DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. FAA-2021-0561; Project Identifier AD-2021-00623-T;
Amendment 39-21647; AD 2021-14-20]
RIN 2120-AA64

Airworthiness Directives; The Boeing Company Airplanes

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final rule; request for comments.

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SUMMARY: 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: 202-493-2251.
Mail: U.S. Department of Transportation, Docket
Operations, M-30, West Building Ground Floor, Room W12-140, 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 110-SK57, Seal Beach, CA 90740-5600;
telephone 562-797-1717; 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 material at the FAA,
call 206-231-3195. It is also available at https://www.regulations.gov
by searching for and locating Docket No. FAA-2021-0561.

Examining the AD Docket

You may examine the AD docket at https://www.regulations.gov by
searching for and locating Docket No. FAA-2021-0561; 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-
231-3959; 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 25.841(b). 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
switch's failure. AD 2012-19-11, Amendment 39-17206 (77 FR 60296,
October 3, 2012).\1\
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\1\ This airworthiness directive was eventually superseded by AD
2015-21-11, Amendment 39-18304 (80 FR 65927, October 28, 2015) (AD
2015-21-11).
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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 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.

FAA's 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 MOM-MOM-21-0292-01B,
dated June 23, 2021. This service information specifies procedures for
repetitive functional tests of the cabin altitude pressure switches,
on-condition 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 operator's 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 operator's fleet that have been tested.

Effect of Certain Installation Procedures on Accomplishment of AD
Requirements

As previously noted, the FAA issued AD 2015-21-11, applicable to
certain Model 737-100, -200, -200C, -300, -400, -500, -600, -700, -
700C, -800, -900, and -900ER series airplanes. AD 2015-21-11 requires,
among other actions, the installation of a redundant cabin altitude
pressure switch in accordance with specified Boeing service
information. The FAA has since approved numerous supplemental type
certificates (STCs) and other means for installing the redundant
pressure switch. As a result of its oversight of these newly-installed
switches, the FAA has determined that use of approved maintenance
procedures for the cabin altitude pressure switch functional test other
than those specified in the task cards identified in Boeing Multi
Operator Message MOM-MOM-21-0292-01B, dated June 23, 2021, is
acceptable for the functional test; therefore, those other procedures
do not require approval of an alternative method of compliance (AMOC).

Differences Between This AD and the Service Information

Although Boeing Multi Operator Message MOM-MOM-21-0292-01B, dated
June 23, 2021, affects ``all 737CL'' airplanes (the 737 Classics
include Model 737-100, -200, -200C, -300, -400, and -500), Boeing did
not send the MOM to Model 737-100, -200 and -200C operators.
Additionally, Boeing did not reference procedures for performing the
cabin altitude pressure switch functional test for Model 737-100, -200,
and -200C series airplanes. There are no Model 737-100 series airplanes
operating worldwide; however, the applicability of this AD includes
those airplanes in the event any of those airplanes are returned to
service in the U.S. The FAA has also included Model 737-200 and -200C
series airplanes in the applicability of this AD. Furthermore, the FAA
requested that Boeing make the service information available to Model
737-200 and -200C operators. Boeing Model 737-200 and -200C operators
may reference 737-200 Airplane Maintenance Manual (AMM) 21-33-11/501
for additional guidance on performing the cabin altitude pressure
switch functional test.
Boeing Multi Operator Message MOM-MOM-21-0292-01B, dated June 23,
2021, uses permissive language, such as ``recommends'' and
``requesting,'' in its ``Functional Test Requirements'' and ``Reporting
Requirements'' sections. However, the regulatory text in paragraphs (g)
and (h) of this AD makes the language in those sections mandatory
unless an exception in paragraph (j) of this AD applies.
Boeing Multi Operator Message MOM-MOM-21-0292-01B, dated June 23,
2021, recommends returning failed pressure switches to the switch
manufacturer. Although the FAA also recommends that operators return
failed pressure switches in order to provide the switch manufacturer
with additional data related to the unsafe condition, this AD does not
require that action.
Although Boeing Multi Operator Message MOM-MOM-21-0292-01B, dated
June 23, 2021, identifies specific AMM task cards for use in
accomplishing the functional test, paragraph (j)(1) of this AD
clarifies that any approved maintenance procedures may be used for the
functional test. This provides the operator an option to use the AMM
task card or any approved maintenance procedure for the functional test
without needing to request an AMOC.
Boeing Multi Operator Message MOM-MOM-21-0292-01B, dated June 23,
2021, specifies certain on-condition actions, including an additional
step while performing the functional test on the switch by increasing
the altitude setting on the switch to an altitude of up to 20,000 feet
if the cabin altitude warning does not activate by 11,000 feet during
the initial functional test. The service information specifies
repeating the functional test at intervals, but does not explicitly
state that the on-condition additional functional testing is limited to
the initial functional test only. Paragraph (j)(3) of this AD requires
the on-condition additional functional test step of increasing the
altitude setting to 20,000 feet only during the initial functional test
(if applicable).
Although Boeing Multi Operator Message MOM-MOM-21-0292-01B, dated
June 23, 2021, specifies that failed switches be replaced with ``new or
serviceable'' switches, this AD requires replacement with
``serviceable'' switches, which include any switches that are eligible
for installation. This is to ensure that any installed switch is
serviceable.

