AD focuses on possible flight crew hypoxia on the Boeing 737

The FAA issued an AD to avoid crew hypoxia and subsequent loss of control of the aircraft
Mario Hagen /

The Federal Aviation Administration (FAA) has issued a new directive to address the failure of cabin altitude pressure switches. This follows a rulemaking decision requiring operators to conduct repetitive functional tests of the cabin altitude pressure switches and taking action if necessary, and reporting said results to the agency. 

The Airworthiness Directive (AD), which was published on December 22, 2022, is intended to tackle the issue on all Boeing 737 aircraft generations, including the 737 MAX. 

Airworthiness Directive following tests 

The latest AD for Boeing’s narrow-body aircraft family comes as a result of a previous directive when the FAA ordered operators of the type to test cabin altitude pressure switches and if required, replace them. However, since the administration also required operators to report these results, the switches’ tendency towards failures was noticed by the FAA. 

The newest directive was ‘prompted by data collected from the reports’ required by the previous AD, which the FAA issued as a final rulemaking on July 20, 2021. Then, the agency ordered airlines flying all Boeing 737 generations to test and replace the switches, if necessary. Since the tests, reports have emerged of ‘latent failures of the cabin altitude pressure switches, and the determination that using certain adapters while performing a functional test may lead to false failures of the cabin altitude pressure switches’, according to the agency’s document. 

‘The FAA is issuing this AD to address the unexpectedly high rate of latent failure of both pressure switches on the same airplane, which could result in the cabin altitude warning system not activating if the cabin altitude exceeds 10,000 feet, resulting in hypoxia of the flight crew, and loss of control of the airplane’, continued the AD. 

Though not a direct result of the cabin altitude warning system failing to activate, Helios Airways Flight 522 is a notable example of a Boeing 737-300 crashing due to flight crew hypoxia. The crew failed to set the pressurization system to the automatic setting, leading to a so-called ‘ghost flight’. The 737 eventually crashed several dozen kilometers north of Athens, Greece, claiming the lives of all 121 people onboard the aircraft. 

The directive affects 2,693 Boeing 737s registered in the United States (US), with a total cost of $85 per functional test of the cabin altitude pressure switches on each aircraft. According to the FAA’s estimates, replacing the switch would cost $1,363 per airplane. 

The AD comes into effect on January 26, 2023. Following this date, compliance with the directive will include testing the switches within 2,000 flight hours of the last test, before 2,000 total flight hours of an aircraft, or within 90 days of the effective date of the FAA’s latest rulemaking. 

‘If, during any functional test, any cabin altitude pressure switch fails to activate at an altitude of between 9,000 and 11,000 feet, replace the switch before further flight,’ concluded the FAA. 

All Boeing 737 generations are affected by the newest AD, including the 737 MAX-8, MAX-9, and MAX 8-200, the latter being the iteration of the aircraft with the highest seat count. The MAX-7 and MAX-10, which are yet to be certified, were not mentioned in the document. 

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