AD 2023-24-04

This AD is effective January 10, 2024.

AD Docket: You may examine the AD docket at regulations.gov under Docket No. FAA-2023-1050; 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.

Joseph Costa, Aviation Safety Engineer, FAA, 3960 Paramount Boulevard, Lakewood, CA 90712; phone: (562) 627-5246; email: joseph.costa@faa.gov.

The FAA issued a notice of proposed rulemaking (NPRM) to amend 14 CFR part 39 by adding an AD that would apply to all Honeywell Model AS907-1-1A and AS907-2-1G engines. The NPRM published in the Federal Register on May 23, 2023 (88 FR 32980). The NPRM was prompted by several reports that Honeywell Model AS907-1-1A and AS907-2-1G engines experienced compressor surge, including an AS907-1-1A dual engine compressor surge, during takeoff climb out through a steep temperature inversion, which resulted in loss of engine thrust control. The FAA determined that the installed ECU software version logic locked the engine inlet total temperature (Tt2) at 60 knots on a takeoff roll and that reference Tt2 remained locked until the aircraft reached 400 feet above ground level (AGL) or the pilot moved the throttle before reaching 400 AGL. The locked Tt2 is mathematically adjusted by the ECU software for altitude and Mach number changes as the takeoff progresses. During a climb to 400 feet AGL with a thermal inversion, the actual engine Tt2 can increase above the Tt2 that is being calculated by the ECU, which causes the compressor guide vanes' (CGVs) and surge bleed valves' (SBVs) positions to be off-schedule for the actual ambient conditions. Significant off-scheduling of the CGVs and the SBVs can lead to a compressor surge event. The compressor surge margin is decreased when scheduling is based on a colder Tt2 temperature than what the engine is actually running. Engine deterioration impacts compressor surge margin and can increase the likelihood of a dual engine compressor surge as the AS907-1-1A and AS907-2-1G engine fleets age. Dual engine power loss due to a temperature inversion may result in significant loss of airplane thrust, which could reduce the climb gradient and result in the airplane's inability to clear obstacles. As a result, the manufacturer updated the software.

In the NPRM, the FAA proposed to require either the replacement of a certain ECU software version installed on AS907-1-1A engines with an updated software version eligible for installation or the replacement of certain ECUs installed on AS907-1-1A engines with ECUs eligible for installation. The NPRM also proposed to require the replacement of certain ECUs installed on AS907-2-1G engines with ECUs eligible for installation. The FAA is issuing this AD to address the unsafe condition on these products.

The FAA received comments from three commenters. The commenters were Bombardier Inc. (Bombardier), Duncan Aviation, and Honeywell. All three commenters requested changes to the proposed AD. The following presents the comments received on the NPRM and the FAA's response to each comment.

Bombardier requested that the FAA update the NPRM so that references to software are consistent. Bombardier noted that in the NPRM SUMMARY section, the phrase “updated software” was used, while other portions of the AD refer to the “software version eligible for installation.”

The FAA acknowledges the inconsistency and has provided detail about the updated software in the Background of this final rule.

Bombardier requested that the FAA include language in the Background paragraph of this AD to explain the effect of Tt2 locking on the SBVs.

The FAA agrees. As a result, the FAA has added language to the Background paragraph of this AD to explain the effect of Tt2 locking and clarify the surge bleed valves that affect surge margin.

Bombardier requested that the FAA update the language in the unsafe condition of the NPRM Background and paragraph (e) of the proposed AD to the following: “may result in significant loss of overall airplane thrust which can lead to inability of the airplane to clear the AFM OEI net flight path (risk of obstacle collision).”

The FAA partially agrees. The unsafe condition description in Background of this final rule was clarified to include “which could reduce the climb gradient and result in the airplane's inability to clear obstacles.” However, paragraph (e) of this AD was not changed as a result of this comment because the consequence remains consistent with the end-level effect if the unsafe condition is not addressed.

Honeywell suggested a change to the second sentence of the Cost of Compliance to remove an erratum.

The FAA agrees and has changed the second sentence of the Costs of Compliance of this final rule to refer to AS907-2-1G engines installed on airplanes of U.S. registry.

Bombardier questioned why the work-hours for removing the ECU for the AS907-1-1A engine are less than the work-hours for removing the ECU for the AS907-2-1G engine.

The FAA acknowledges that the estimated work-hours should be the same for removing an ECU from both engine models. However, the FAA estimated one ECU will need to be replaced in the AS907-1-1A fleet. Therefore, the FAA estimated 1 work-hour to replace the ECU and 1 work-hour to complete the power assurance run for one engine. The FAA did not change this AD as a result of this comment.

Honeywell noted that the structure of the Estimated Costs table is such that the total or individual engine model fleet cost to the U.S. registered operators is not clearly presented.

The FAA agrees and has clarified the Costs of Compliance of this final rule. The cost per product columns reflect the estimated costs per engine. The AS907-1-1A cost to replace ECU software was moved to the Estimated Costs table to show a total fleet cost assuming all ECUs receive the software upgrade. The cost to replace the AS907-1-1A ECU remains unchanged. The FAA has no way of knowing how many AS907-1-1A engine operators will replace the ECU instead of the software.

Duncan Aviation and Honeywell requested that the FAA change the expression of compliance times throughout Tables 1 and 2 of the NPRM to be more concise. Each of the two commenters suggested slightly different phrasing of compliance times. Each suggested removal of the terms “before exceeding” and “hours time-in-service (TIS).” Duncan Aviation and Honeywell requested the order of “whichever occurs first” and “after the effective date of this AD” be reversed. Duncan Aviation stated that “hours TIS” could be misinterpreted as time since new.

The FAA agrees that the expression of compliance times should be clear. The FAA clarifies that “hours TIS” is equivalent to flight hours. The FAA changed the compliance times in Table 1 to paragraph (g)(1) and Table 2 to paragraph (g)(2) of this AD to read as follows: Within X months/years or Y flight hours after the effective date of this AD, whichever occurs first.

Honeywell requested that the FAA change the ECU software version number in paragraph (g)(1) of the proposed AD from “AS907_1011” to “AS907_1001.” Bombardier requested that the FAA change the ECU software part version number in paragraph (g)(1) of the proposed AD from “AS907_1011” to “AS907-1001.”

The FAA agrees with changing the ECU software version number in paragraph (g)(1) to “AS907_1001” because “AS907_1001” is the correct nomenclature.

Bombardier requested that the FAA change the part/version numbers defined as eligible for installation. Bombardier requested that the definitions change from excluding certain part/version numbers to requiring certain part/version numbers because older versions of software will not address the unsafe condition.

The FAA partially agrees. While these items may be the only items currently eligible for installation, the definitions also allow future approved items. This mitigates the need for an alternative method of compliance (AMOC) for future software versions or ECU P/Ns. There is no version of software older than AS907_1001 for the AS907-1-1A engine. The FAA did not change this AD as a result of this comment.

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, and any other changes described previously, this AD is adopted as proposed in the NPRM. None of the changes will increase the economic burden on any operator.

The FAA estimates that this AD affects a total of 853 engines installed on airplanes of U.S. registry. The FAA estimates that 175 AS907-2-1G engines installed on airplanes of U.S. registry will require replacing two ECUs per engine.

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

The FAA has included all known costs in its cost estimate. According to the manufacturer, however, some of the costs of this AD may be covered under warranty, thereby reducing the cost impact on affected operators.

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 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.

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

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

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