Background The FAA issued a notice of proposed rulemaking (NPRM) to amend 14 CFR part 39 by adding an AD that would apply to all Bombardier, Inc., Model CL-600-2A12 (601) and CL-600-2B16 (601-3A, 601-3R, and 604 Variants) airplanes. The NPRM was published in the Federal Register on August 5, 2025 (90 FR 37434). The NPRM was prompted by AD CF-2025-06, dated January 20, 2025 (Transport Canada AD CF-2025-06) (also referred to as the MCAI), issued by Transport Canada, which is the aviation authority for Canada. The MCAI states that new or more restrictive airworthiness limitations have been developed. The flightcrew of a Challenger airplane started the auxiliary power unit (APU) during the approach and allowed it to run for approximately 10 minutes after landing. When the flightcrew shut the APU down, an APU FIRE warning message was posted on the engine indicating and crew alerting system (EICAS). The APU fire suppression was discharged; however, the fire was not fully extinguished following the discharge of the fire bottles. Upon further investigation, it was discovered that a fuel solenoid valve was leaking, and the APU muffler drainage was blocked leading to an accumulation of fuel in the muffler. The fuel in the APU muffler was ignited by the high-temperature exhaust gases in the muffler. In the NPRM, the FAA proposed to require revising the existing maintenance or inspection program, as applicable, to incorporate new or more restrictive airworthiness limitations. The FAA is issuing this AD to address an accumulation of fuel in the APU muffler caused by a drainage block in the muffler. This condition, if not corrected, could lead to an accumulation of fuel in the muffler being ignited by the high-temperature exhaust gases in the muffler, and could subsequently lead to an uncontrolled fire in the APU bay. You may examine the MCAI in the AD docket at regulations.gov under Docket No. FAA-2025-1728. Discussion of Final Airworthiness Directive Comments The FAA received a comment from Bombardier. The following presents the comment received on the NPRM and the FAA's response to the comment. Request To Clarify Source of Fuel Leakage Bombardier noted the Background section of the proposed AD states that upon further investigation, it was discovered that a fuel solenoid valve was leaking. Bombardier stated it was suspected that there was an internal fuel leakage from the fuel solenoid valve, not that the valve was found leaking. The FAA acknowledges Bombardier's request. The FAA notes that the information in the Background section of the proposed AD restates the information provided in the MCAI, so no change to the AD is necessary in this regard. Conclusion These products have been approved by the civil aviation authority of another country and are approved for operation in the United States. Pursuant to the FAA's bilateral agreement with this State of Design Authority, that authority has notified the FAA of the unsafe condition described in the MCAI referenced above. The FAA reviewed the relevant data, considered any comments received, and determined that air safety requires adopting this AD as proposed. Accordingly, the FAA is issuing this AD to address the unsafe condition on these products. Except for minor editorial changes, this AD is adopted as proposed in the NPRM. None of the changes will increase the economic burden on any operator. Material Incorporated by Reference Under 1 CFR Part 51 The FAA reviewed the following Bombardier material: - Temporary Revision No. 5-2-6, dated June 28, 2024, which includes new Task 49-15-01-101*, "Functional Test of the Auxiliary Power Unit (APU) Muffler Drain." - Temporary Revision No. 5-2-30, dated June 28, 2024, which includes new Task 49-15-01-101*, "Functional Test of the Auxiliary Power Unit (APU) Muffler Drain." - Temporary Revision No. 5-2-74, dated June 28, 2024, which includes new Task 49-15-01-101*, "Functional Test of the Auxiliary Power Unit (APU) Muffler Drain (A/C 5301 to 5630, Post SB 604-49-006, and A/C 5631 and Subs)." - Temporary Revision No. TR 5-271, dated September 30, 2024, which includes, among other tasks, new Task 49-15-01-101*, "Functional Test of the APU Muffler Drain." - Temporary Revision No. TR 5-285, dated September 30, 2024, which includes, among other tasks, new Task 49-15-01-101*, "Functional Test of the APU Muffler Drain." (The asterisk (or "one star") with the last three digits of the task numbers listed above indicates that the task is an airworthiness limitation task.) Temporary Revision Nos. 5-2-6, 5-2-30, 5-2-74, TR 5-271, and TR 5-285 introduce, among other tasks, functional tests of the APU muffler drain. These documents are distinct since they apply to different airplane configurations. This material 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 550 airplanes of U.S. registry. The FAA estimates the following costs to comply with this AD: The FAA has determined that revising the maintenance or inspection program takes an average of 90 work-hours per operator, although the agency recognizes that this number may vary from operator to operator. Since operators incorporate maintenance or inspection program changes for their affected fleet(s), the FAA has determined that a per-operator estimate is more accurate than a per-airplane estimate. Therefore, the agency estimates the average total cost per operator to be $7,650 (90 work-hours × $85 per work-hour). 