The Federal Aviation Administration (FAA) recently issued two new NPRMs (Notices of Proposed Rule Making), one on Sept. 19, 2024 (Docket No. FAA-2024-2143; Project Identifier AD-2024-00008-A), and the second on Sept. 23, 2024 (Docket No. FAA-2024-2142; Project Identifier AD-2024-00033-A).
The two NPRMs are an expansion of AD 2020-26-16 that mandated inspections for cracks in the wing attachment bolt holes in the left and right wings of some single-engine Piper PA-28 and PA-32 aircraft.
NPRM 2024-2142 applicability
The most recently released NPRM (2024-2142) is an advance warning that a newly proposed AD will apply to all the PA-28 and PA-32 aircraft that were affected by AD 2020-26-16 (which would be superseded). NPRM 2142 will also:
…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.
The NPRM does contain one subtraction from the eligibility list (but keep reading; you’re not off the hook yet, PA-32 owners):
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.
The “separate rulemaking action” mentioned in the quote above is NPRM 2024-2143. The following 2024-2143 subheading gives applicable models for that NPRM.
NPRM 2024-2143 applicability
NPRM 2024-2143’s applicability includes the PA-32 models removed from NPRM 2024-2142 above, and numerous models not previously affected by AD 2020-26-16. The list for this NPRM includes the PA-28-140, PA-28-150, PA-28-160, PA-28-180, PA-28S-160, PA-28S-180, PA-28-236, PA-28-201T, PA-32-300, PA-32R-300, PA-32RT-300, PA-32RT-300T, PA-32-301FT, 3 PA-2-301XTC, PA-32R-301 (HP), PA-32R-301 (SP), PA-32R-301T, PA-32-301, and PA-32-301T airplanes.
With the exception of the PA-32-300 (which includes all serial numbers greater than 32-7940001), all serial numbers for all the listed models are to be included in the AD proposed by NPRM 2024-2143.
Findings following inspection reports after AD 2020-26-16
Both NPRMs describe the history of inspections under AD 2020-26-16:
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.
No information was included on the type of usage or the airframe total time in service of the airplanes with the confirmed fatigue cracks.
But this small number of confirmed cracks was not the only problem discovered.
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.
The NPRMs expands on this thread with the following verbiage:
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.
After discussing the details of bolt hole cracks discovered to date, the NPRMs also note the possibility of overload cracks:
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.
Evidence causes FAA to propose bolt hole inspections for the entire PA-28 and PA-32 Fleet
The NPRMs explain the reasoning behind expansion of the inspection requirements:
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.
It was found that inspections that utilized non-destructive eddy current testing for cracks in two bolt holes in certain PA-28 and PA-32 models revealed more issues than expected. Therefore, the FAA is calling for a change in the formula used to determine when a PA-28 or PA-32 must be inspected, and also wants to require that all the PA-28 and PA-32 fleet must be inspected.
In the FAA’s search for factors that could increase the possibility of metal fatigue, an important production process was implicated.
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
This discovery is quite significant. The NPRMs continue with information from Piper:
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.
This discovery revealed that the manufacturing method at Piper, a technique that has been used for decades, contributed to the stresses in the target area of the inspections.
NPRM 2024-2142 required actions
The new determination formula
NPRM 2024-2142 explains how AD 2020-26-16 determined when a bolt hole eddy current inspection was required:
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 Factored Service Hours (FSH) equation. After AD 2020-26-16 was published, further analysis completed by Piper estimated this factor was significantly lower.
(Note that the original AD also included charter flying in the severe use category.—Ed.)
This new analysis is significant because it indicates that airplanes not used in charter or flight instruction are more susceptible to bolt-hole cracking and other wing spar material anomalies than was originally thought when AD 2020-26-16 became law.
NPRM 2142 assigns affected aircraft to Group 1 or Group 2. See the table below for applicability.
Determining Calculated Service Hours (CSH)
The CSH formula for Group 1 aircraft is as follows:
The CSH formula for Group 2 aircraft is:
Group 1 airplanes with unmodified spars must get the eddy current wing spar bolt hole inspection within 100 hours time in service (TIS) of reaching 3,000 total CSH.
The CSH trigger number for the eddy current inspection for Group 2 airplanes is 4,500 total CSH.
Repetitive inspections for Group 1 airplanes in Tier A (3,000 to 7,499 CSH) is every 1,750 CSH; Tier B (7,500 to 11,999 CSH) is every 700 CSH, Tier C (12,000 through 13,499 CSH) is every 500 CSH; Tier D (14,000 through 25,000 CSH) requires compliance with paragraph (j) of the AD, which requires spar replacement or installation of a spar reinforcement kit.
Repetitive inspections for Group 2 airplanes without modified wing spars are at 400 CSH for wings with 4,500 through 11,999 CSH. At 12,000 CSH, one must replace the spar or install the spar reinforcement kit.
The FAA has provided tables describing rquired actions for Group 1 and Group 2 aircraft as well.
