COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Fokker F100 (F28 Mk 100) Series Aeroplanes AD/F100/33 Wheel-Speed Sensor Wiring 4/99 Applicability: F.28 Mk.0100 aeroplanes listed in Fokker Service Bulletins (SB) F100-32-067 Revision 1 dated 6 July 1998 and F100-32-037 Revision 1 dated 4 December 1998. Requirement: 1. If not previously accomplished, modify the grounding of the wheel speed sensor wiring shielding in a0cordance with Part 1, 2, 3, or 4, as applicable, of the Accomplishment Instructions of Fokker SB F100-32-067 dated 12 March 1993 or Revision 1 dated 6 July 1998. 2. Install new electrical grounding on the wheel speed sensor channel of the anti-skid control box in accordance with Part 1, 2, or 3, as applicable, of the Accomplishment Instructions of Fokker SB F100-32-037 Revision 2 dated 4 December 1998. Note: Netherlands AD 1998-100/2 refers. Compliance: 1. Prior to 1 August 1999. 2. Prior to 15 February 2000. This Airworthiness Directive becomes effective on 22 April 1999. Background: The RLD received a report concerning an incident with a F.28 Mk.0100, where during approach, the liftdumpers inadvertently deployed. Preliminary investigation revealed that the outboard wheel speed signal was induced to 'high' by electromagnetic interference (EMI). The liftdumper arming test did not detect the signal because it was not transmitted until the skid control box was powered-up on landing gear DOWN selection. Further analysis by the RLD of the liftdumper system protection against EMI, together with investigation into the Flight Control Computer failure mode which caused the EMI, have shown that aeroplanes with a post SB F100-32-067 configuration are significantly less susceptible to EMI on the wheel speed signals. This directive requires modification of the wheel speed sensor wiring shielding and the installation of new electrical grounding of the anti-skid system to prevent EMI. The RLD has advised that these modification are considered a partial solution to the problem and that further rulemakeing is anticipated.