Interim Action

The FAA considers this AD to be an interim action. The reporting
that is required by this AD will enable the airplane manufacturer to
obtain better insight into the nature, cause, and extent of the switch
failures, and eventually to develop final action to address the unsafe
condition. Once final action has been identified, the FAA might
consider further rulemaking.

Justification for Immediate Adoption and Determination of the Effective
Date

Section 553(b)(3)(B) of the Administrative Procedure Act (APA) (5
U.S.C. 551 et seq.) authorizes agencies to dispense with notice and
comment procedures for rules when the agency, for ``good cause,'' finds
that those procedures are ``impracticable, unnecessary, or contrary to
the public interest.'' Under this section, an agency, upon finding good
cause, may issue a final rule without providing notice and seeking
comment prior to issuance. Further, section 553(d) of the APA
authorizes agencies to make rules effective in less than thirty days,
upon a finding of good cause.
An unsafe condition exists that requires the immediate adoption of
this AD without providing an opportunity for public comments prior to
adoption. The FAA has found that the risk to the flying public
justifies forgoing notice and comment prior to adoption of this rule
because, as previously noted, the unexpectedly high rate of latent
failure, of both pressure switches on the same airplane, could result
in the cabin altitude warning system not activating if the cabin
altitude exceeds 10,000 feet, resulting in hypoxia of the flightcrew,
and loss of control of the airplane. Accordingly, notice and
opportunity for prior public comment are impracticable and contrary to
the public interest pursuant to 5 U.S.C. 553(b)(3)(B).
In addition, the FAA finds that good cause exists pursuant to 5
U.S.C. 553(d) for making this amendment effective in less than 30 days,
for the same reasons the FAA found good cause to forgo notice and
comment.

Comments Invited

The FAA invites you to send any written data, views, or arguments
about this final rule. Send your comments to an address listed under
ADDRESSES. Include Docket No. FAA-2021-0561 and Project Identifier AD-
2021-00623-T at the beginning of your comments. The most helpful
comments reference a specific portion of the final rule, explain the
reason for any recommended change, and include supporting data. The FAA
will consider all comments received by the closing date and may amend
this final rule because of those comments.
Except for Confidential Business Information (CBI) as described in
the following paragraph, and other information as described in 14 CFR
11.35, the FAA will post all comments received, without change, to
https://www.regulations.gov, including any personal information you provide.
The agency will also post a report summarizing each substantive verbal
contact received about this final rule.

