AD FAA-2012-0862

We are superseding Airworthiness Directive (AD) 2008-08-25 that applied to certain The Boeing Company Model 747-400 and 747-400F series airplanes. AD 2008-08-25 required installing drains and drain tubes to eliminate water accumulation in the drip shield above the M826 cardfile in the main equipment center. This new AD requires installing modified drain tubes, relocating wire bundle routing, installing a new drip shield and drip shield deflectors, and replacing insulation blankets. For certain airplanes, this new AD also concurrently requires sealing the drain slot, installing spuds, and installing drain tubes. This AD was prompted by reports of continued water damage to diode fire card 285U0072-1 in the M826 automatic fire overheat logic test system cardfile following a false FWD CARGO FIRE message, with no change in frequency, which resulted in an air turn back. We are issuing this AD to prevent water from exiting over the edge of the existing drip shield and contaminating electrical components in the M826 cardfile, which could result in an electrical short and potential loss of several functions essential for safe flight.