On Thursday, the US Federal Aviation Administration (FAA) issued a directive for all Boeing 737 operators to inspect their aircraft’s cabin altitude pressure switches for possible failures. The directive applies to all 737 series, including but not limited to the Classic, NG, and MAX.
Over 9,300 aircraft should be inspected
According to Reuters, a recent FAA directive is mandating that all operators of 737 aircraft, regardless of the series, conduct inspections of cabin altitude pressure switches to address possible failures. These inspections are to include repetitive tests of the switches, as ordered by the directive, with replacements being made if needed.
With the broad scope of the directive, the issue could affect more than 9,300 Boeing 737s worldwide. The FAA has jurisdiction over the 2,502 737s registered in the United States, but also any aircraft flying in US airspace and anywhere else it has been delegated authority (this includes international waters). Despite these limits, most civil aviation authorities around the world look to the FAA for guidance, taking cues from this American agency.
The specific issue and risk requiring attention
Sources note that the directive was prompted after an operator reported experiencing failures on both pressure switches during their on-wing functional tests of three different 737 models. This testing was conducted in September of 2020.
If the aircraft’s two switches should fail, the FAA says that the cabin altitude warning system would not activate if the cabin altitude exceeds 10,000 feet (3,050 m). It should be made clear that risk would only occur if both switches were to fail at the same time a depressurization event was to take place.
At 10,000 feet and above, low oxygen levels become dangerous to those onboard, with cabins typically being pressurized to 8,000 feet (2,438 m) or lower.
Simple Flying reached out to Boeing, which had the following statement:
“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”
The manufacturer has yet to experience any in-flight dual-switch failures- only during functional tests.
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A known issue
Reuters reports that Boeing had initially reviewed the issue and its expected failure rate (at an unspecified date), finding that it did not pose a safety issue.
However, the FAA and Boeing, after subsequent investigation and analysis, determined in May that “the failure rate of both switches is much higher than initially estimated, and therefore does pose a safety issue.” For its part, the FAA notes that it does not have enough information to determine the cause of this unexpectedly high failure rate.
It was back in 2012 that the FAA mandated that all 737 aircraft utilize two switches to provide a measure of redundancy due to the importance of the switches’ functions.
The FAA states that tests must be conducted within 2,000 flight hours since the last test of the cabin altitude pressure switches or within 90 days of the directive’s effective date. Tests should also be conducted on airplanes that have yet to fly 2,000 hours.
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