Narrative:

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 circuit breaker (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.

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Original NASA ASRS Text

Title: Maintenance employee reported finding wire chafing behind the fwd lav due to installation of incorrect spacers on a B737-800.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.