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 ADDRESSES . Include “Docket No. FAA-2024-2142; Project Identifier AD-2024-00033-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 amend the proposal because of those comments.

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Confidential Business Information

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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 FSH 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 follows from that determination. The FAA evaluated the inspection reports submitted by operators as required by AD 2020-26-16 and determined that additional action is needed, including requiring repetitive inspections of the lower main wing spar bolt holes for crack(s) and non-crack damage and replacement or modification of the main wing spar, using CSH instead of FSH to determine times for required actions, and revising the applicability by removing certain serial-numbered Piper Model PA-32-300 airplanes and all Model PA-32R-300, PA-32RT-300, and PA-32RT-300T airplanes because those airplanes would be included in the applicability of a proposed separate rulemaking action. The FAA also determined that Piper Model PA-32S-300 airplanes should be added to the applicability.

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. 1372, dated April 3, 2024 (Piper SB No. 1372), 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 of the same operator 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. While AD 2020-26-16 addressed the immediate safety concern, data indicates that more airplanes will need to be inspected and, due to aging, the airplanes already inspected will need additional inspections. This includes the need to expand inspections to include Piper Model PA-32S-300 airplanes in the applicability of this proposed AD because these airplanes share a similar structural design of the main wing spar with the airplane 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 aircraft 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. This often takes the form of repetitive inspections to be able to capture the formation of a detectable crack, requiring repair or replacement. The FAA has also determined that inspections alone are not sufficient to keep the fleet risk acceptably low long-term. Cracks are more likely to develop with aging of the main wing spar, so over time it becomes more likely that cracks will exist throughout the fleet and could be missed by inspection, due in part to the inherent imperfections of the inspection method; therefore, replacement or modification of the spars is needed. Both the FAA and Piper attempted to determine an inspection program that would manage risk to an acceptable level using inspection alone; however, no method could be found that did not eventually require spar replacement.

Ensuring further damage is not caused by the inspection itself is important, especially with repetitive inspections; however, inspecting for fatigue cracks as well as other hole anomalies is critical and outweighs the risk associated with repetitive inspections. Additionally, repeated inspections inherently allow for continued direct observation of the bolt holes over time and correcting non-crack damage if necessary. Piper has developed service actions, most recently in Piper SB No. 1345, Revision A, and Piper SB No. 1372, 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 all models that share a similar structural design by utilizing the same baseline spar. 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 current proposed requirements separate models into two groups, each requiring its own actions with a separate action for a third group under a separate proposed rulemaking action. The airplane models discussed share similar spar structure, while one group experiences higher operational loads than the other, due mostly to differences in gross weight and maximum airspeed. The remaining Piper Model PA-28 and PA-32 airplanes that would not be included in the applicability of this proposed AD either experience lower operational loads or have additional structure, both effectively lowering the stress experienced in the subject bolt holes of the baseline spar.

Hours Calculation

The potential for fatigue cracking of the main wing spar bolt holes is highly influenced by the usage profile of the spar and airplane. For airplanes included in the applicability of this proposed AD, the primary usage is either flight instruction or personal use. Flight instruction consists of more hours spent flying at lower altitudes where the airplane is exposed to gusts and more takeoffs and landings than is typical for personal usage. These characteristics lead to reaching the fatigue life of the spar more quickly than personal usage, which generally involves a large time spent in cruise and fewer takeoffs and landings. As a result of the usage differences, the same TIS for an airplane used in flight instruction is not equivalent to the same time for a personal-use airplane.

An FSH equation was established in AD 2020-26-16 to account for the differences in usage and reduce the burden on personal use airplanes. This equation is still the best method available to account for different usage when determining when inspections should occur. In the United States, airplanes used in flight instruction for hire are required by FAA regulation to receive 100-hour inspections. Because instructional usage is unable to be tracked directly, a count of these inspections is used as a method for determining instructional usage to differentiate it from personal usage.

When AD 2020-26-16 was issued, available analysis indicated instructional usage was approximately 17 times more severe than personal usage, leading to this factor appearing in the FSH equation. After AD 2020-26-16 was published, further analysis completed by Piper estimated this factor was significantly lower. This has resulted in a new equation, now referred to as CSH in Piper SB No. 1372. The CSH calculation has the same instructions and intent as the FSH from AD 2020-26-16, but the value in the equation has been updated based on improved analysis and data. For Group 1 airplanes this proposed AD would require using a value of 3 instead of the value of 2 that is specified in Piper SB No. 1372, and a value of 2 would be used for Group 2 airplanes, as specified in Piper SB No. 1372.

Determination of Inspection and Spar Retirement or Modification Timing

The fleet corrective actions analyzed consist of an initial inspection, followed by repetitive inspections until a time is reached when the spar should be modified or replaced. The FAA has worked with Piper to develop the specific timing for these actions using actual service data to determine current and future risk of fatigue cracks developing, and analysis using the physical properties of the structure to estimate formation and growth of cracks in the critical area of the spar. These efforts have resulted in inspection timing estimates that provide opportunities to locate cracks before reaching a critical size.

