Air TransportFAA issues airworthiness directive for Boeing 737 MAX after cabin overheating events
The Federal Aviation Administration (FAA) has issued a new airworthiness directive (AD) for all Boeing 737 MAX 8, 737 MAX 9 and 737 MAX 8200 aircraft following two in-flight events involving excessive cabin and flight deck temperatures that could not be controlled using existing procedures.
The directive, effective February 24, requires operators to revise the airplane flight manual (AFM) within 30 days to include new non-normal procedures for flight crews in the event a specific circuit breaker in the standby power control unit (SPCU) trips. The FAA is accepting public comments on the rule through April 10.
According to the agency, investigations traced the problem to the BAT BUS SECT 2 circuit breaker (CB3062) in the SPCU. That circuit provides power to air conditioning and cabin pressurization systems. If the breaker trips, it can generate an unintended electrical ground signal that commands the ram air deflector doors to close.
When those doors close, airflow to the air conditioning heat exchangers is reduced, limiting the cooling of hot bleed air supplied by the engines. As a result, the system can deliver excessively hot air to the cabin and flight deck, potentially leading to uncontrollable high temperatures.
The FAA said that if not addressed, the condition could result in injury or incapacitation of flight crew and passengers and affect the ability to maintain safe flight and landing.
The directive requires airlines to add procedures covering a controlled descent, an attempt to reset the tripped circuit breaker and, if unsuccessful, the selection of engine bleed switches to OFF. The measure is considered interim action while Boeing develops a permanent modification.
The FAA bypassed its usual notice-and-comment process prior to adoption, citing safety concerns that justify immediate implementation. The agency estimates the directive affects 771 US-registered aircraft and that the required manual revision will cost approximately $85 per airplane.
Boeing said it supports the FAA’s directive and has identified the root cause as a ground wire fault in the air conditioning system. The manufacturer is developing an engineering solution that will be incorporated into the 737 MAX 8 and 9 and made available for the 737 MAX 7 and 10 before their certification. Previous-generation 737 models are not affected, the company said.
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