The Federal Aviation Administration (FAA) has issued a new airworthiness directive (AD) for the Boeing 737 MAX after identifying another electrical fault that can lead to excessive temperatures in the cabin and cockpit.
The directive applies to all in-service 737 MAX 8, MAX 8-200 and MAX 9 aircraft and takes effect on July 16. It follows an earlier FAA directive issued in February that addressed a related problem involving the aircraft's environmental control system.
According to the FAA, the latest action responds to reports of in-flight overheating events that flight crews could not control. The agency said tripped circuit breakers can prevent ram air from reaching the air conditioning system's heat exchangers, leaving hot engine bleed air insufficiently cooled before it enters the cockpit and passenger cabin.
The new directive addresses circuit breakers associated with the environmental control system, while the previous rule focused on the standby power control unit. Although the electrical faults differ, both can produce the same result: an uncontrollable rise in cockpit and cabin temperatures.
Airlines must revise their flight manuals to include two additional non-normal checklists describing the procedures crews should follow if the malfunction occurs. The instructions include descending to the lowest safe altitude, resetting the affected circuit breakers and, if necessary, switching off engine bleed air.
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The FAA described the directive as an interim measure and said immediate action was justified without the usual public comment period because of the potential safety risk.
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Boeing developed the operational procedures included in the directive and is working on a permanent engineering fix. The manufacturer has said the modification is planned for the 737 MAX 8, MAX 8-200 and MAX 9 fleets and will also be incorporated into the 737 MAX 7 and MAX 10 before those two variants enter commercial service.
