DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 39
[Docket No. FAA-2020-0331; Product Identifier 2020-NM-019-AD; Amendment
39-21397; AD 2021-02-14]
Airworthiness Directives; The Boeing Company Airplanes
AGENCY: Federal Aviation Administration (FAA), DOT.
ACTION: Final rule.
SUMMARY: The FAA is adopting a new airworthiness directive (AD) for
certain The Boeing Company Model 737-600, -700, -700C, -800, -900, and
-900ER series airplanes. This AD was prompted by a report that the
necessary sealant was not applied to the side of body (SOB) slot as a
result of a production drawing that provided unclear SOB slot sealant
application instructions. This AD requires a general visual inspection
for insufficient sealant in the SOB slot, and related investigative and
corrective actions. The FAA is issuing this AD to address the unsafe
condition on these products.
DATES: This AD is effective March 30, 2021.
The Director of the Federal Register approved the incorporation by
reference of a certain publication listed in this AD as of March 30, 2021.
ADDRESSES: 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 on
the internet at https://www.regulations.gov by searching for and
locating Docket No. FAA-2020-0331.
Examining the AD Docket
You may examine the AD docket on the internet at https://www.regulations.gov
by searching for and locating Docket No. FAA-2020-
0331; 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 address for Docket Operations is U.S. Department of
Transportation, Docket Operations, M-30, West Building Ground Floor,
Room W12-140, 1200 New Jersey Avenue SE, Washington, DC 20590.
FOR FURTHER INFORMATION CONTACT: James Laubaugh, Aerospace Engineer,
Propulsion Section, FAA, Seattle ACO Branch, 2200 South 216th St., Des
Moines, WA 98198; phone and fax: 206-231-3622; email:
The FAA issued a notice of proposed rulemaking (NPRM) to amend 14
CFR part 39 by adding an AD that would apply to certain The Boeing
Company Model 737-600, -700, -700C, -800, -900, and -900ER series
airplanes. The NPRM published in the Federal Register on May 6, 2020
(85 FR 26893). The NPRM was prompted by a report indicating that the
necessary sealant was not applied to the SOB slot as a result of a
production drawing providing unclear SOB slot sealant application
instructions on certain The Boeing Company Model 737-600, -700, -700C,
-800, -900, and -900ER series airplanes. The NPRM proposed to require a
general visual inspection for insufficient sealant in the SOB slot, and
related investigative and corrective actions.
The FAA is issuing this AD to address fuel leaking into the air
distribution mix bay (ADMB), which if not addressed, could possibly
lead to an ignition of flammable fluid vapors, fire, or explosion, or
fuel vapor inhalation by passengers and crew.
The FAA gave the public the opportunity to participate in
developing this final rule. The following presents the comments
received on the NPRM and the FAA's response to each comment.
United Airlines stated it had no technical objection to the NPRM.
Effects of Winglets on Accomplishment of the Proposed Actions
Aviation Partners Boeing stated that the installation of blended or
split scimitar winglets per Supplemental Type Certificate (STC)
ST00830SE does not affect compliance with the proposed actions.
The FAA agrees with the commenter that STC ST00830SE does not
affect the accomplishment of the manufacturer's service instructions.
Therefore, the installation of STC ST00830SE does not affect the
ability to accomplish the actions required by this AD. The FAA has not
changed this AD in this regard.
Request To Reference Later Revision of Multi Operator Message (MOM)
Boeing requested that the FAA revise the proposed AD to reference a
later revision of the MOM, specifically, Boeing Multi Operator Message
MOM-MOM-20-0049-01B(R2), with an anticipated publication date of July
31, 2020. Boeing stated that the revision has improved illustration and
work instructions, clarifies how to distinguish between certain
application flaws and more severe flaws and deterioration, and provides
relief from certain corrective actions for certain secondary fuel
barrier coating conditions.
