Summary
We propose to supersede Airworthiness Directive (AD) 2012-19- 11, which applies to certain The Boeing Company Model 737 airplanes. AD 2012-19-11 currently requires incorporating design changes to improve the reliability of the cabin altitude warning system by installing a redundant cabin altitude pressure switch, replacing the aural warning module (AWM) with a new or reworked AWM, and changing certain wire bundles or connecting certain previously capped and stowed wires as necessary. For certain airplanes, AD 2012-19-11 also requires prior or concurrent incorporation of related design changes by modifying the instrument panels, installing light assemblies, modifying the wire bundles, and installing a new circuit breaker, as necessary. Since we issued AD 2012-19-11, we have determined that certain airplanes were not included in the requirement to incorporate related design changes. This proposed AD would add, for certain airplanes, a requirement to incorporate related design changes. This proposed AD also, for certain airplanes, no longer gives credit for accomplishing certain previous actions. We are proposing this AD to prevent the loss of cabin altitude warning, which could delay flightcrew recognition of a lack of cabin pressurization, and could result in incapacitation of the flightcrew due to hypoxia (a lack of oxygen in the body), and consequent loss of control of the airplane.