Transport Canada CF-2019-23

Mandates a software upgrade for the HSTECU to verify the motor brake assemblies (MBA) for braking capability during the power-up test. During an unscheduled inspection, a mis-installed no-back pawl was discovered on a Horizontal Stabilizer Trim Actuator (HSTA). The no-back mechanism is a primary means to prevent back driving of the HSTA, and the MBA are the secondary means. If not corrected, unavailability of the no-back mechanism in combination with loss of, or degraded HSTA MBA braking capability, could lead to a loss of the airplane.