Airworthiness Directives; Piper Aircraft, Inc. Airplanes

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 the ADDRESSES section. Include “Docket No. FAA-2024-2143; Project Identifier AD-2024-00008-A” 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 revise 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 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 Fred Caplan, Aviation Safety Engineer, FAA, 1701 Columbia Avenue, College Park, GA 30337. 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 issued AD 2020-26-16, Amendment 39-21371 (86 FR 3769, January 15, 2021) (AD 2020-26-16), for certain Piper Model PA-28-151, PA-28-161, PA-28-181, PA-28-235, PA-28R-180, PA-28R-200, PA-28R-201, PA-28R-201T, PA-28RT-201, PA-28RT-201T, PA-32-260, PA-32-300, PA-32R-300, PA-32RT-300, and PA-32RT-300T airplanes. AD 2020-26-16 was prompted by an accident involving wing separation on a Piper Model PA-28R-201 airplane. An investigation by the National Transportation Safety Board (NTSB) revealed a fatigue crack in a visually inaccessible area of the lower main wing spar cap. The applicability of the NPRM for AD 2020-26-16 included additional Piper model airplanes with similar main wing spar structures as the Model PA-28R-201. Based on airplane usage history, the FAA determined that only those airplanes with a higher risk for fatigue cracks (airplanes with a significant history of operation in flight training or other high-load environments) should be subject to the inspection requirements proposed in that NPRM.

AD 2020-26-16 requires calculating the factored service hours for each main wing spar to determine when an inspection is required, inspecting the lower main wing spar bolt holes for cracks, and replacing any cracked main wing spar. The agency issued AD 2020-26-16 to detect and correct fatigue cracks in the lower main wing spar cap bolt holes.

Actions Since AD 2020-26-16 Was Issued

The preamble to AD 2020-26-16 explains that the FAA considers the requirements “interim action” and was considering further rulemaking. The FAA has now determined that further rulemaking is necessary, and this proposed AD and a separate proposed rulemaking action (Docket No. FAA-2024-2142) that would supersede AD 2020-26-16 follows from that determination. Similar to AD 2020-26-16, this proposed AD is also considered to be an interim action that would determine the need for additional actions in the fleet addressed currently. The FAA evaluated the inspection reports submitted by operators as required by AD 2020-26-16 and determined that wing spars from additional Piper airplane models should be inspected.

Since the FAA issued AD 2020-26-16, the FAA has analyzed the accident history of the airplanes affected by AD 2020-26-16 and other Piper airplanes operated in a similar fashion. The following paragraphs communicate the FAA's findings on this subject.

Accident History

Fatigue cracking was present in the main wing spars of Piper Model PA-28-181, Model PA-28R-201, and Model PA-28-161 airplanes involved in the following accidents. The following NTSB reports are related to this issue and can be found on ntsb.gov.

• NTSB Accident Number FTW87FA088: March 30, 1987—Marlin, TX—Piper Model PA-28-181—7,490 hours time-in-service (TIS). This accident was determined to have been caused by fatigue cracking in the outboard bolt holes of the main wing spar. This airplane's primary usage was a “Pipeline Patrol” mission.
• NTSB Accident Number NYC93FA140: August 2, 1993—Provincetown, MA—Piper Model PA-28-181—11,683 hours TIS. This accident was determined to have been caused by structural overloading related to weather, but fatigue cracks were present near the outboard bolt holes. This airplane's usage history included personal use, flight instruction, and charter flights.
• NTSB Accident Number ERA18FA120: April 4, 2018—Daytona Beach, FL—Piper Model PA-28R-201—7,691 hours TIS. This accident was determined to have been caused by fatigue cracking in the outboard bolt holes of the main wing spar. This airplane's primary usage was flight instruction.

Bolt Hole Cracks and Other Findings

Following the release of AD 2020-26-16, the FAA and Piper received over 2,800 bolt-hole eddy current inspection reports. The inspections performed in the field revealed a mix of observations that warrant further discussion. Of the total inspections, over 100 reported a positive eddy current indication, with several including pictures of the bolt hole showing the source of the indication.

