Summary
We propose to adopt a new airworthiness directive (AD) for certain Model 737 airplanes. This proposed AD would incorporate design changes to improve the reliability of the cabin altitude warning system by requiring installation of a redundant switch of the cabin altitude pressure, replacing the aural warning module (AWM) with a new or reworked AWM, changing certain wire bundles, and connecting certain previously capped and stowed wires, as necessary. This proposed AD, for certain airplanes, would also require modifying the instrument panels, installing light assemblies, modifying the wire bundles, and installing a new circuit breaker, as necessary. This proposed AD was prompted by a report of a lack of cabin pressurization event caused by the flightcrew not receiving an aural warning because of the failure of the cabin altitude pressure switch. We are proposing this AD to prevent failure of the flightcrew to recognize and react to a lack of cabin pressurization, which could result in incapacitation of the flightcrew due to hypoxia (lack of oxygen in the body), and consequent loss of control of the airplane.