Airworthiness Directives; Various Transport Category Airplanes Manufactured by McDonnell Douglas

This amendment adopts a new airworthiness directive (AD), applicable to various transport category airplanes manufactured by McDonnell Douglas. This AD requires a one-time test of the fire extinguishers for the engine and auxiliary power unit (APU), as applicable, to determine the capability of the Firex electrical circuits to fire discharge cartridges, and troubleshooting actions if necessary. This action is necessary to prevent failure of the fire extinguishers to fire discharge cartridges, which could result in the inability to put out a fire in an engine or in the APU. This action is intended to address the identified unsafe condition.