Piper later conducted more detailed inspections in a study of 24 main wing spars with 20 having positive eddy current indications. Out of the 20 positive indications, 3 were identified as fatigue cracks, where 1 was confirmed by Piper, and 2 were confirmed by the NTSB. The remaining were determined to be features not consistent with a crack, and 1 overstress crack as confirmed by the NTSB.

Though not all are confirmed, many of the indications are likely not fatigue cracks but are a variety of anomalies in the hole. These can include corrosion pitting, scratches, gouges, and threading marks possibly caused by forceful insertion and removal of the close-fit bolts without proper unloading of the wing or other reasons. While these may not present as fatigue cracks at the time of inspection, anomalies in the hole create a stress concentration where cracks can begin to grow. Therefore, it is still crucial to inspect the critical bolt holes for these issues and take corrective action to prevent the formation of fatigue cracks. Piper Service Bulletin No. 1345, Revision A, dated September 17, 2021 (Piper SB No. 1345, Revision A); and Piper Service Bulletin No. 1412, dated May 7, 2024 (Piper SB No. 1412), include procedures for distinguishing between indications caused by hole damage or other anomalies from those caused by cracks.

In addition to the various forms of non-crack hole damage, the inspections revealed several cracks in and around the bolt holes. As part of the AD 2020-26-16 inspection reports, 6 cracks were found, including 2 later verified by NTSB lab examination and 1 verified by Piper (from the Piper study referenced above), and 3 visible cracks in photos. Other known cracks include those found in an airplane in the same fleet as the 2018 accident airplane, a separately submitted crack finding confirmed with dye penetrant, and a crack located on the lower spar cap surface running alongside the inspection bolt holes. Given these findings, additional cracks may be present among the other unconfirmed reported indications.

Other cracks have been discovered that may be caused by overload rather than by fatigue. While use of the airplane within its limits should not cause an overload crack, some crack findings have revealed that airplanes have been operated outside their limits. Though cracks due to overload are not the primary source of this corrective action, this emphasizes the need for and importance of inspecting the spar bolt holes for evidence of any cracking.

Long-Term Continued Operational Safety

The AD 2020-26-16 inspection report results indicated that additional inspections are needed to manage the safety of the fleet. Data indicates that more airplanes will need to be inspected, including the need to expand inspections to Piper airplane models that share a similar structural design of the main wing spar beyond the models addressed in AD 2020-26-16.

Crack development is a function of many factors, including the design of the structure, how severely the aircraft is flown, and manufacturing processes. Small imperfections may exist in any aircraft structure from an early age; however, through operation, these imperfections may slowly grow into fatigue cracks. Fatigue cracks have the effect of weakening the structure and its ability to support the stresses the airplane was originally designed to handle.

The 2018 accident, along with other accidents in this fleet attributed to fatigue cracking, and the AD 2020-26-16 inspection reports, indicate an aging fleet that requires intervention to ensure any fatigue cracking does not reach a critical state prior to being detected.

Ensuring further damage is not caused by an inspection itself is important; however, inspecting for fatigue cracks as well as other hole anomalies is critical and outweighs the risk associated with doing the inspections. Piper has developed service actions, most recently in Piper SB No. 1345, Revision A; and Piper SB No. 1412, that mitigate inspection-induced damage by emphasizing proper unloading of the wing for both bolt and wing removal and replacement, if necessary, along with other instructions for ensuring care of the bolt holes.

Corrective Action Development

Each requirement outlined in this proposed AD has been developed to both address the unsafe condition and limit the number of required inspections, reducing the burden on operators where possible. A brief discussion of each aspect of the requirements continues below.

Airplane Model Grouping

The inspection data received via the reporting requirement in AD 2020-26-16, along with testing of the baseline spar common to all Piper Model PA-28 and PA-32 airplanes, has shown that inspections should be extended to include Piper airplane models that share similar structural design but were not included in the applicability of AD 2020-26-16. It is likely that a significant contributing factor in the formation of cracks found in the main wing spar bolt attachment area is the cold bending of the spar to achieve the wing's dihedral. This method of forming the spar dihedral combined with the proximity to the wing attachment bolt holes leads to high residual stress in that area. The potential for fatigue cracking in and around the bolt holes, as well as higher variability in crack location and severity, is higher under this constant additional stress.

