Canadian investigators caution that the risks spotlighted in the November 2014 landing gear collapse of a Jazz Aviation Bombardier Q400 are not limited to the twin turboprop.
The twin-engine turboprop, flying as Air Canada Express Flight 8481, experienced a tire rupture on takeoff from Calgary, Alberta, and diverted to Edmonton International Airport. On landing there in the dark, the deflated and unbalanced right tire spun up, creating a vibration that caused the right gear to unlock and collapse. As the gear collapsed, all four right propeller blades sheared off, with one blade entering the cabin.
Three passengers were injured when the blade pierced the cabin, making projectiles of plastic pieces from the sidewalls and windows.
In its final report on the incident, the Transportation Safety Board of Canada (TSB) cautioned that a gap in certification regulations means other aircraft types could be at risk of a similar occurrence.
“If there are no specific requirements for dynamic vibration testing of components or completed airframes, there is a risk that similar or other aircraft systems could fail during high-vibration conditions,” the TSB said.
Canadian certification rules do not list vibration criteria for aircraft certification, other than references to aerodynamic-induced vibration on the airframe in flight, despite a broader requirement that any malfunction of a component that could affect safe flight, landing or the operation of the aircraft under adverse conditions be “improbable.”
The investigation also revealed a gap in safety procedures for tire blowouts: The Q400 aircraft flight manual does not address landing with a flat tire. While Bombardier has guidance telling crews to move passengers from seats directly in line with the propellers in the event of an unsafe landing gear condition, on the flight when the accident occurred, the gear was properly down and locked and there were no associated warnings in the cockpit.
Investigators also determined that right-side tires on Q400s tend to fail more frequently than others, an effect they tied in part to “hard” right turns under power as aircraft depart gates when no passenger bridges are used. “This hard braking and turning may cause an extreme shearing force on the tread areas and sidewalls of the right tire,” the TSB said.
The failed tire, made by Dunlop, had been retreaded once. It had most likely struck a hard object during a previous flight, according to the TSB. After the rupture, portions of the tire struck the aft fuselage.
Following the accident, Jazz stopped using retreaded tires on the Q400, changed pilot procedures to “avoid braking and tire pivot whenever possible” and modified taxiing techniques to lessen stress on the main landing gear tires.
The TSB said other Q400 operators also changed procedures to mitigate the effect of sharp right turns on the ramps near gates.