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Federal Register / Vol. 86, No. 136 / Tuesday, July 20, 2021 / Rules and Regulations
DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration 14 CFR Part 39
Docket No. FAA20210561; Project Identifier AD202100623T; Amendment 3921647; AD 20211420
RIN 2120AA64

Airworthiness Directives; The Boeing Company Airplanes Federal Aviation Administration FAA, DOT.
ACTION: Final rule; request for comments.
AGENCY:

The FAA is adopting a new airworthiness directive AD for all The Boeing Company Model 737 airplanes.
This AD was prompted by reports of latent failures of the cabin altitude pressure switches. This AD requires repetitive functional tests of the pressure switches, and on-condition actions, including replacement, if necessary. The FAA is issuing this AD
to address the unsafe condition on these products.
DATES: This AD is effective July 20, 2021.
The Director of the Federal Register approved the incorporation by reference of a certain publication listed in this AD
as of July 20, 2021.
The FAA must receive comments on this AD by September 3, 2021.
ADDRESSES: You may send comments, using the procedures found in 14 CFR
11.43 and 11.45, by any of the following methods:
Federal eRulemaking Portal: Go to https www.regulations.gov. Follow the instructions for submitting comments.
Fax: 2024932251.
Mail: U.S. Department of Transportation, Docket Operations, M
30, West Building Ground Floor, Room W12140, 1200 New Jersey Avenue SE, Washington, DC 20590.
Hand Delivery: Deliver to Mail address above between 9 a.m. and 5
p.m., Monday through Friday, except Federal holidays.
For service information identified in this final rule, contact Boeing Commercial Airplanes, Attention:
Contractual & Data Services C&DS, 2600 Westminster Blvd., MC 110SK57, Seal Beach, CA 907405600; telephone 5627971717; internet https
www.myboeingfleet.com. You may view this service information at the FAA, Airworthiness Products Section, Operational Safety Branch, 2200 South 216th St., Des Moines, WA. For information on the availability of this
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SUMMARY:

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material at the FAA, call 2062313195.
It is also available at https
www.regulations.gov by searching for and locating Docket No. FAA2021
0561.
Examining the AD Docket You may examine the AD docket at https www.regulations.gov by searching for and locating Docket No.
FAA20210561; or in person at Docket Operations between 9 a.m. and 5 p.m., Monday through Friday, except Federal holidays. The AD docket contains this final rule, any comments received, and other information. The street address for Docket Operations is listed above.
FOR FURTHER INFORMATION CONTACT:
Nicole Tsang, Aerospace Engineer, Cabin Safety and Environmental Systems Section, FAA, Seattle ACO
Branch, 2200 South 216th St., Des Moines, WA 98198; phone and fax: 206
2313959; email: Nicole.S.Tsang@
faa.gov.
SUPPLEMENTARY INFORMATION:
Background The FAA requires every proposed transport category airplane design with a pressurized cabin to include a system that warns the flightcrew of cabin depressurization. 14 CFR 25.841b. On Boeing Model 737 airplanes, such warning systems include a cabin altitude pressure switch. The functions of this pressure switch are twofold: To detect if a certain cabin pressure altitude has been exceeded; and if so, to send a signal to the parts of the system that provide aural and visual warnings to the flightcrew. When this switch fails, it fails latently; that is, without making the failure known to the flightcrew or maintenance personnel. Due to the importance of the functions provided by this switch, in 2012 the FAA mandated that all Boeing Model 737 airplanes utilize two switches, to provide redundancy in case of one switchs failure. AD 20121911, Amendment 3917206 77 FR 60296, October 3, 2012.1
The FAA has received reports of latent failures of these cabin altitude pressure switches. In September 2020, an operator reported that on three of its airplanes, both pressure switches failed the on-wing functional test. The affected switches were on three different models of the Boeing 737.
The airplane manufacturer investigated, and initially found, for reasons that included the expected 1 This airworthiness directive was eventually superseded by AD 20152111, Amendment 39
18304 80 FR 65927, October 28, 2015 AD 2015
2111.

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failure rate of the switches, that it did not pose a safety issue. Boeing decided in November 2020 that the failures were not a safety issue. Subsequent investigation and analysis led the FAA
and the airplane manufacturer to determine, in May of 2021, that the failure rate of both switches is much higher than initially estimated, and therefore does pose a safety issue.
The FAA does not yet have sufficient information to determine what has caused this unexpectedly high failure rate, so a terminating corrective action cannot yet be developed. However, a latent failure of both pressure switches could result in the loss of cabin altitude warning, which could delay flightcrew recognition of a lack of cabin pressurization, and result in incapacitation of the flightcrew due to hypoxia a lack of oxygen in the body, and consequent loss of control of the airplane. Therefore addressing these failures requires immediate action. The FAA is issuing this AD to address the unsafe condition on these products.
FAAs Determination The FAA is issuing this AD because the agency has determined the unsafe condition described previously is likely to exist or develop in other products of the same type design.
Related Service Information Under 1
CFR Part 51
The FAA reviewed Boeing Multi Operator Message MOMMOM21
029201B, dated June 23, 2021. This service information specifies procedures for repetitive functional tests of the cabin altitude pressure switches, oncondition actions including follow-on functional testing and replacement of failed switches, sending a report to Boeing about any pressure switches that fail the initial functional test, and reporting to Boeing the airplanes in the operators fleet that have been tested.
This service information is reasonably available because the interested parties have access to it through their normal course of business or by the means identified in the ADDRESSES section.
AD Requirements This AD requires accomplishing the actions specified in the service information already described, except as discussed under Differences Between this AD and the Service Information.
This AD also requires reporting to Boeing the results of the first functional test if any pressure switch failed, and sending reports to Boeing of the airplanes in the operators fleet that have been tested.

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