SUPPLEMENTARY INFORMATION:

Discussion

We issued an NPRM that proposed to amend 14 CFR part 39 to add an AD that would apply to the specified products. The NPRM was published in the Federal Register on September 16, 2016 (81 FR 63725). The NPRM was prompted by the Japan Civil Aviation Bureau (JCAB), which is the aviation authority for Japan, AD No. TCD-8783-2016, dated June 28, 2016 (referred to after this as “the MCAI”), to correct an unsafe condition for certain Mitsubishi Heavy Industries (MHI) Models MU-2B-20, MU-2B-25, MU-2B-26, MU-2B-30, MU-2B-35, and MU-2B-36 airplanes.

As part of the MHI MU-2B aging aircraft program, one-piece and three-piece main wings were subjected to detailed teardown inspections, and cracks were found in the wing spacer plates attached to the forward lower spar area at wing station 580. It was determined that the cracks resulted from fatigue caused by flight loads.

Japan is the State of Design for the MHI airplane models that the MCAI AD applies to: Models MU-2B-20, MU-2B-25, and MU-2B-26 with serial numbers (S/Ns) 102 and 121 through 347, except 313 and 321; and Models MU-2B-30, MU-2B-35, and MU-2B-36 with S/Ns 502 through 696, except 652 and 661. The United States is the State of Design for MHI Models MU-2B-10, MU-2B-15, MU-2B-25, MU-2B-26, MU-2B-26A, and MU-2B-40 with S/Ns 313SA, 321SA, and 348SA through 459SA; and Models MU-2B-36A and MU-2B-60 with S/Ns 661SA and 697SA through 1569SA airplanes. Japan is the State of Design for Models MU-2B-10 and MU-2B-15 airplanes, but has recently removed these models from the MHI Japanese type certificate. These models remain on the FAA type certificate; however, none of these airplanes are currently on the U.S. registry.

The NPRM proposed to require repetitively inspecting the wing spacer plates for cracks until they were replaced with an improved design wing spacer plates. The proposed actions were intended to detect and correct cracks in the wing spacer plates, which could result in reduced structural integrity of the wings and loss of control.

Actions Since the NPRM Was Issued

Since issuance of the NPRM, we have received data from operators who completed the inspections specified in MHI MU-2 Service Bulletin No. 245, dated April 21, 2016, and MU-2 Service Bulletin No. 107/57-005, dated May 3, 2016. During the inspections, no cracking in a primary wing structure has been detected. We have determined that damage is contained to the wing spacer plates without affecting the primary structure. Our analysis of fleet data also demonstrates that the wing spar and the wing spar cap maintains conformity with the structural requirements of the type certificate after complete fracture of the wing spacer plate. Neither the JCAB nor the manufacturer has provided the FAA with sufficient data that an unsafe condition exists. Therefore, we have determined that AD action is not appropriate, and the NPRM should be withdrawn.

After we received numerous comments on the NPRM stating there is no unsafe condition, we requested additional information from JCAB to demonstrate that the cracks found in the spacers reduce the structural integrity of primary structure. JCAB replied that it took AD action because it is uncertain how the fatigue strength will be affected over the life of the airplane. In addition to the lack of conclusive data that there is an unsafe condition, we considered that removing the wing in order to perform the proposed corrective action may be more detrimental to the aircraft than the cracks themselves.

Withdrawal of this NPRM constitutes only such action and does not preclude the agency from issuing future rulemaking on this issue, nor does it commit the agency to any course of action in the future.

Regulatory Findings

Since this action only withdraws an NPRM, it is neither a proposed nor a final rule and therefore, not covered under Executive Order 12866, the Regulatory Flexibility Act, or DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979).

The Withdrawal

Accordingly, the notice of proposed rulemaking, Docket No. FAA-2016-9139, which published in the Federal Register on September 16, 2016 (81 FR 63725), is withdrawn.

84 FR 6708