The hard landing of a Jazz Aviation Bombardier Q400 in 2017 substantially damaged the aircraft and generated enough load to trigger a switch designed to activate only after a crash, according to a report from the Transportation Safety Board of Canada.
The report notes that the company was unaware of the damage and operated the aircraft prior to making repairs.
The incident involved Jazz flight 7977, a Q400 (registration C-GYJZ) from Montreal to Billy Bishop Toronto City airport on the evening of 9 November last year.
Immediately prior to touchdown on Billy Bishop's Runway 26, wind shear caused the aircraft's speed to decline, leading the pilot flying to increase power to the Q400's Pratt & Whitney Canada PW150A turboprops, says the TSB's report released on 9 July.
The aircraft then touched down on the runway and a pilot reduced the throttle, but the aircraft lifted again when more wind shear caused the airspeed to increase, the TSB says.
The wind shear then eased, and the aircraft, now also lacking lift from the idle engines, dropped and hit the runway hard, generating what investigators estimate was no less than 5.7g of vertical load.
That force "exceeded the design criteria for the landing gear" and triggered an "inertia switch" designed to activate when subjected to a force of 5.5g, says the TSB.
That switch cuts power to the flight data and cockpit voice recorders, ensuring those recorders do not erase data after a crash, the TSB says.
The pilots received a warning in the cockpit of a problem with the flight recorder.
After taxiing the Q400 to the gate, the pilots contacted Jazz maintenance personnel, who said they did not believe the landing caused the issue with the flight recorders.
The pilots were unsure if the incident classified as "hard landing", which would require an inspection by mechanics.
Those mechanics would need to travel to Billy Bishop from Toronto Pearson International airport, a time-consuming process that would delay departure of the return flight beyond the airport's 22:00 curfew, causing a cancellation, the TSB says.
The pilots therefore conducted a visual inspection of the aircraft themselves, found nothing abnormal, and decided to operate the return flight, landing uneventfully back in Montreal.
An inspection at Montreal, however, revealed the Toronto landing exceeded the Q400's "design criteria", caused buckling of the aircraft's skin below the windows on the right side and damaged the right-side landing gear.
Jazz declines to comment further to FlightGlobal, calling the report "thorough" and noting that it outlines improvements made by the company.
The TSB's report says the company has taken steps to prevent such incidents.
Jazz has provided additional training to help crews recognise hard landings, improved procedures related to suspected or actual hard landings, updated flight crew guidance and taken steps to prevent departures in cases of uncertain airworthiness, the TSB says.
But the report notes that neither Jazz nor Bombardier provides pilots with firm means to determine when a hard landing occurred. Though maintenance manuals do classify hard landings as those exceeding "vertical acceleration thresholds", pilots have no means to determine when those thresholds have been exceeded, the TSB says.