Airworthiness Directives; Various Transport Category Airplanes Manufactured by McDonnell Douglas

The FAA is revising an existing airworthiness directive (AD) that applies to various transport category airplanes manufactured by McDonnell Douglas. We issued that AD to require 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 new AD removes certain transport category airplanes from the applicability of the existing AD. This AD results from reports indicating that fire extinguishers for the engine and auxiliary power unit had failed to discharge when commanded. We are issuing this AD 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.