FAA Mandate Targets 737 Cabin Pressure Switches 

WASHINGTON—The FAA is mandating more frequent inspections of Boeing 737 cabin altitude pressure switches over concerns that latent failures could leave pilots and maintenance personnel unaware that the crucial warning system is not working.

In an immediately adopted airworthiness directive (AD) due out July 20, the agency will order affected operators to inspect both switches on each 737 every 2,000 flight hours (FH). The current interval is every 6,000 FH.

Required by regulations on transport-category aircraft with pressurized cabins, the switches serve several functions. They detect when the cabin pressure altitude exceeds a specified limit—10,000 ft. in the 737’s case—and send signals that trigger aural and visual warnings to pilots.

The 737 was designed with one switch. In 2012, the FAA mandated the addition of a second switch for redundancy and because their failures are latent, with no warning or indication to pilots or maintenance personnel. 

The inspection-interval change comes based on Boeing’s recommendation following analysis of in-service switch reliability. Boeing acted based on an operator’s report that both switches failed on three different 737s during functional checks in September 2020. Boeing initially determined, “for reasons that included the expected failure rate of the switches,” that changes were not required, the FAA explained in the directive’s preamble.

Follow-up analysis found that “the failure rate of both switches is much higher than initially estimated,” the FAA said. In June, Boeing alerted operators, recommending the more frequent inspections. No in-service incidents have been linked to dual switch failures.

“Safety is our highest priority, and we fully support the FAA’s direction, which makes mandatory the inspection interval that we issued to the fleet in June,” Boeing said in a statement. 

The FAA directive affects 2,500 U.S.-registered 737s, the FAA said. Worldwide adoption of the mandate is likely, which would subject 8,124 aircraft to the directive, including 877 in storage, Aviation Week’s Fleet Discovery shows.