COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Fokker F27 Series Aeroplanes AD/F27/146 Stabilizer De-Icing Monitoring System 6/98 Applicability: Model F.27 Mk.200, Mk.400M, Mk.500, Mk.500F, Mk.500RF and Mk.600 aeroplanes, with serial numbers 10603 through 10692 inclusive. Requirement: Install a horizontal and vertical stabilizer de-icing monitoring system in accordance with Fokker Service Bulletin F27/30-44 dated 20 February 1998. Note: RLD AD 1998-019 (A) refers. Compliance: Within the next 18 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 (Fokker 50) operators have reportedly 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. Early model F.27 'Friendship' aeroplanes are equipped with a tailplane de-icing system which enables the flight crew to monitor the correct functioning of each individual 'boot' segment. However, later model F27 aeroplanes have a similar tail de-icing system as that fitted to Fokker 50 aeroplanes, where the operation of the de- icing system is not monitored. Consequently, information on the system's operational status is not available to the flight crew and any failure would not be detected. This directive introduces a system to monitor the operational status of the horizontal and vertical stabilizer de-icing, which will provide a warning in the cockpit when failure to correctly inflate any 'boot' occurs.