What happened
A Cessna 152 departed Saskatoon, Saskatchewan, for a flight intended to practice stalls, spins, and slow flight. The crew operated within the CYA 306(T) training area, performing various maneuvers between 4,000 and 4,500 feet above sea level. During the session, the aircraft entered a spin at approximately 4,000 feet. Radar data and witness accounts indicate the aircraft continued its descent, eventually reaching an altitude of about 200 feet above ground level near Vanscoy. A witness observed the aircraft momentarily in a level attitude before it banked sharply to the left and struck a field. Both occupants, a highly experienced pilot and a student instructor, were fatally injured.
The investigation
The investigation focused on the aircraft's mechanical condition and the physiological state of the crew. While the aircraft's control systems and engine were found to be functional, investigators identified a small hole in the number 4 cylinder exhaust stack. This leak allowed exhaust gases to accumulate in the engine cowling and potentially migrate into the cockpit.
Toxicology results showed carbon monoxide levels in both pilots, with the pilot-in-command measuring between three and nine per cent and the student between zero and five per cent. When accounting for the reduced oxygen availability at 4,000 feet, the effective physiological impact was higher. Additionally, the post-mortem examination of the pilot-in-command revealed mild atherosclerosis, which could potentially increase susceptibility to angina when combined with carbon monoxide exposure.
Findings
- The aircraft entered a secondary stall during an attempt to recover from the initial spin, but the low altitude prevented successful recovery.
- An exhaust leak in the number 4 cylinder stack allowed carbon monoxide to enter the cabin.
- The crew was exposed to low levels of carbon monoxide, which may have contributed to physiological effects.
- The training maneuver was continued below the regulatory minimum altitude for aerobatic maneuvers for reasons that remain undetermined.
- The instructions for the installed carbon monoxide detector were obscured by its mounting, making it difficult for the crew to monitor for leaks during flight.