The Federal Aviation Administration (FAA) has recently issued a directive to operators of all Boeing Co 737 series airplanes 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.
It was prompted after an operator reported 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.
Airplane cabins are pressurized to the equivalent of not more than 8,000 feet (2438 m).
Boeing said it supports “the FAA’s direction, which makes mandatory the inspection interval that we issued to the fleet in June." The FAA directive did not report any in-flight failures of the switches.
The FAA said on Thursday that 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.
Subsequent investigation and analysis led the FAA and Boeing to determine in May that “the failure rate of both switches is much higher than initially estimated, and therefore does pose a safety issue." Boeing declined to say what the failure rate was.
The FAA added it “does not yet have sufficient information to determine what has caused this unexpectedly high failure rate."
Due to the importance of functions provided by the switch, the FAA in 2012 mandated all Boeing 737 airplanes utilize two switches to provide redundancy in case of one switch’s failure.
The directive covers all versions of the 737 jetliners, including the MAX, but is unrelated to any issues related to the MAX’s return to service last November.