Airworthiness Directive · Amendment 39-15928 · Boeing Model 737-300, -400, and -500 Series Airplanes
Boeing Model 737-300, -400, and -500 Series Airplanes
This AD applies to Boeing Model 737-300, -400, and -500 series airplanes, certificated in any category; as identified in Boeing Alert Service Bulletin 737-21A1156, Revision 2, dated December 11, 2008.
We are adopting a new airworthiness directive (AD) for certain Boeing Model 737-300, -400, and -500 series airplanes. This AD requires modifying 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). This AD 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. We are issuing this AD 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 airplane.
Comply with this AD within the compliance times specified, unless already done.
Within 24 months after the effective date of this AD: Modify the control power wiring of the normal supply fan and the low flow sensor for the equipment cooling system of the EFIS, by doing all the applicable actions specified in the Accomplishment Instructions of Boeing Alert Service Bulletin 737-21A1156, Revision 2, dated December 11, 2008.
(g)(1) Actions done before the effective date of this AD in accordance with Boeing Alert Service Bulletin 737-21A1156, Revision 1, dated October 23, 2007, are acceptable for compliance with the corresponding requirements of this AD.
(2) For Groups 1 and 2 airplanes identified in Boeing Alert Service Bulletin 737-21A1156, Revision 1, dated October 23, 2007: Actions done before the effective date of this AD in accordance with Boeing Alert Service Bulletin 737-21A1156, dated June 20, 2006, are acceptable for compliance with the corresponding requirements of this AD.
