Summary
This document proposes the adoption of a new airworthiness directive (AD) that is applicable to certain Saab Model SAAB 2000 series airplanes. This proposal would require replacement of the ignition exciter in the auxiliary power unit with a part that is designed to operate better in cold weather. This proposal is prompted by two occurrences of the auxiliary power unit (APU) failing to start after flight in cold soak conditions. The actions specified by the proposed AD are intended to prevent such APU failure, which could result in the inability of the APU to restart the engines in the event both engines quit operating during flight.