Safety Issue

In response to this occurrence, the aircraft manufacturer:

• issued a multi-operator message that contained advice and instructions for the inspection and interim repair of main equipment centre dripshields in 747-400 aircraft
• published a service bulletin on 4 November 2009, advising that the following modifications be made to 747-400 dripshields:

– add raised flanges to the cooling exhaust holes in the plenum to prevent any water inside the plenum from spilling onto equipment

– install additional drain lines to remove water from the plenum

– repair and reinforce the dripshield gutter

– seal and reinforce the plenum interfaces

– apply a waterproof, fibreglass-reinforced overcoat.

• released a service letter to all operators of the 747-400 aircraft, advising that operators:

– establish maintenance programs for the drain system based on operator experience on the frequency of blockages and over-flow events

– visually check the condition of the forward drain mast connection hose or hoses, repeated at an interval based on operator experience

– perform a visual check and, if necessary, perform a functional test of the forward drain mast

– perform an inspection and clean up of any contaminated areas after liquid is spilled in the galley.

• released a service letter to all operators of the 747-400 aircraft, advising that operators:

– establish maintenance programs for the drain system based on operator experience on the frequency of blockages and over-flow events.

The aircraft operator advised that, as a result of this occurrence, it:

• carried out a fleet inspection of 747-400 dripshields and made temporary repairs in response to any faults found during those inspections

– damage was identified in the dripshields of 18 out of 30 aircraft inspected, and consisted of small cracks around the fasteners and cracks in the seals along the three dripshield joints

• commenced a program to conduct a strengthening and sealing modification to the MEC dripshield plenum on its 747-400 aircraft
• conducted permanent repairs on the dripshield gutters of its 747-400 fleet
• changed the MEC equipment area maintenance procedures to specify dripshield inspection criteria and to provide feedback on the long-term suitability of the modifications and repairs.

Subsequent to this occurrence, the United States (US) Federal Aviation Administration (FAA):

• adopted airworthiness directive AD 2009 22 14 (effective 8 December 2009), requiring the installation of larger dripshields and drain lines in the electrical/electronic equipment centres of aircraft manufacturer’s 747 200C and 747 200F aircraft to prevent water contamination of essential electrical/electronic units. The aircraft manufacturer’s procedure referenced by the airworthiness directive was first published on 15 February 2007 and revised on 9 October 2008.
• issued a notice of proposed rulemaking (NPRM) on 5 May 2010 stating: We propose to adopt a new airworthiness directive (AD) for certain Model 747-400 and 747-400D series airplanes. This proposed AD would require installing aluminium gutter reinforcing brackets to the forward and aft dripshield gutters of the main equipment center (MEC); and adding a reinforcing fiberglass overcoat to the top surface of the MEC dripshield, including an inspection for cracking and holes in the MEC dripshield, and corrective actions if necessary. This proposed AD also provides for an option to install an MEC dripshield drain system, which, if accomplished, would extend the compliance time for adding the reinforcing fiberglass overcoat to the top surface of the MEC dripshield. This proposed AD results from a report indicating that an operator experienced a multi-power system loss in-flight of 1, 2, and 3 alternating current (AC) electrical power systems located in the MEC. We are proposing this AD to prevent water penetration into the MEC, which could result in the loss of flight critical systems. Comment on the NPRM was required by 6 July 2010.