The proposed initial inspection time requirements for Group 1 and Group 2 airplanes have been determined using data from AD 2020-26-16 inspection reports and prior inspections consisting of known crack findings and known inspections that did not find an anomaly that exceeds the eddy current size threshold, to estimate times when a crack will exist in each group's fleet. An initial inspection time is then set just prior to when cracks would be expected in the fleet. This proposed initial inspection serves as a baseline inspection and will be followed by repetitive inspections to ensure a crack is not missed.

The proposed repetitive inspection programs have been set differently for each airplane group. For Group 1 airplanes, a tiered approach of inspecting more often with increasing age has been proposed as a method of reducing the burden on lower TIS airplanes. It is possible to inspect less used airplanes less often because these carry the least risk of developing a fatigue crack within the population. As age increases, and therefore risk increases, the proposed inspections are set to be more frequent. A similar approach would be possible for Group 2 airplanes, but the inspection intervals are estimated to be much shorter based on inspection data, structural design, and airplane performance, so stepping down with age would not be practical.

As an airplane ages, the likelihood or risk of a crack developing increases. A potential solution to this would be to continue to increase the frequency of inspections to ensure a crack is found before reaching a critical length; however, frequent and increased inspections are not a practical or safe approach due to the inherent risk in repeated bolt removal and reinsertion. At the fleet level, an ever-increasing age of the fleet means a higher risk that cracks are present in the fleet and may be missed, even with frequent inspections. This leaves a solution of removing or modifying the highest age spars to reduce and maintain the fleet risk, therefore spar retirement or modification where possible is proposed.

These proposed fleet inspection requirements must be adjusted as needed to ensure cracks are found before they reach a critical size. A recent inspection of a Model PA-28-181 airplane main wing spar was performed at a time earlier than required by AD 2020-26-16 and revealed cracks at a TIS earlier than anticipated. A section of the main wing spar was sent to an independent materials lab, and under high magnification normally spaced fatigue striations were found, confirming this crack was caused by fatigue. This new finding required the FAA to adjust the Group 1 inspection schedules to earlier times to ensure a similar crack would be found in time.

Although this airplane was used almost exclusively for flight instruction, reducing the inspection times resulted in additional burden on all airplanes, including those for personal usage. Therefore, for this proposed AD, the FAA determined that for Group 1 airplanes a value of 3 should be used in the CSH instead of the value of 2 that is specified in Piper SB No. 1372. An increase in this value from 2 to 3 serves to mitigate the increased burden on Group 1 airplanes resulting from the reduced inspection times and provides relief for operators of personal-use airplanes.

Spar Modification and Replacement Options

Piper has developed a reinforcement kit as an option and alternative to retirement of the main wing spar, applicable to certain Group 1 airplanes. There is currently no spar modification developed for Piper Model PA-28R-180 and PA-28R-200 airplanes that are included in Group 1 or any airplanes designated as Group 2 airplanes, as specified in Piper SB No. 1372. The proposed inspection times after installing a reinforcement kit would provide an extended life of the main wing spar and longer intervals between the proposed repetitive inspections. The reinforcement kit provides additional structure that lowers and distributes the stress in the critical areas, allowing for slower crack growth. Piper used damage tolerance analysis when developing the inspection times after a reinforcement kit is installed.

Finally, new wing spars are available from Piper that have machined the spar dihedral bend instead of the cold bending process, eliminating the residual stress factor in these spars. These new wing spars have a different life limit and will not require any inspections.

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 SB No. 1372. This material specifies procedures for reviewing airplane maintenance records to determine the number of 100-hour inspections completed on the airplane since new and any record of main wing spar replacement; calculating the service hours; doing eddy current inspections of the main wing spar bolt holes for crack(s) and non-crack damage; repairing bolt holes with non-cracking damage; for certain airplanes installing a main wing spar reinforcement kit; and replacing a main wing spar.

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 ADDRESSES.

Proposed AD Requirements in This NPRM

This proposed AD would retain none of the requirements of AD 2020-26-16. This proposed AD would revise the applicability by removing certain serial-numbered Model PA-32-300 airplanes and all Piper Model PA-32R-300, PA-32RT-300, and PA-32RT-300T airplanes because those airplanes will be included in a separate proposed rulemaking action. This proposed AD would add serial-numbered Model PA-28R-200 and PA-28R-201 airplanes to the applicability to include all serial numbers and would also add Piper Model PA-32S-300 airplanes to the applicability. This proposed AD would require accomplishing the actions specified in the material already described, except as discussed under “Differences Between this AD and the Referenced Material.” This proposed AD would also require reporting inspection results to Piper and the FAA if any cracks are found during any inspection.

Differences Between This Proposed AD and the Referenced Material

For Group 1 airplanes, to determine the CSH, instead of using the value of 2 provided in the simplified formula in Part 1, paragraph 2.b of the Instructions in Piper SB No. 1372, this proposed AD would require using a value of 3.

In addition, for Group 1 airplanes, the compliance times for the initial and repetitive inspections and the replacement or modification of the main wing spars specified in paragraphs (i) and (j) of this proposed AD are different from what is in Table 1 of Piper SB No. 1372.

Costs of Compliance

The FAA estimates that this AD, if adopted as proposed, would affect 10,665 airplanes of U.S. registry.

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