What happened
On a day in mid-afternoon, a MD60 and0N helicopter, registered as C-FTAZ, was performing heli-slinging duties approximately 25 nautical miles northwest of Zama Lake, Alberta. Operated by Rupert's Land Operations Inc., the aircraft was in the process of returning to a staging area after picking up the final bag of a seismic equipment load. While flying roughly 3 nautical miles from the pick-up site, the tail boom suddenly separated from the main fuselage. The aircraft crashed into a clearing about 1600 feet from the initial lift site, with the 115-foot longline and its load still attached. The pilot, who was the sole occupant of the aircraft, sustained fatal injuries. There was no fire following the impact.
The investigation
Investigators focused their analysis on the structural integrity of the tail boom attachment fittings and the effectiveness of existing inspection protocols. The investigation examined the sequence of mechanical failures that led to the separation of the tail boom from the fuselage. The TSB also reviewed the 25-hour inspection procedures, which had been modified via an Alternate Method of Compliance (AMOC) to allow for visual inspections rather than the borescope method previously required by Airworthiness Directive AD2001-24-51.
Findings
- The primary cause of the accident was fatigue cracking at the upper right tail boom attachment fitting.
- This initial failure caused a massive transfer of structural loads to the remaining three attachment fittings and the aft ring structure.
- Fatigue also developed in the locknut flexible beam elements of the upper left fitting, which weakened the fastener and caused the upper left bolt to pull out.
- The failure of these two upper fittings created an overload that led to the separation of the two lower fittings, resulting in the total detachment of the tail boom and subsequent loss of aircraft control.
- The visual inspections permitted under the AMOC failed to detect the damage prior to the structural failure, potentially because debris near the fittings obscured the cracks.
Safety action
Following the accident, Rupert's Land Operations Inc. voluntarily grounded its MD600N fleet to replace attachment fittings. Subsequent actions included a service bulletin from McDonnell Douglas Helicopters Inc. (MDHI) mandating borescope inspections and the replacement of fittings, as well as a technical bulletin regarding fuselage and tail boom strengthening. Additionally, the FAA issued an Airworthiness Directive requiring the creation of new inspection holes and the replacement of aluminum upper right fittings with steel components.