area, approximately 3 nautical miles from the pick-up site. While en route, the tail boom separated from the fuselage, and the helicopter crashed into a clearing about 1600 feet from the pick-up site. The sling load remained attached to the aircraft via a 115-foot longline. The tail boom was found approximately 240 feet from the main wreckage. The accident occurred at about 1400 mountain standard time. The pilot sustained fatal injuries. There was no post-impact fire.
Investigation Findings
The investigation determined that fatigue cracking of the upper right attach fitting initiated the failure sequence. This cracking transferred loads to the aft ring structure and the remaining three tail boom attach fittings. Subsequently, fatigue developed in the locknut flexible beam elements of the upper left attach fitting, compromising the fastener strength and allowing the upper left bolt to pull out. The failure of the two upper attach fittings caused overload and separation of the lower two attach fittings, resulting in tail boom detachment and loss of aircraft control. Safe recovery following such structural failure was considered unlikely.
The pilot held a commercial helicopter pilot licence and a valid aircraft maintenance engineer licence, though not employed as such at the time. The pilot had accumulated 9827 total flight hours, including 2056.6 hours on the MD600N. The most recent pilot proficiency check was on 30 June 2005.
The mandated 25-hour inspection per Airworthiness Directive AD2001-24-51 required borescope inspection of the upper fittings. This was later augmented by an Alternate Method of Compliance (AMOC) allowing visual inspections. The 25-hour visual inspections did not identify damage to the tail boom attach fittings before structural failure; debris around the fitting may have obscured a crack.
Safety Actions Taken
Rupert's Land Operations Inc. voluntarily grounded its MD600N fleet pending replacement of attachment fittings per Service Bulletin SB600N-43. On 13 April 2006, MDHI issued SB600N-043 requiring borescope inspection of all four attach fittings and replacement within 25 hours. On the same date, MDHI issued Technical Bulletin TB600N-007R1 to strengthen the tail boom attach fittings and upper longerons. On 27 April 2006, the FAA issued AD2006-08-12 requiring additional inspection holes, inspection of all attach fittings and related structure, replacement of the aluminum upper right fitting with steel fittings, painting of the inspection area, and replacement of existing nut plates within specified times.