Airworthiness Directives; The Boeing Company Airplanes

SUPPLEMENTARY INFORMATION:

Background

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 737-8 and Model 737-9 airplanes. The NPRM published in the Federal Register on February 14, 2024 (89 FR 11231). The NPRM was prompted by a Boeing review of the SPCU design where a single point of failure exists internal to the SPCU. In the NPRM, the FAA proposed to require installing four diodes and changing wire bundles in the P5 panel, as well as performing installation and power tests and applicable on-condition actions. The FAA is issuing this AD to address a potential single point of failure in the SPCU, which can result in a non-latent loss of the entire battery bus and consequent un-annunciated loss of control and indication of both engine anti-ice (EAI) systems.

Discussion of Final Airworthiness Directive

Comments

The FAA received comments from eight commenters who supported the NPRM without change.

The FAA received additional comments from Alaska Airlines and two individuals. The following presents the comments received on the NPRM and the FAA's response to each comment.

Request for a “Credit for Previous Actions” Paragraph

Alaska Airlines requested that the proposed AD be revised to include a “Credit for Previous Actions” paragraph that gives credit as specified in paragraph (h)(2), “Exceptions to Service Information Specifications,” of the proposed AD. Alaska requested credit for “ACTION 3” in the Action column of the table in the “Compliance” paragraph of Boeing Alert Requirements Bulletin 737-30A1083 RB, dated November 18, 2022, for airplanes on which the requirements bulletin was embodied prior to the effective date of this AD.

The FAA disagrees with revising this AD to include a “Credit for Previous Actions” paragraph in this AD. Paragraph (f), “Compliance,” of this AD provides credit for required actions performed before the effective date of this AD. The FAA has not changed this AD as a result of this comment.

Request for Clarification of AD Actions

An individual was supportive, but requested clarification of whether the proposed AD adequately addresses the unsafe condition to prevent the single point of failure addressed by the proposed AD. The commenter also questioned what is being done so this condition does not occur again, and whether this is the only issue with this airplane design. The commenter also requested clarification of whether the affected airplanes would be grounded until the proposed AD actions are completed.

The FAA worked with Boeing to develop appropriate actions, i.e., the diode installation and wire bundle change, using the procedures specified in Boeing Alert Requirements Bulletin 737-30A083 RB, dated November 18, 2022, to address the identified unsafe condition in this AD. This particular issue is the only one addressed by this AD. The FAA continuously evaluates operator and manufacturer reports and other data in order to promote safe type certification and production. Operators are required to comply with all applicable actions of an AD within the required compliance time and cannot later undo those required actions. The FAA's safety analysis has determined that the compliance time for corrective action in this AD provides an acceptable level of safety. The FAA has not changed this AD as a result of this comment.

Request for FAA's Approach to Continued Operational Safety

Another individual stated that the AD “prompted a review of” the SPCU and “found a design flaw.” The commenter acknowledged that the proposed AD would address the underlying problem, but questioned how long the condition has existed, whether the SPCU review addresses all systems on these airplanes, and whether affected airplanes remain in service until the concerns are addressed. This commenter stated that a deeper review into other models and ways to address these issues must be investigated.

The FAA provides the following clarification. This AD did not prompt the SPCU review; rather, after the SPCU review conducted by Boeing, the FAA determined that an AD was necessary and appropriate to address the unsafe condition. The FAA continuously receives and evaluates performance and safety data from operators and manufacturers on all type-certificated airplanes. The FAA takes corrective action—whether advisory such as an advisory circular or mandatory such as an AD—as warranted by the facts. The FAA has not changed this AD as a result of this comment.

Conclusion

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 Boeing Alert Requirements Bulletin 737-30A083 RB, dated November 18, 2022. This material specifies procedures for installing four diodes and changing wire bundles in the P5 panel, as well as performing an anti-ice diode installation test and an engine anti-ice and wing anti-ice power test and applicable on-condition actions. On-condition actions include doing applicable corrective actions until the tests are passed.

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 205 airplanes of U.S. registry. The FAA estimates the following costs to comply with this AD: