Summary
This document proposes the adoption of a new airworthiness directive (AD) that is applicable to certain Boeing Model 737-600, -700, -700C, -800, and -900 series airplanes equipped with certain Honeywell start converter units (SCU). This proposal would require replacement of the SCU of the auxiliary power unit (APU) located in the electrical and electronics (E/E) compartment with a new, improved SCU. This action is necessary to prevent overheating of the electrical connector of the SCU, which could create an ignition source and possible fire in the E/E compartment and cause damage to certain electrical wire bundles on the E2-2 shelf. Such damage could result in loss of power from the APU generator, failure of electrically powered airplane systems, and consequent reduction in the ability of the flight crew to control the airplane in certain adverse operating conditions. This action is intended to address the identified unsafe condition.