COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Fokker F50 (F27 Mk 50) Series Aeroplanes AD/F50/77 Stabilizer De-Icing Monitoring System 6/98 Applicability: Model F.27 Mk.050 (Fokker 50) aeroplanes with serial numbers 20103 through 20245 inclusive, 20248, 20250 through 20255 inclusive, 20257 through 20269 inclusive, 20272 through 20275 inclusive, 20278, 20280 through 20283 inclusive, 20285, 20287 through 20302 inclusive, 20304, 20305 and 20307 through 20311 inclusive. Requirement: Install a horizontal and vertical stabilizer de-icing monitoring system in accordance with Fokker Service Bulletin SBF50-30-025 dated 31 October 1997. Note: RLD AD 1997-113 (A) refers. Compliance: For aeroplanes without any tail de-icing distributor valve heating system installed - Within the next 10 calendar months after the effective date of this directive. For aeroplanes that have a tail de-icing distributor valve heating system installed, either in accordance with Fokker Service Bulletin SBF50-30-024 dated 26 March 1997 or Fokker Engineering Report TT95.55175 - Within the next 16 calendar months after the effective date of this directive. This airworthiness directive becomes effective on 21 May 1998. Background: The Netherlands airworthiness authority has advised that a number of F.27 Mk.050 operators have experienced problems with the tail de-icing system. After prolonged operation at very low outside air temperatures, one or more sections of the de-icing boots do not inflate when the system is selected ON. Investigation shows that interrupted boot operation may be caused by freezing of one or more distributor valves. Fokker has developed an optional service bulletin (SBF50.30.024), which introduces heater muffs on the affected distributor valves, to decrease the likelihood of the valves freezing. However, on early models, the operation of the de-icing system is not monitored. This directive introduces a system to monitor the operation status of the horizontal and vertical stabilizer de-icing, and provide a warning in the cockpit when operation is interrupted.