1AD/DHC-6/57 - Voltage Regulators - Inspection and Provision of Ground Wires

COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY

SCHEDULE OF AIRWORTHINESS DIRECTIVES

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DHC-6 (Twin Otter) Series Aeroplanes

AD/DHC-6/57 Voltage Regulators - Inspection and

Provision of Ground Wires

6/85

Applicability: All model DHC-6 Aircraft with S/Nos 531 through 814 (Post Modification No.

6/1590).

Requirement: Inspect the Generator Field Control Relay Panel for the existence of wires between Terminal Block TB-1A, Terminals A4 and C4 and Ground at intercostal C6FS1442-

36. If wires coded 1P80B20N and 2P80B20N exist between terminals A4 and C4 repectively and ground then no further action is required. If no wires exist at these locations, install wires in accordance with the requirements of DeHavilland of Canada

S.B No 6/473. Compliance: Prior to 31 July 1985.

Background: It is believed that during factory incorporation of Mod 6/1590 the two ground wires may have been omitted. The two wires provide “latching” for relays associated with generator overvoltage protection. Failure of these relays to “latch in” after a generator overvoltage may cause the generators to enter a cyclic mode with rising and falling voltages being applied to the aircraft electrical system.