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, (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 reference, Safety. 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. § 39.13 [Amended] 2. The FAA amends § 39.13 by adding the following new airworthiness directive: 2025-24-11 Bombardier, Inc.: Amendment 39-23208; Docket No. FAA-2025-1728; Project Identifier MCAI-2025-00076-T. (a) Effective Date This airworthiness directive (AD) is effective January 16, 2026. (b) Affected ADs None. (c) Applicability This AD applies to all Bombardier, Inc., Model CL-600-2A12 (601) and CL-600-2B16 (601-3A, 601-3R, and 604 Variants) airplanes, certificated in any category. (d) Subject Air Transport Association (ATA) of America Code 49, Auxiliary Power (e) Unsafe Condition This AD was prompted by a determination that new or more restrictive airworthiness limitations are necessary. The FAA is issuing this AD to address an accumulation of fuel in the auxiliary power unit (APU) muffler caused by a drainage block in the muffler. This condition if not corrected, could lead to an accumulation of fuel in the muffler being ignited by the high-temperature exhaust gases in the muffler, and could subsequently lead to an uncontrolled fire in the APU bay. (f) Compliance Comply with this AD within the compliance times specified, unless already done. (g) Maintenance or Inspection Program Revision Within 90 days after the effective date of this AD, revise the existing maintenance or inspection program, as applicable, to incorporate the information specified in the applicable tasks identified in figure 1 to paragraph (g) of this AD. The initial compliance time for doing the tasks is at the time specified in the applicable tasks identified in figure 1 to paragraph (g) of this AD, or within 90 days after the effective date of this AD, whichever occurs later. Figure 1 to Paragraph (g)-Temporary Revisions and Tasks ER12DE25.000 Note 1 to figure 1 to paragraph (g): The asterisk (or "one star") with the last three digits of the task numbers listed in table 1 to paragraph (g) of this AD indicates that the task is an airworthiness limitation task. (h) No Alternative Actions or Intervals After the existing maintenance or inspection program has been revised as required by paragraph (g) of this AD, no alternative actions ( e.g., inspections) or intervals may be used unless the actions and intervals are approved as an alternative method of compliance (AMOC) in accordance with the procedures specified in paragraph (i)(1) of this AD. (i) Additional AD Provisions The following provisions also apply to this AD: (1) Alternative Methods of Compliance (AMOCs): The Manager, International Validation Branch, FAA, has the authority to approve AMOCs for this AD, if requested using the procedures found in 14 CFR 39.19. In accordance with 14 CFR 39.19, send your request to your principal inspector or responsible Flight Standards Office, as appropriate. If sending information directly to the manager of the International Validation Branch, send it to the attention of the person identified in paragraph (j) of this AD and email to: AMOC@faa.gov . Before using any approved AMOC, notify your appropriate principal inspector, or lacking a principal inspector, the manager of the Flight Standards Office. (2) Contacting the Manufacturer: For any requirement in this AD to obtain instructions from a manufacturer, the instructions must be accomplished using a method approved by the Manager, International Validation Branch, FAA; or Transport Canada; or Bombardier's Transport Canada Design Approval Organization (DAO). If approved by the DAO, the approval must include the DAO-authorized signature. (j) Additional Information For more information about this AD, contact Joseph Catanzaro, Aviation Safety Engineer, FAA, 1600 Stewart Avenue, Suite 410, Westbury, NY 11590; phone: 516-228-7300; email: 9-avs-nyaco-cos@faa.gov. (k) Material Incorporated by Reference (1) The Director of the Federal Register approved the incorporation by reference of the material listed in this paragraph under 5 U.S.C. 552(a) and 1 CFR part 51. (2) You must use this material as applicable to do the actions required by this AD, unless this AD specifies otherwise. (i) Bombardier Temporary Revision No. 5-2-6, dated June 28, 2024. (ii) Bombardier Temporary Revision No. 5-2-30, dated June 28, 2024. (iii) Bombardier Temporary Revision No. 5-2-74, dated June 28, 2024. (iv) Bombardier Temporary Revision No. TR 5-271, dated September 30, 2024. (v) Bombardier Temporary Revision No. TR 5-285, dated September 30, 2024. (3) For Bombardier material identified in this AD, contact Bombardier Business Aircraft Customer Response Center, 400 Côte-Vertu Road West, Dorval, Québec H4S 1Y9, Canada; telephone 514-855-2999; email ac.yul@aero.bombardier.com; website bombardier.com. (4) You may view this material 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. (5) You may view this material at the National Archives and Records Administration (NARA). For information on the availability of this material at NARA, visit www.archives.gov/federal-register/cfr/ibr-locations or email fr.inspection@nara.gov. Issued on November 24, 2025. Steven W. Thompson, Acting Deputy Director, Compliance & Airworthiness Division, Aircraft Certification Service.