The Federal Aviation Administration (FAA) has a directive to operators of all Boeing 737 series planes to conduct inspections to address possible failures of cabin altitude pressure switches.
The directive requires operators to conduct repetitive tests of the switches and replace them if needed. The directive covers 2,502 U.S.-registered airplanes and 9,315 airplanes worldwide. The FAA directive follows an operator reporting in September that both pressure switches failed the on-wing functional test on three different 737 models.
The FAA said failure of the switches could result in the cabin altitude warning system not activating if the cabin altitude exceeds 10,000 feet (3,050 m), at which point oxygen levels could become dangerously low.
Boeing said it supports “the FAA’s direction, which makes mandatory the inspection interval that we issued to the fleet in June.”
According to the FAA, tests must be conducted within 2,000 flight hours since the last test of the cabin altitude pressure switches, before airplanes have flown 2,000 hours, or within 90 days of the directive’s effective date.
Boeing initially reviewed the issue, including the expected failure rate of the switches, and found it did not pose a safety issue.
Due to the importance of functions provided by the switch, the FAA had in 2012 mandated all Boeing 737 airplanes utilize two switches to provide redundancy in case of one switch’s failure.
in May this year, FAA and Boeing determined that “the failure rate of both switches is much higher than initially estimated, and therefore does pose a safety issue.” The FAA has confirmed that it “does not yet have sufficient information to determine what has caused this unexpectedly high failure rate.”
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