Summary
The FAA proposes to supersede Airworthiness Directive (AD) 2017-12-07, which applies to certain The Boeing Company Model 737-800, -900, and -900ER series airplanes. AD 2017-12-07 requires replacing the affected left temperature control valve and control cabin trim air modulating valve. Since the FAA issued AD 2017-12-07, the agency determined that the affected parts may be installed on airplanes outside the original applicability of AD 2017-12-07. This proposed AD would retain the requirements of AD 2017-12-07, expand the applicability to include those other airplanes, and add a new requirement for certain airplanes to identify and replace the affected parts. The FAA is proposing this AD to address the unsafe condition on these products.