DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. FAA-2021-0660; Project Identifier AD-2021-00398-T]
RIN 2120-AA64

Airworthiness Directives; Learjet Inc. Airplanes

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Notice of proposed rulemaking (NPRM).

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SUMMARY: The FAA proposes to adopt a new airworthiness directive (AD)
for certain Learjet Inc. Model 45 airplanes. This proposed AD was
prompted by a report of a fuel leak due to a cracked fuel line between
the engine fuel control and the engine fuel flow meter. This proposed
AD would require replacing the existing fuel flow meter bracket
assembly with a redesigned bracket assembly and reporting information
to the FAA. The FAA is proposing this AD to address the unsafe
condition on these products.

DATES: The FAA must receive comments on this proposed AD by September
27, 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 NPRM, contact Learjet
Inc., One Learjet Way, Wichita, KS 67209; phone: (316) 946-2000; email:
ac.ict@aero.bombardier.com; website: https://businessaircraft.bombardier.
com/en/aircraft/Learjet.html. You may view this service information at the
FAA, Airworthiness Products Section, Operational Safety Branch, 901 Locust
Kansas City, MO 64106. For information on the availability of this
material at the FAA, call (816) 329-4148.

Examining the AD Docket

You may examine the AD docket at https://www.regulations.gov by
searching for and locating Docket No. FAA-2021-0660; 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 NPRM, any comments
received, and other information. The street address for Docket
Operations is listed above.

FOR FURTHER INFORMATION CONTACT: James Galstad, Aviation Safety
Engineer, Wichita ACO Branch, FAA, 1801 S Airport Road, Wichita, KS
67209; phone: (316) 946-4135; email: james.galstad@faa.gov or Wichita-COS@
faa.gov; or Thomas Teplik, Aviation Safety Engineer, Wichita ACO
Branch, FAA, 1801 S Airport Road, Wichita, KS 67209; phone: (316) 946-
4196; email: thomas.teplik@faa.gov or Wichita-COS@faa.gov.

SUPPLEMENTARY INFORMATION:

Comments Invited

The FAA invites you to send any written relevant data, views, or
arguments about this proposal. Send your comments to an address listed
under ADDRESSES. Include "Docket No. FAA-2021-0660; Project Identifier
AD-2021-00398-T" at the beginning of your comments. The most helpful
comments reference a specific portion of the proposal, 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 proposal 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 NPRM.

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 NPRM 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 NPRM, 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 NPRM. Submissions containing CBI should be sent to James
Galstad, Aviation Safety Engineer, or Thomas Teplik, Aviation Safety
Engineer, Wichita ACO Branch, FAA, 1801 S Airport Road, Wichita, KS
67209. Any commentary that the FAA receives which is not specifically
designated as CBI will be placed in the public docket for this
rulemaking.

Background

The FAA received a report of a fuel leak due to a cracked fuel line
between the engine fuel control and the engine fuel flow meter on a
Learjet Model 45 (Learjet 45) airplane. There are four different Model
45 configurations: Model 45 (Learjet 40), Model 45 (Learjet 45), Model
45 (Learjet 70), and Model 45 (Learjet 75). They all are susceptible to
cracked fuel lines with possible fuel leakage because the fuel flow
meter bracket and fuel line is common to each model. Further analysis
of the fleet of all the 45 models revealed similar failures in this
area including the following: 16 fuel line failures, 2 instances of
multiple inlet attaching bolts breaking, 9 leaking fuel controls, a
broken gearbox strut, 4 cracked No. 6 bearing oil supply lines, and 7
cracked engine oil tanks. The FAA evaluated the flammable fluid leaks
and broken parts and determined that they may have resulted from
vibration.

Following the above report and analysis, Learjet designed a new
engine fuel flow meter bracket and incorporated it during production.
The unsafe condition, if not addressed, could result in an engine
installation fire, which could progress to an uncontrolled fire and
consequent loss of control of the airplane.

FAA's Determination

The FAA is issuing this NPRM after determining that the unsafe
condition described previously is likely to exist or develop on other
products of the same type design.

Related Service Information Under 1 CFR Part 51

The FAA reviewed Bombardier Learjet 40 Service Bulletin (SB) SB 40-
73-01, Revision 1, Bombardier Learjet 45 SB 45-73-2, Revision 1,
Bombardier Learjet 70 SB 70-73-01, Revision 1, and Bombardier Learjet
75 SB 75-73-01, Revision 2, all dated January 9, 2017. This service
information specifies procedures for replacing the existing fuel flow
meter bracket assembly with a redesigned bracket assembly with pad fuel
flow meter that has an increased material thickness. These documents
are distinct since they apply to different airplane models. 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 ADDRESSES.

Proposed AD Requirements in This NPRM

This proposed AD would require accomplishing the actions specified
in the service information already described, except as discussed under
"Differences Between this Proposed AD and the Service Information."

Differences Between This Proposed AD and the Service Information

This proposed AD would require reporting certain maintenance
information to the FAA, where the service information does not. The
information provided in the reports would be related to contributing
factors that the FAA found showed a correlation between the reported
engine fan vibration levels and the cracking fuel line between engine
fuel control and the engine fuel flow meter and a correlation between
the cracking fuel line and a certain batch of fan disks. In addition,
the FAA found that a contributing factor could be the susceptibility of
the fuel flow meter bracket assembly and the susceptibility of the
bracket assembly with pad fuel flow meter to the vibration induced. The
requested reporting information would allow the FAA to determine
whether further rulemaking action would be necessary to mitigate the
unsafe condition.

Also, the effectivity of Bombardier Learjet 45 SB 45-73-2, Revision
1, dated January 9, 2017, begins with serial number 45-005. This
proposed AD would also apply to airplane serial numbers 45-002 through
45-004 because, although these three airplanes are not currently in
service, they are subject to the unsafe condition. Thus, it is
necessary to include them in the event they are returned to service.

Costs of Compliance

The FAA estimates that this AD, if adopted as proposed, would
affect 443 airplanes of U.S. registry.

The FAA estimates the following costs to comply with this proposed
AD:

Estimated 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 current 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 take
approximately 9 hours per response, including the time for reviewing
instructions, searching existing data sources, gathering and
maintaining the data needed, and 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

The FAA determined that this proposed AD would not have federalism
implications under Executive Order 13132. This proposed AD would not
have a substantial direct effect on the States, on the relationship
tween 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 this proposed
regulation:

(1) Is not a "significant regulatory action" under Executive
Order 12866,

(2) Would not affect intrastate aviation in Alaska, and

(3) Would 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
reference, Safety.

The Proposed Amendment

Accordingly, under the authority delegated to me by the
Administrator, the FAA proposes to amend 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: