What happened
On 13 March 1994, an ATR 42, registration C-GIQV, was performing a scheduled flight from Val D'Or Airport to Montreal International (Dorval) Airport. The aircraft was carrying 23 passengers and 3 crew members under instrument flight rules.
While climbing through approximately 17,000 feet, a violent explosion occurred, resulting in the loss of power from the right engine and immediate cabin depressurization. The fracture of the No. 2 propeller blade caused it to penetrate the aircraft fuselage, creating a vertical slit measuring 104 cm by 2.5 cm. The force of the imbalance caused the propeller and reduction gearbox to separate from the engine, tearing away engine mounts and the air inlet.
Following the event, the crew executed emergency procedures, including the engine failure checklist and managing the cabin altitude. The pilot-in-command elected to continue to Dorval to minimize flight maneuvers and potential further structural damage. The aircraft landed at 1116 EST without further incident. While there were no fatalities, two passengers required medical examination for ear discomfort caused by the pressure change.
The investigation
The investigation focused on the mechanical failure of the propeller and the manufacturing of the blades. Investigators examined the engine mounts, the fuselage damage, and the internal structure of the propeller blades.
Technical analysis revealed that the fracture originated inside the taper bore of the blade, making it impossible to detect via external inspections. The investigation also looked into the manufacturing processes of the blades, specifically the use of cork and the discontinuation of shotpeening in 1987. The investigators also reviewed the maintenance history, including the 7,500-hour major inspection performed on the aircraft.
Findings
- The primary cause of the blade fracture was corrosion pitting within the taper bore.
- This corrosion was triggered by a chemical reaction between water and chlorine deposits found on the cork used to hold lead wool in place.
- The chlorine was a byproduct of the bleaching process used during the manufacture of the cork.
- The lack of shotpeening in the taper bore area, which had been discontinued in 1987, reduced the blade's resistance to crack propagation.
- The force generated by the propeller imbalance exceeded the structural limits of the engine mounts, leading to the separation of the engine components.