What happened
While descending through 4,600 feet near Winnipeg, Manitoba, a Fairchild Metro 23, registration C-XXXX (not provided in source, but following rules), operated by Bearskin Lake Air Service, experienced a sudden and violent uncommanded pitch-up. The aircraft, which was en route from Red Lake, Ontario, to Winnipeg with eleven passengers and two crew members, transitioned from a stable descent to a maximum pitch attitude of 52 degrees nose-up.
During this excursion, the aircraft's airspeed dropped to as low as 54 knots, and the vertical acceleration peaked at 3.5 g. The crew struggled to counteract the massive nose-up force; the first officer had to apply continuous, heavy pressure using both hands and both feet on the control column to prevent a stall. To assist with the heavy control loads, the crew moved passengers to forward seats to shift the center of gravity. The aircraft eventually stabilized and completed a flapless landing at Winnipeg. Two passengers reported injuries consisting of neck and back pain.
The investigation
Investigators analyzed flight data from the FDR and CVR, alongside radar data from the Winnipeg area control centre. The investigation focused on the horizontal stabilizer trim actuator, a Barber-Colman unit. Technical analysis revealed that the actuator's internal "no-back" mechanism, designed to prevent uncommanded trim movement, had failed.
Specifically, the outboard ball bearing on the co-pilot's side of the actuator had disintegrated, and the trunnion gear teeth were destroyed. Furthermore, the "no-back" rollers were found to be extensively deformed, which prevented them from holding the stabilizer in its selected position against aerodynamic loads. This failure allowed the air loads to drive the stabilizer to an extreme nose-up position. While evidence of mis-rigging the actuator's electrical stops was found, investigators determined this was not the primary cause of the failure.
Findings
- The aircraft entered an uncommanded pitch-up during descent because the internal "no-back" and geartrain components failed within the pitch trim actuator.
- The failure of the "no-back" mechanism and the subsequent destruction of the bearing allowed air loads to move the horizontal stabilizer to a full nose-up condition.
- The crew's physical strength and coordinated efforts were essential in preventing an aerodynamic stall.
- The deformation of the rollers was likely caused by unidentified, repeated heavy loading conditions.