In an attempt to support less onerous inspections and to understand the causal factors, Piper investigated the residual stresses in the critical bolt-hole area. That investigation showed that the residual stress due to the spar cold bending process is a significant contributing factor in reducing the fatigue life of the spar bolt holes. An additional outcome of this investigation is a change to all new manufactured spars having machined dihedral bends to eliminate the residual stresses in the critical area.

Though there are differences between all Model PA-28 and PA-32 airplanes, such as additional reinforcing structure and lower operational loads, all airplane models share this same baseline spar with the cold bent dihedral. Differing characteristics allow for a grouping and tailoring of the requirements for each airplane model, but all airplane models need to be inspected. The airplane models in the applicability of this proposed AD are not the same airplane models that are included in the applicability of the proposed rulemaking action (Docket No. FAA-2024-2142) that would supersede AD 2020-26-16, and the proposed required actions are different between these two proposed rulemaking actions.

The remaining Piper Model PA-28 and PA-32 airplanes that would not be included in the applicability of this proposed AD either experience higher operational loads or have less structure. Both of these conditions increase the stress experienced in the subject bolt holes of the baseline spar and thus are subject to the proposed rulemaking action (Docket No. FAA-2024-2142) to supersede AD 2020-26-16.

Determination of Inspection Compliance Time

The proposed compliance time for the eddy-current inspection specified in this proposed AD was based on an inspection report received in response to AD 2020-26-16 that showed a crack indication in a Model PA-32-300 wing spar, later verified by Piper as a crack. Some airplanes in the proposed applicability of this AD may have been inspected as part of the requirements of AD 2020-26-16; however, if cracks in the wing spar are not expected as early due to the structural differences discussed above, these inspections may not yield the intended insight into the state of the wing spars. Therefore, the current proposed compliance time was set near and prior to the time-in-service of this confirmed crack finding in a wing spar of the same population as those in the airplanes in the applicability of this proposed AD.

Wing spars on the affected Piper airplanes could develop cracks that, if not addressed, would result in a wing separating from the fuselage in flight.

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.

Material Incorporated by Reference Under 1 CFR Part 51

The FAA reviewed Piper Service Bulletin 1412, dated May 7, 2024. This material specifies procedures for doing a one-time eddy current inspection of the lower wing spar bolt holes for crack(s) and replacing any cracked main wing spar. This material also includes instructions to report the results of the inspection to Piper. 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.

Proposed AD Requirements in This NPRM

This proposed AD would require reviewing airplane maintenance records to determine if an eddy current inspection of the lower main wing spar bolt holes was done and depending on the result, doing an eddy current inspection of the lower wing spar for crack(s) if not previously done or if done prior to 12,000 hours TIS, and replacing any cracked main wing spar. This proposed AD would also require sending all inspection results to Piper and the FAA.

Differences Between This Proposed AD and the Referenced Material

Piper SB 1412 specifies to contact Piper for disposition if any non-crack damage is found in the main wing spar bolt holes or any crack(s) or non-crack damage is found in the spar box bolt holes but this proposed AD would require contacting either the Manager, East Certification Branch, FAA, or the Piper Organization Designation Authorization (ODA) for instructions and doing those actions. To be approved, the repair method, modification deviation, or alteration deviation must meet the certification basis of the airplane, and the approval must specifically refer to the proposed AD.

Piper SB 1412 specifies using its feedback form to report the inspection results but this proposed AD would require using the form included as Appendix 1 to this proposed AD.

Interim Action

The FAA considers that this proposed AD would be an interim action. The proposed inspection reports would provide the FAA with additional data for determining the number of cracks present in the fleet. After analyzing the data, the FAA may take further rulemaking action.

Costs of Compliance

The FAA estimates that this AD, if adopted as proposed, would affect 10,927 airplanes of U.S. registry.

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