AD FAA-2015-1990

We are adopting a new airworthiness directive (AD) for certain The Boeing Company Model 737-400 series airplanes, as modified by a certain supplemental type certificate. This AD was prompted by the discovery of a design drawing error regarding placards that identified incorrect squibs and pressure switches for certain fire extinguisher bottles. This AD requires a detailed inspection of certain cargo placards to determine if they are the correct placards and in the correct location, a detailed inspection of the harnesses to verify that they are marked and installed correctly, and corrective action if necessary. We are issuing this AD to detect and correct incorrectly installed harnesses for the cargo fire suppression system bottles, which could result in an incorrect activation sequence of the bottles, the inability to suppress a cargo fire quickly, and a possible uncontrollable fire.