The FAA agrees to reference a later revision of the service
information. However, Boeing has released additional later revisions
that provide additional clarity. Boeing Multi Operator Message MOM-MOM-
20-0049-01B(R4), dated September 28, 2020, provides clarification on
the definitions of sufficient sealant in the SOB slot, fillet seal in
the SOB slot, and secondary fuel barrier. It also provides
clarifications to the inspection area, and revisions were made to
improve the identification of applicable figures. The changes in the
revision provide relief on repairs of serviceable secondary fuel
barrier coating. This later revision 4 contains no substantive change
from MOM-MOM-20-0049-01B(R1), dated January 29, 2020. The FAA has
revised paragraphs (g) and (i) of this AD to reference Boeing Multi
Operator Message MOM-MOM-20-0049-01B(R4), dated September 28, 2020. The
FAA has also added paragraph (j) to this AD to provide credit for
accomplishing certain earlier revisions of this service information
before the effective date of this AD.
Request To Define Type of Inspection Required
Delta Air Lines (Delta) requested clarification on what type of
inspection is needed to accomplish the inspection requirements. Delta
noted that in the General Notes section of the attachment to Boeing
Multi Operator Message MOM-MOM-20-0049-01B(R2), dated August 4, 2020,
the MOM provides a definition of a detailed inspection. However, Delta
noted that because "detailed inspection" is not used in other parts
of the service information, it seems the definition of a detailed
visual inspection should have been provided instead. Delta stated that
this inconsistency could lead to an inability to comply with the
instructions in the MOM.
The FAA agrees to clarify. As stated previously, the FAA has
revised this AD to reference Boeing Multi Operator Message MOM-MOM-20-
0049-01B(R4), dated September 28, 2020, and in this later revision,
both the steps and General Notes refer to "detailed inspection" as
well as "general visual inspection." The FAA has removed paragraph
(g) of the proposed AD as this definition is no longer needed. The FAA
has redesignated subsequent paragraphs accordingly.
Request To Clarify Figures
Alaska Airlines (Alaska) and Delta requested that the figures
within Boeing Multi Operator Message MOM-MOM-20-0049-01B(R1), dated
January 29, 2020, be revised to add clarity. Alaska and Delta stated
that the figures lack clear reference links from the text to the
figures. Delta also stated that not addressing the unclear figures
could lead to incorrect accomplishment of the SOB slot inspection.
The FAA agrees that the figures are unclear and the internal
reference links need to be revised. As stated previously, this AD has
been revised to reference Boeing Multi Operator Message MOM-MOM-20-
0049-01B(R4), dated September 28, 2020, which contains improved figure
references. No further changes to this AD have been made.
Request To Clarify Inspection Area and Acceptance Criteria for Sealant
Alaska, Delta, and Southwest Airlines requested clarification on
what is considered to be acceptable or insufficient sealant in the SOB
slot. Alaska and Delta requested that the inspection area and
acceptance criteria for sealant in the SOB slot and secondary fuel
barrier be clarified. Southwest Airlines proposed a compliance table be
added to the proposed AD to define acceptable sealant conditions, and
Alaska supported Southwest Airlines' idea. Delta also suggested that
the unclear criteria for sealant could lead to incorrect accomplishment
of the SOB slot inspection.
The FAA agrees that the criteria for acceptable sealant are
unclear. As described above, the FAA has revised this AD to refer to
Boeing Multi Operator Message MOM-MOM-20-0049-01B(R4), dated September
28, 2020, which clarifies the area of inspection and the acceptable
conditions of the sealant.
Request To Clarify an Inspection Area
Delta requested clarification on the inspection area addressed in
certain portions (Part 2, Step 3 and View 2D21's flagnote 63) of Boeing
Multi Operator Message MOM-MOM-20-0049-01B(R1), dated January 29, 2020.
Delta stated that those parts could be interpreted to specify that the
entire front spar of the center tank must be inspected for the fillet
seal and SOB slot, or only the SOB slot area.
