Summary
We propose to adopt a new airworthiness directive (AD) for certain The Boeing Company Model 757 and Model 767 airplanes. This proposed AD was prompted by a standby power relay failure and subsequent illumination of the ''STANDBY BUS OFF'' light, which led the flightcrew to set the standby power switch to the ''BAT'' position, isolating the battery and standby buses, disabling the battery charger, and eventually causing the main battery to be depleted. This proposed AD would require doing wiring changes and installing a new air/ground relay to the battery charger system. We are proposing this AD to prevent discharge of the main battery, which could result in multiple system degradation, reduced airplane controllability, and runway excursion upon landing.