The FAA is ordering inspections of Boeing 737 passenger oxygen supply systems, fast-tracking Boeing-recommended steps to flag a material-related failure that could prevent the key emergency systems from working.
In an airworthiness directive (AD) set for publication July 10, the FAA will order inspections of all U.S.-registered 737 Next Generation and 737 MAX models in service. Other regulators are likely to adopt the mandate. The Aviation Week Network Fleet Discovery database shows about 7,600 affected aircraft in the global fleet, including 200 listed as stored.
The visual inspections target thermal pads on retention straps that are part of passenger service unit (PSU) oxygen generator installations. Boeing has received “multiple reports” of straps failing, which can cause the generator to shift and potentially malfunction, the AD said. It did not provide details on the reports or any related in-service incidents.
“This condition, if not addressed, could result in shifted PSU oxygen generators that might become non-functional, which could result in an inability to provide supplemental oxygen to passengers during a depressurization event,” the agency said.
While the FAA directive flags all 737 NGs and 737 MAXs, Boeing said the issue was traced to an August 2019 adhesive change. The new adhesive “has been found, under certain circumstances, to have allowed units to shift up to three-quarters of an inch,” Boeing said in a statement. “We have gone back to the original adhesive for all new deliveries to ensure the generators remain firmly in place, as intended."
Boeing declined to say whether the issue has been linked to any in-service incidents.
Each 737 has about 60 oxygen generators. Two retention straps “with either PSA or non-PSA thermal pads” secure each generator to the PSU.
“For all reported failures, the PSA thermal pad configurations were under the retention straps,” the FAA said. Part of Boeing’s recommended instructions is installing straps with non-PSA thermal pads.
All FAA ADs are issued to correct what the agency determines are “unsafe conditions.” Most are both based on manufacturer recommendations issued to affected operators and issued in draft form for public input. Incorporating changes into a final AD often takes a year or longer.
The 737 PSU directive, which will not include a pre-publication comment period, is based on bulletins Boeing sent affected operators in mid-June advising the inspections. While the directive is not an emergency order requiring immediate action nor is it effective right away—its effective date is 15 days after publication—the rapid turnaround suggests the agency considers the issue significant.
Affected operators must reference Boeing’s bulletins for compliance instructions, including inspection deadlines, the FAA said. The bulletins have not been made public. Boeing declined to discuss its inspection recommendations.
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