The FAA agrees that the specifications of the area to be inspected
should be clearer. As discussed above, the FAA has revised this AD to
reference Boeing Multi Operator Message MOM-MOM-20-0049-01B(R4), dated
September 28, 2020, which clarifies that only the SOB slot area must be
inspected as shown in Figure 1 of Boeing Multi Operator Message MOM-
MOM-20-0049-01B(R4), dated September 28, 2020. No additional changes to
this AD have been made.
Request To Add Service-Based Compliance Time
Southwest Airlines requested that a service-based compliance time
be added to the calendar-based compliance time proposed in the NPRM.
Southwest Airlines proposed that the compliance time be re-written to
require compliance within "9 months or 2,000 flight cycles from AD
effective date, whichever occurs later." Southwest Airlines pointed
out that due to the reduced flight schedules in response to the COVID-
19 pandemic, many airplanes are in long-term storage. Southwest
Airlines added that some of these long-term storage facilities might
not be capable of providing heavy maintenance, and airplanes would need
to be ferried to a facility where these actions may be accomplished.
The FAA disagrees with adding the 2,000 flight cycles to the
compliance time. In developing an appropriate compliance time, the FAA
considered the safety implications, parts availability, and normal
maintenance schedules. In consideration of all of these factors, the
FAA determined that the compliance time, as proposed, represents an
appropriate interval for the general visual inspection for insufficient
sealant in the SOB slot within the fleet, while still maintaining an
adequate level of safety. However, under the provisions of paragraph
(k) of this AD, the FAA will consider requests for approval of an
extension of the compliance time if sufficient data are submitted to
substantiate that the new compliance time would provide an acceptable
level of safety. The FAA has not changed this AD in this regard.
Request To Extend Compliance Time
Alaska requested that an additional compliance time deferral of 24
months be allowed under the following conditions: No fuel contamination
in the ADMB; no external leak or signs of a previous external leak
around the SOB slot; insufficient sealing conditions temporarily
corrected with PR-1826 B-1/4 sealant or SF5387 secondary fuel barrier
materials, in accordance with certain service information instructions;
and repetitive inspections for fuel contamination of the ADMB done at
intervals not to exceed 12 months until the repair specified in
paragraph (g) of this AD is completed. Alaska stated that the extended
compliance time would relieve some of the operational impact and would
aid in planning for the time-intensive repair during a heavy
The FAA does not agree to provide a 24-month compliance time for
the repair. The technical specifications provided for the proposed
alternative sealant for temporary repair do not provide the FAA with
enough information to determine that the temporary repair would comply
with the certification basis of the airplane and provide an acceptable
level of safety when incorporated into the existing fuel system design.
There is also insufficient information on how the procedures for use in
the temporary repair would differ from the permanent repair. However,
under the provisions of paragraph (k) of this AD, the FAA will consider
requests for approval of an extension of the repair compliance time if
sufficient data are submitted to substantiate that the conditions and
temporary repair would provide an acceptable level of safety. The FAA
has not changed this AD in this regard.
Request To Allow Removing Certain Other Parts To Gain Inspection Access
American Airlines and Delta requested a revision to the proposed AD
to allow removing the air return grilles or sidewall panels to expose a
hole in a beam above the SOB slot area so the inspection of the sealant
can be done through the opening. Both commenters stated that the access
procedure specified in Boeing Multi Operator Message MOM-MOM-20-0049-
01B(R1), dated January 29, 2020, removes seats and cabin floor panels
to gain access to the inspection area. American Airlines also stated
that allowing the alternative access procedure for the inspection would
reduce the amount of time necessary to accomplish an inspection of the
SOB slot, and that accomplishing any required corrective action would
then require the removal of seats and cabin floor panels to gain access
to the SOB slot area.
The FAA agrees that the inspection could be accomplished by
removing the air return grilles or sidewall panels. As discussed
previously, the FAA has revised this AD to reference Boeing Multi
Operator Message MOM-MOM-20-0049-01B(R4), dated September 28, 2020,
which does contain this additional method of compliance for accessing
the inspection area. The FAA has not made any additional revisions to
this AD in this regard.
