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 CAR 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 F100 (F28 Mk 100) Series Aeroplanes AD/F100/44 No 1 Pitot Tube AC Sensing Circuit 7/2001 Applicability: All Model F.28 Mk.0070 and Mk.0100 aeroplanes with either Fokker Service Bulletin (SB) F100-30-019 or SB F100-30-020 embodied. Requirement: 1. Perform an operational check of the No 1 Pitot Tube Heating System, to establish whether both tube and mast heaters are serviceable, in accordance with Part 1 of the Accomplishment Instructions of Fokker SB F100-30-25 Revision 1 dated 14 March 2001. 2. If, as a result of the Requirement 1 operation check, either heating element is found to be inoperative, replace the defective Pitot Tube with a serviceable unit of the same part number. 3. Modify the No 1 Pitot Tube Alternating Current Sensing Circuit in accordance with Part 2, 3, 4, 5 or 6, as applicable, of the Accomplishment Instructions of Fokker SB F100-30-25 Revision 1. Note: RLD AD 2001-041 (A) refers Compliance: 1. Within three calendar months after the effective date of this Directive and thereafter at intervals not to exceed 12 months until the Requirement 3 modification is incorporated. 2. Before further flight after the Requirement1 check has been completed. 3. Within three calendar years after the effective date of this Directive. This Airworthiness Directive becomes effective on 12 July 2001. Background: The RLD has advised that recently an operator reported the loss of indicated airspeed (IAS) on the captain's speed indicator during a flight in icing conditions. Inspection revealed an inoperative No 1 pitot tube heater caused by a short circuit in the heater element. Further investigation revealed that a short circuit in the No 1 pitot tube heaters can stay undetected, due to the characteristics of the DC/AC inverter supplying the AC current to the heater. This condition, if not corrected, may lead to further occurrences of undetected short circuits in the pitot tube heaters, with consequent loss the captain's air speed indication. Fokker F100 (F28 Mk 100) Series Aeroplanes AD/F100/44 (Continued) This Directive requires repetitive checking of the No 1 pitot tube heaters until modification of the current monitoring circuit is accomplished. The modification is designed to ensure that short circuits of the pitot tube heaters are detected. Eugene Paul Holzapfel Delegate of the Civil Aviation Safety Authority 21 May 2001 The above AD is notified in the Commonwealth of Australia Gazette on 13 June 2001.