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/F100/16 Elevator BCU Backlash Remover Mechanism 4/99 Applicability: F.28 Mk.0100 aircraft, all serial numbers, if equipped with Menasco Aerospace Elevator Booster Control Unit (BCU), P/N 23400-4 or P/N 23400-5, serial numbers MC-001 through MC-288, that have not been modified in accordance with Fokker Service Bulletin (SB) F100-27-061 dated 2 March 1994 and Menasco Aerospace SB 23400-27-15 dated October 1993. Requirement: 1. Perform an operational check for backlash in the elevator mechanical control system in accordance with Part 1 of the Accomplishment Instructions of Fokker SB F100-27-052 Revision 1 dated 29 March 1994. 2. Inspect the elevator BCU backlash remover mechanism P/N 12777-( ) in accordance with Part 2 of the Accomplishment Instructions of Fokker SB F100-27- 052 Revision 1 dated 29 March 1994. Note: If no anomalies are detected during this inspection, continue with appropriate trouble-shooting procedures in accordance with the Aircraft Maintenance Manual. 3. Replace any affected bolts with airworthy parts in accordance with the Aircraft Maintenance Manual. Note: RLD BLA 93-051/3(A) refers. Compliance: 1. Before 500 flight cycles or 60 calendar days, whichever occurs first, since the last inspection in accordance with Requirement 1. Thereafter at intervals not to exceed 500 flight cycles or 60 calendar days, whichever occurs first. 2. Before further flight if, during an inspection in accordance with Requirement 1, any backlash is detected in the control column. 3. Before further flight if, during an inspection in accordance with Requirement 2, either bolt P/N NAS6204C22D or P/N NAS6204C13D cannot move freely or is found corroded. This Airworthiness Directive becomes effective on 22 April 1999. Background: During maintenance on an F.28 Mk.0100, the pivot bolts and bushes of the backlash remover lever mechanism on the elevator Booster Control Unit were found to be corroded. A subsequent investigation uncovered similar cases with other operators. This particular mechanism prevents backlash in the elevator control forces when the elevator BCU is not hydraulically powered, providing the pilot with full manual control of the elevator system. Corrosion on these bolts and bushes causes the backlash remover mechanism to stick, which results in deteriorated elevator control when the BCU is in MANUAL mode.