A Mechanic reports he unknowingly; installed a Fan Bypass Check valve 180 degrees out in the Diffuser/ Plenum assembly; for the Right-hand Pack; on a B737-800; even though the AMM shows the Check valve is 'Murphy Proof'; and cannot be installed upside down or 180 degrees out.
Synopsis
A Mechanic reports he unknowingly; installed a Fan Bypass Check valve 180 degrees out in the Diffuser/ Plenum assembly; for the Right-hand Pack; on a B737-800; even though the AMM shows the Check valve is 'Murphy Proof'; and cannot be installed upside down or 180 degrees out.
Narrative
Took over a Primary Heat Exchanger replacement job; Right-hand Pack on a B737-800. There was a Non-Routine for a cracked Plenum. When we took over the job; the previous workers were gone and all the parts were on a table disassembled. The new part arrived. The AMM we used was XXX. On page X; step G (X); (a) thru (c) does not 'Note' the proper positioning of the Fan Bypass Check Valve in the Plenum. Also; the Figures (Drawings) for the Plenum/Diffuser assembly; figure XXX; sheet 1 & 2; show that it is 'Murphy Proof'; and cannot be installed upside down; or 180 degrees out. On Figure XXX sheet 2; there is no Forward; Aft; Left-hand Assembly; Right-hand Assembly identifications. We installed the assembly as to what we assumed was the proper way per the AMM and the our Work Card. It was found out later due to Right-hand Pack problems; that the Fan Bypass Check Valve was installed 180 degrees out in the Diffuser assembly. I feel that the AMM and the Figure should be clarified to show and tell you more info on how the Diffuser should be installed onto the Plenum.
NASA callback
Reporter stated there was no way to determine; from looking at the drawings (Figures) from the AMM; which direction the rectangular shaped; hinged; single flapper Fan Check valve should be positioned. There were no springs attached to the flapper. Although the mount bolts were offset; he still managed to install the Check valve 180 degrees out. All the Maintenance Manual (MM) says is to 'put in proper position'. There was little information from the shift turnover.Reporter stated the aircraft later had a chronic placard for right pack overheat at altitude; but when on the ground; everything seemed to work fine. At altitude; the Ram Air Doors for the Packs are not opened very much; so cooling air flow is reduced. After further troubleshooting; Mechanics were able to determine the Check valve; which is mounted inside a Diffuser Duct that connects to both the Primary and Secondary Heat Exchangers inside a larger air Plenum was incorrectly installed. Reporter stated he doesn't remember which way the flapper valve should face; but his carrier's Engineering has issued an MM Revision to include language for correct installation.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.