Airworthiness Directives; Bombardier, Inc. Model BD-100-1A10 (Challenger 300) Airplanes

We are adopting a new airworthiness directive (AD) for all Bombardier, Inc. Model BD-100-1A10 (Challenger 300) airplanes. This AD was prompted by reports that the horizontal stabilizer trim actuator (HSTA) no-back and the number 1 motor brake assembly (MBA) can both fail dormant. This AD requires revising the airplane maintenance schedule to include new functional tests of the HSTA no-back and HSTA brake system. We are issuing this AD to prevent dormant failure of the HSTA no-back and the number 1 MBA, which along with additional component failure could result in an uncontrollable horizontal stabilizer surface runaway without the ability to retrim, and consequent loss of the airplane.