AD/F50/99 Electrical Power AC Contactors (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 F50 (F27 Mk 50) Series Aeroplanes

AD/F50/99 Electrical Power - AC Contactors 8/2008

Applicability:

F27 Mark 050 and Mark 0502 aircraft, all serial numbers, unless modified during production to incorporate Fokker ECR 51780 or whilst in service in accordance with Fokker Service Bulletin SBF50-24-030 dated 6 November 2003 or later revision approved by the European Aviation Safety Agency (EASA).

Requirement:

1.    Remove the standard nuts and lock washers from electrical power contactors part number (P/N) SG02206, install new self-locking nuts and perform the applicable tests on the AC Bus Transfer System in accordance with the Accomplishment Instructions of Fokker Services SBF50-24-031 dated 29 January 2008 or later revision approved by the EASA.

Note 1:  The replacement of contactors P/N SG02206 with improved versions in accordance with SBF50-24-030 or later revision approved by the EASA is acceptable as an alternate method of compliance for Requirement 1 provided that the replacement is accomplished within 36 months after the effective date of this Directive.

2.    Contactors P/N SG02206 may not be installed on any aircraft as a replacement part, unless it has been modified in accordance with Goodrich Power Systems SB SG02206-24-01 dated 4 March 2008 or later revision approved by the EASA.

Note 2:  EASA AD 2008-0091 refers.

Compliance:

For Requirement 1 - Within 36 calendar months after the effective date of this Directive.

For Requirement 2 - After 31 May 2011.

This Airworthiness Directive becomes effective on 31 July 2008.

Background:

Several incidents have been reported where an electrical burning smell was noted in the cockpit, originating from the Electrical Power Centre.

Troubleshooting revealed a partly molten terminal, which normally attaches a wire or bus bar to a stud of an Electrical Power Contactor, P/N SG02206.  Furthermore, heat damage to the contactor stud itself was found.  Material investigation revealed that the terminal, which was attached to the stud, was not properly torque tightened when the incident occurred.

Loss of torque is considered to have occurred during operation, for reasons not fully understood.  Further loosening may have taken place in-service under influence of vibration.  As a result, poor contact caused electrical arcing during which extremely high temperatures were developed, leading to partial melting of the terminal.

Investigation of some other burned contactors revealed evidence (flat spring lock washer) of a fully torqued terminal/stud connection when the overheating occurred.  The exact cause for the increase in temperature in the contactor and the terminal/stud could not be determined.  However, it could not be excluded that an increase of the temperature inside the contactor could lead to reduction of the reliability of the contactor stud/terminal connection due to loss of lock washer tension.  The affected Electrical Power Contactor is used on several locations in the electrical power system, i.e. Generator Line Contactor (GLC), Bus Tie Contactor (BTC), Auxiliary Power Contactor (APC) and External Power Contactor (EPC).

This condition, if not corrected, could lead to further cases of overheating of terminals and studs of Electrical Power Contactors P/N SG02206, possibly resulting in the loss of electrical power systems, electrical arcing and fire/smoke in the cockpit.

James Coyne
Delegate of the Civil Aviation Safety Authority

13 June 2008