Airworthiness Directives; The Boeing Company Airplanes

We are adopting a new airworthiness directive (AD) for certain The Boeing Company Model 747-400 series airplanes. This AD was prompted by reports of water leaking into electrical and electronic equipment in the main equipment center (MEC). This AD requires modifying the floor panels; removing drains; installing floor supports, floor drain trough doublers, drain troughs, and drains; and sealing and taping the floor panels. We are issuing this AD to prevent water from entering the MEC, which could result in an electrical short and potential loss of several functions essential for safe flight.