AD/F100/81 Autopilot Disconnect Unit (Cth)

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 39.001(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 F100 (F28 Mk 100) Series Aeroplanes

AD/F100/81 Autopilot Disconnect Unit 9/2006

Applicability:	F28 Mark 0070 and 0100 aeroplanes.
Requirement:	Modify the aeroplane in accordance with Section 3 - Accomplishment Instructions of Fokker Service Bulletin SBF100-22-050 dated 25 April 2006 or later EASA approved revision.
Note:  CAA-NL AD NL-2006-010 (EASA Approval 2006-0195) refers.
Compliance:	Within 36 months after the effective date of this Directive.
This Airworthiness Directive becomes effective on 31 August 2006.
Background:	The Netherlands Civil Aviation Authority (CAA-NL) has advised that to date, there have been at least 10 reported events on Fokker 70 (F28 Mark 0070) and Fokker 100 (F28 Mark 0100) aircraft where the flight crew manually overpowered the autopilot, inadvertently neglecting to disengage the autopilot.
Detailed investigation of these incidents has shown that this usually occurs in a high workload environment that demands immediate manual control of the aircraft by the pilot flying, eg terrain warning.  When the autopilot is not disengaged, the elevator servomotor is overpowered and the horizontal stabilizer is moved by the Automatic Flight Control & Augmentation System (AFCAS) auto-trim in a direction opposite to the (manual) deflection of the elevator, causing high elevator control forces.  This condition, if not corrected, could cause the stabilizer to move to an extreme out-of-trim position, creating the (remote) possibility of loss of control of the aircraft, due to the extreme control loads.

In the original design of AFCAS, operation of the control wheel-mounted stabilizer trim switches has no effect when the autopilot is engaged.  Based on the assumption that stabilizer trim switches will be operated by the pilot flying when encountering high control forces, an Autopilot Disconnect Unit (ADU) has been developed that disconnects the autopilot when the stabilizer trim switches are operated.  Since a potentially unsafe condition has been identified that may exist or develop on aircraft of this type design, this Directive requires the installation of ADUs and associated wiring changes.

James Coyne
Delegate of the Civil Aviation Safety Authority

21 July 2006