Summary
We propose to supersede an existing airworthiness directive (AD) that applies to certain The Boeing Company Model 747-400 and 747- 400F series airplanes. The existing AD currently requires installing drains and drain tubes to eliminate water accumulation in the dripshield above the M826 cardfile in the main equipment center. Since we issued that AD, we received 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. This proposed AD would instead require installing drain tubes, relocating wire bundle routing, installing a new drip shield and drip shield deflectors, and replacing insulation blankets. For certain airplanes, this proposed AD would also concurrently require sealing the drain slot, installing spuds, and installing drain tubes. We are proposing this AD to prevent water from exiting over theedge 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.