Confidential Business Information

CBI is commercial or financial information that is both customarily
and actually treated as private by its owner. Under the Freedom of
Information Act (FOIA) (5 U.S.C. 552), CBI is exempt from public
disclosure. If your comments responsive to this AD contain commercial
or financial information that is customarily treated as private, that
you actually treat as private, and that is relevant or responsive to
this AD, it is important that you clearly designate the submitted
comments as CBI. Please mark each page of your submission containing
CBI as ``PROPIN.'' The FAA will treat such marked submissions as
confidential under the FOIA, and they will not be placed in the public
docket of this AD. Submissions containing CBI should be sent to 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: 206-231-3959; email: Nicole.S.Tsang@faa.gov. Any
commentary that the FAA receives that is not specifically designated as
CBI will be placed in the public docket for this rulemaking.

Regulatory Flexibility Act

The requirements of the Regulatory Flexibility Act (RFA) do not
apply when an agency finds good cause pursuant to 5 U.S.C. 553 to adopt
a rule without prior notice and comment. Because the FAA has determined
that it has good cause to adopt this rule without notice and comment,
RFA analysis is not required.

Costs of Compliance

The FAA estimates that this AD affects 2,502 airplanes of U.S.
registry. The FAA estimates the following costs to comply with this AD:

Estimated Costs

In addition, the FAA has determined that preparing and sending a
monthly report of tested airplanes takes about 1 work-hour per
operator. Since operators are required to submit this report for their
affected fleet(s), the FAA has determined that a per-operator estimate
is more appropriate than a per-airplane estimate. Therefore, the FAA
estimates the average total cost of the monthly report to be $85 (1
work-hour x $85) per report, per operator.
The FAA estimates the following costs to do any necessary on-
condition actions that would be required based on the results of the
functional test. The FAA has no way of determining the number of
aircraft that might need these actions:

On-Condition Costs

Paperwork Reduction Act

A federal agency may not conduct or sponsor, and a person is not
required to respond to, nor shall a person be subject to a penalty for
failure to comply with a collection of information subject to the
requirements of the Paperwork Reduction Act unless that collection of
information displays a currently valid OMB Control Number. The OMB
Control Number for this information collection is 2120-0056. Public
reporting for this collection of information is estimated to be
approximately 1 hour per response, including the time for reviewing
instructions, searching existing data sources, gathering and
maintaining the data needed, completing and reviewing the collection of
information. All responses to this collection of information are
mandatory. Send comments regarding this burden estimate or any other
aspect of this collection of information, including suggestions for
reducing this burden to: Information Collection Clearance Officer,
Federal Aviation Administration, 10101 Hillwood Parkway, Fort Worth, TX
76177-1524.

Authority for This Rulemaking

Title 49 of the United States Code specifies the FAA's authority to
issue rules on aviation safety. Subtitle I, section 106, describes the
authority of the FAA Administrator. Subtitle VII: Aviation Programs
describes in more detail the scope of the Agency's authority.
The FAA is issuing this rulemaking under the authority described in
Subtitle VII, Part A, Subpart III, Section 44701: General requirements.
Under that section, Congress charges the FAA with promoting safe flight
of civil aircraft in air commerce by prescribing regulations for
practices, methods, and procedures the Administrator finds necessary
for safety in air commerce. This regulation is within the scope of that
authority because it addresses an unsafe condition that is likely to
exist or develop on products identified in this rulemaking action.

Regulatory Findings

This AD will not have federalism implications under Executive Order
13132. This AD will not have a substantial direct effect on the States,
on the relationship between the national government and the States, or
on the distribution of power and responsibilities among the various
levels of government.
For the reasons discussed above, I certify that this AD:
(1) Is not a ``significant regulatory action'' under Executive
Order 12866, and
(2) Will not affect intrastate aviation in Alaska.

List of Subjects in 14 CFR Part 39

Air transportation, Aircraft, Aviation safety, Incorporation by
reference, Safety.

Adoption of the Amendment

Accordingly, under the authority delegated to me by the
Administrator, the FAA amends 14 CFR part 39 as follows:

PART 39--AIRWORTHINESS DIRECTIVES

1. The authority citation for part 39 continues to read as follows:

Authority: 49 U.S.C. 106(g), 40113, 44701.

Sec. 39.13 [Amended]

2. The FAA amends Sec. 39.13 by adding the following new airworthiness
directive: