Summary
This document proposes the supersedure of an existing airworthiness directive (AD), applicable to certain Boeing Model 747- 400, 757-200, 767-200, and 767-300 series airplanes. That AD currently requires repetitive checks to detect certain failures in the warning electronic unit (WEU) or modular avionic warning electronic assembly (MAWEA); repetitive tests to detect any failure of tactile, visual, or aural alerts generated by the WEU or MAWEA; and corrective action, if necessary. This action would make these requirements applicable to other airplanes on which the defective power supplies may be installed, eliminate the repetitive tests for certain airplanes, and increase the interval for the repetitive tests for certain other airplanes. This action also would require replacement of the existing power supplies in the WEU or MAWEA with modified, new, or serviceable power supplies. The actions specified by the proposed AD are intended to prevent failure of the WEU or MAWEA power supplies, which could result in loss of visual, aural, and tactile alerts to the flightcrew. Absence of such alerts could result in the flightcrew being unaware that an immediate or appropriate action should be taken in the event of an unsafe condition.