B737-800
Training / Qualification
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Maintenance; Flight Deck / Cabin / Aircraft Event Other / Unknown
Flight Crew Diverted; General Maintenance Action
Procedure; Aircraft; Incorrect / Not Installed / Unavailable Part; Human Factors
Aircraft X arrived [at] ZZZ with chronic cabin pressurization problems. The outflow valve indication showed the pressure outflow valve rapidly changing from full closed to full open position. The flight crew had to don their emergency O2 (Oxygen) masks due to not being able to control pressurization in Auto or in Manual modes. The aircraft had to make a diversion into ZZZ for troubleshooting and repairs. During troubleshooting; it was found that wire bundle X had 6 areas of chafing behind the fwd lav; causing the pressurization circuit to short to ground. At closer investigation it was discovered that the wrong standoff spacers were installed on the wire bundles that run up the entire frame behind the fwd lav and this caused wear on the bundle due to contact with the lav module. The proper spacers were installed and all wiring repaired. In this case it was a critical pressurization control circuit that was affected; it could have easily been any number of other circuits potentially causing an inflight fire. A cockpit fire in this area would be catastrophic as this area is located between the fwd lav and the Captain's P18 CB (Circuit Breaker) panel behind the coat closet wall in the cockpit; yet inaccessible to the crew. Due to the heightened awareness and concerns of aging wire in aircraft it is requested to accomplish a one-time [fleet] inspection of this area to ensure the proper spacer installation and no wire bundle contact or damage present.
The reporter stated a wire bundle behind the sidewall of the FWD lav in between the fuselage and the lav wall was chaffed in 6 locations.  The chaffing occurred along the middle area of the lav wall where the wire bundle came in contact with fastener heads attached to the lav module.  The standoff that was used was measured and the length was greater than the installation drawing called for.  Reportedly; this condition appeared to be original from the manufacturer and therefore may exist on multiple aircraft and operators. The wire bundle starts at the pressure controller panel in the E&E bay and runs through the ceiling of the FWD cargo pit then up the side of the fuselage behind the lav wall.  Reporter stated inspection access can be obtained through the panel located along the side of the fuselage though the cockpit coat closet.  The wire bundle is located approximately 1-2 inches aft of the FWD edge of the lav module.
Maintenance employee reported finding wire chafing behind the fwd lav due to installation of incorrect spacers on a B737-800.