AD/B737/358 Electronic Flight Instrument System Cooling (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.

Boeing 737 Series Aeroplanes

AD/B737/358 Electronic Flight Instrument System Cooling 8/2009

Applicability:	Model 737-300, -400 and -500 series aeroplanes, as identified in Boeing Alert Service Bulletin (ASB) 737-21A1156, Revision 2, dated 11 December 2008.
Requirement:	Modify the control power wiring of the normal supply fan and the low flow sensor for the equipment cooling system of the electronic flight instrument system (EFIS), by doing all the applicable actions specified in the Accomplishment Instructions of ASB 737-21A1156, Revision 2.
For all aeroplanes - Actions accomplished before the effective date of this Directive in accordance with Boeing ASB 737-21A1156, Revision 1, dated 23 October 2007, are acceptable for compliance with the corresponding requirements of this Directive.
For Groups 1 and 2 aeroplanes identified in Boeing ASB 737-21A1156, Revision 1 - Actions accomplished before the effective date of this Directive in accordance with Boeing ASB 737-21A1156, dated 20 June 2006, are acceptable for compliance with the corresponding requirements of this Directive.
Later revisions of the ASB 737-21A1156, approved by the United States Federal Aviation Administration (FAA) as an Alternate Method of Compliance (AMOC) to FAA AD 2009-12-05 are considered acceptable for compliance with the equivalent Requirements of this Directive.
Note:  FAA AD 2009-12-05 Amdt 39-15928 refers.
Compliance:	Within 24 months after the effective date of this Directive.
This Airworthiness Directive becomes effective on 30 July 2009.

Background: This Directive results from a report of loss of both the normal EFIS cooling supply and the indication of EFIS cooling loss due to a single failure of the battery bus, causing eventual power-down of the EFIS displays.  The standby attitude indication is also powered by this battery bus.  This Directive is issued to prevent loss of all attitude indications from both the standby indicator and EFIS displays, which could decrease the ability of the flightcrew to maintain the safe flight and landing of the aeroplane.

James Coyne
Delegate of the Civil Aviation Safety Authority

16 June 2009