Request To Allow Use of Alternative Sealant
Alaska stated that the sealant AC-360 B1/2, which is specified as
the sealant to use to fill a void, is discontinued. The FAA infers that
Alaska wants the AD to be revised to remove reference to the
The FAA agrees that the sealant AC-360 B1/2 has been discontinued.
As discussed above, this AD has been revised to reference Boeing Multi
Operator Message MOM-MOM-20-0049-01B(R4), dated September 28, 2020,
which removes the reference to the sealant AC-360 B1/2, but retains the
reference to sealant BMS 5-142, which is the appropriate sealant to
use. The FAA has not made any further changes to this AD in this
Request To Use Faster-Curing Sealants
Alaska requested that the FAA allow alternative sealants that cure
faster than the ones specified in the service information. Alaska
suggested allowing the use of sealant PR-1826 B1/4 in place of BMS 5-
142, and SF5387 in place of BMS 5-81. Alaska also provided technical
data sheets for the proposed sealants. Alaska noted that use of the
slower-curing sealants could result in an airplane being out of service
for up to 34 hours, and that time could be reduced by use of faster-
curing sealants that provide an equivalent level of safety.
The FAA agrees that the alternative sealants would provide a faster
cure time. However, the FAA does not have enough data to determine if
the alternative sealants comply with the certification basis of the
airplane and if they will provide an acceptable level of safety when
incorporated into the existing fuel system design. The FAA will
consider requests for alternative sealants as an alternative method of
compliance if requested using the procedure specified in paragraph (k)
of this AD. The FAA has not changed the AD in this regard.
The FAA reviewed the relevant data, considered the comments
received, and determined that air safety and the public interest
require adopting this final rule with the changes described previously
and minor editorial changes. The FAA has determined that these minor
Are consistent with the intent that was proposed in the
NPRM for addressing the unsafe condition; and
Do not add any additional burden upon the public than was
already proposed in the NPRM.
The FAA also determined that these changes will not increase the
economic burden on any operator or increase the scope of this final rule.
Related Service Information Under 1 CFR Part 51
The FAA reviewed Boeing Multi Operator Message MOM-MOM-20-0049-
01B(R4), dated September 28, 2020. This service information describes
procedures for a general visual inspection for insufficient sealant in
the SOB slot. The service information also describes procedures for
related investigative actions including a general visual inspection of
the ADMB for fuel contamination, a check for external leaks of the
center fuel tank external surfaces inside the pressure boundary, and an
internal leak check of the center fuel tank to identify the leakage
path(s). The service information also describes procedures for
corrective actions including removal of all insulation blankets below
the crease beam (left side to right side), clean-up of all fuel
contamination, repair of any leak, preparation of the SOB slot for
sealing, application of sealant, and repair of the secondary fuel
barrier. 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.
Costs of Compliance
The FAA estimates that this AD affects 731 airplanes of U.S.
registry. The FAA estimates the following costs to comply with this AD:
COST PER PRODUCT
COST ON U.S. OPERATORS
|Inspection for sealant
||30 work-hours x $85 perhour = $2,550.
The FAA estimates the following costs to do any necessary repairs
that would be required based on the results of the inspection. The FAA
has no way of determining the number of aircraft that might need these
COST PER PRODUCT
|Repair of sealant
|| 2 work-hours x $85 per hour = $170.
|Insulation blanket replacement
|| 24 work-hours x $85 per hour = $2,040.
|| 6 work-hours x $85 per hour = $510.
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.
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
(2) Will not affect intrastate aviation in Alaska, and
(3) Will not have a significant economic impact, positive or
negative, on a substantial number of small entities under the criteria
of the Regulatory Flexibility Act.
List of Subjects in 14 CFR Part 39
Air transportation, Aircraft, Aviation safety, Incorporation by
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