What happened
On 20 March 2004, a Baby Belle amateur-built helicopter, registration C-FZQN, departed from a farm near Ralph, Saskatchewan, for a local visual-flight-rules mission. The pilot intended to inspect various road and highway intersections for snow accumulation. Shortly after takeoff, while flying northwest at an altitude of approximately 500 feet, debris began falling from the aircraft. The helicopter experienced a vertical drop and a steep nose-down pitch, ultimately crashing into a farm field. The pilot, who was the sole occupant, sustained fatal injuries, and the aircraft was destroyed by a post-impact fire.
The investigation
The investigation examined the wreckage and the mechanical state of the aircraft. Although a fire prevented the analysis of fuel samples and instrument readings, investigators identified that the main rotor blades had separated from the main mast yoke. Marks on the yoke suggested mast bumping had occurred. The investigation also uncovered debris, including parts of the tail boom and the tail rotor, scattered up to 500 metres from the primary wreckage site. Analysis of the tail rotor gear box shaft revealed an overload failure. Furthermore, a section of the horizontal stabilizer spar showed a complete fracture at a rivet hole, with evidence of 75 per cent fatigue cracking around the circumference of the tube.
Findings
Investigators determined that the separation of the horizontal stabilizer led to a loss of control and a sudden upward pitch of the tail boom. This movement caused the main rotor blades to bend downward, resulting in the tail structure striking the rotor disc. This interference caused the in-flight breakup of the aircraft.
Several factors contributed to this structural failure:
- A previous flight involving the separation of a tail-rotor blade had induced excessive vibrations in the stabilizer, leading to the bending of the horizontal stabilizer spar.
- The use of an unrecommended maintenance practice—cold straightening of the stabilizer spar—concentrated stresses at the first rivet hole, which facilitated fatigue cracking.
- The installation of an end cap at the root end of the stabilizer spar obscured the fatigue crack from inspection.
Safety action
Following the investigation, the manufacturer of the Baby Belle kit released a technical bulletin to operators. This bulletin provided information regarding the accident and outlined recommended inspection criteria, specifically advising operators to remove the end cap at the root end of the stabilizer spar if it is currently installed.