COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES British Aerospace BAe 3100 (Jetstream) Series Aeroplanes AD/JETSTREAM/22 Ground Idle Increase 13/87 DM Applicability: All Jetstream 3100 series aircraft S/N 601 to 763. Requirement: Action in accordance with BAe Alert Service Bulletin 76-A-JM7620. Compliance: For Part A of requirement document: Within 200 hours time in service or by 20 November 1987, whichever occurs first. For Part B: As advised in requirement document. Background: The propeller manufacturer has discovered unexpected natural frequency variations during routine propeller blade inspection. Some of these frequencies are high enough to approach the engine resonant frequency at ground idle in adverse (tail wind) conditions with the resulting high stress introducing possible fatigue damage to the blades. Increasing ground idle to a minimum of 72% will avoid these resonant frequencies.