AIRWORTHINESS DIRECTIVE For the reasons set out in the background section, the CASA delegate whose signature appears below issues the following Airworthiness Directive (AD) under subregulation 39.1 (1) of CASR 1998. The AD requires that the action set out in the requirement section (being action that the delegate considers necessary to correct the unsafe condition) be taken in relation to the aircraft or aeronautical product mentioned in the applicability section: (a) in the circumstances mentioned in the requirement section; and (b) in accordance with the instructions set out in the requirement section; and (c) at the time mentioned in the compliance section. Fokker F50 (F27 Mk 50) Series Aeroplanes AD/F50/87 Pitot & Static Port Heating Wiring 8/2004 Applicability: Model F.27 Mk.050, Mk.0502 and Mk.604 aeroplanes, all serial numbers. Requirement: Modify the wiring of the pitot and static port heating in accordance with Section 2 - Accomplishment Instructions of Fokker Service Bulletin F50-30-028 dated 4 March 2004 or Revision 1 dated 26 April 2004. Note 1: Fokker Services Manual Change Notification - Maintenance Documentation (MCNM) F50-048 dated 4 March 2004 also pertains to this subject. Note 2: CAA-NL AD 2004-038 (A) Corr refers. Compliance: Within the next 36 months after the effective date of this Directive. This Airworthiness Directive becomes effective on 5 August 2004. Background: The Netherlands CAA has received a report from a Fokker 50 operator of intermittent events where, during autopilot engagement in IAS-Mode decent, the aircraft suddenly pitched nose down resulting in an overspeed situation. Investigation revealed a failed heater element in the left hand upper pitot-static tube, which was not detected by the current sensor, and as a result no "PITOT HEAT" alert was generated. In a single air data computer (ADC) configuration only pressure from the left hand upper pitot head is used. In this instance freezing of the pitot head resulted in erroneous ADC airspeed output Fokker F50 (F27 Mk 50) Series Aeroplanes AD/F50/87 (continued) Currently, to detect failures, each heating element is connected in series with a current sensor, which is designed to detect low or zero heater current. This Directive requires the modification of the heater circuit wiring to extend detection capability to include heating element short circuits to ground and heating element failures that result in a low insulation resistance. James Coyne Delegate of the Civil Aviation Safety Authority 23 June 2004 The above AD is notified in the Commonwealth of Australia Gazette on 14 July 2004.