Group 1:
Group 2:
Spar replacement and modification requirements
Table 4 of the proposed AD requires main wing spar modification or replacement prior to 14,000 CSH for Group 1 airplanes (unless they have Piper’s Wing Spar Reinforcement kit installed, in which case the main wing spars must be replaced at 25,000 CSH). The spars replacement interval for Group 2 aircraft is 12,000 CSH.
NPRM 2024-2143 required actions
As noted above, NPRM 2024-2143 will create a new AD, encompassing many additional PA-28 and PA-32 models.
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 FAA estimates that if the AD proposed in this NPRM becomes law, it would affect 10,927 airplanes in the U. S. registry.
The proposed AD would require a bolt-hole eddy current inspection on affected aircraft if: a) the airframe has accumulated 12,000 hours or more on the wing spar, or b) has missing or incomplete maintenance records (this matters, because if it can be determined that the eddy current inspection of the main spar was done prior to the effective date of the AD at 12,000-hour TIS or greater, and in accordance with Piper SB 1345 or 1345A, the owner or operator may take credit for the inspection).
The inspections must be completed within 100 hours after the main wing spar accumulates 12,000 hours time in service, whichever occurs later, or, for airplanes with unknown TIS hours on the main wing spar, within 100 hours TIS or 60 days after the effective date of the AD, whichever occurs later.
Like NPRM 2142, the aircraft maintenance records (logbooks) must be reviewed within 30 days after the effective date of the AD to determine wing spar total time (it’s possible that the spar was changed in the past) and whether the airplane was used in commercial operations. For NPRM calculations, commercial operations are indicated by the existence of 100-hour inspections in the logbooks.
Note that the aircraft models in NPRM 2024-2143 are not subject to spar life limits.
What is the eddy current inspection procedure?
The two NPRMs refer to the same inspection procedure. The FAA reviewed Piper Service Bulletin 1412, dated May 7, 2024. This bulletin 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 on how to report the results of the inspection to Piper.
Inspection costs and estimated cost of repairs
In NPRM 2024-2143, the FAA estimates the following costs:
1 hour for maintenance record (logbook) research
If any necessary actions are required based on the results of the records review, add the following:
1 hour (at $600 per hour) for the eddy current inspection of the left and right lower main wing spars, plus $20 parts cost
4 hours for access, bolt removal, bolt reinstallation, inspection, and closing up
1 hour to report inspection results
If necessary:
40 work hours to replace the main wing spar and $10,983 parts cost.
The same estimated costs are listed in 2142. 2142 also adds costs for repairing holes with non-crack damage (2 hours at $85 per hour for a total of $170, plus $20 parts cost), and for installing a modification/reinforcement kit (190 hours at $85 per hour for a total of $16,150, plus $4,000 parts cost).
Ferry permits
The NPRMs do permit the issuance of a ferry permit to move the aircraft to a location to be inspected. They do not permit the issuance of a ferry permit on an airplane to a maintenance facility after a crack has been detected, so it’s prudent to have the inspection done where remediation work can be performed if needed.
Inspection results reporting
The NPRMs require the inspection results be reported (by the inspecting mechanic) to the FAA (and Piper Aircraft) using a form that will be contained in the respective ADs, and within specific time constraints. The NPRMs wording follows:
Report the inspection results to the FAA, East Certification Branch, and to Piper Aircraft, Inc., using Appendix 1, “Inspection Results Form,” of this AD,
(1) If the action was done on or after the effective date of this AD, submit the report within 30 days after the action was done.
(2) If the action was done before the effective date of this AD, submit the report within 30 days after the effective date of this AD.
Comments may be submitted
The FAA invites comments on these two NPRMs, but with short deadlines. For NPRM 2143, Nov. 4 is the last day; for NPRM 2142, Nov. 7 is the deadline. Instructions on how to submit comments are included in the text of the NPRMs (see Resources for links). According to the FAA, “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.”
(Piper has produced a strong response against the proposed ADs, which we have linked in Resources. Owners of Piper aircraft affected by these proposals should read both the NPRMs and Piper’s opinion carefully. –Ed.)
I have submitted a comment suggesting that the comment period is too short and thus doesn’t give owners and operators who don’t subscribe to FAA NPRMs (or who get this information through Piper Flyer magazine) the opportunity to comment.
Text in both NPRMs states that these inspections are an interim action; no final action has been determined.
Steve Ells is contributing editor and tech rep for Piper Flyer Association. He has been an A&P/IA for 45-plus years. He is a commercial pilot with instrument and multiengine ratings and loves utility and bush-style airplanes and operations. He’s a former associate editor for AOPA Pilot. He owns Ells Aviation and lives in Templeton, California. Send questions and comments to editor@www.piperflyer.com.
RESOURCES
NPRM 2024-2142
NPRM 2142 (Docket FAA-2024-2142)
tinyurl.com/NPRM-2142
Piper’s Response to NPRM 2142
tinyurl.com/Pipers-Response-to-NPRM-2142
Public Comments on NPRM 2142
tinyurl.com/NPRM-2142-Comments
NPRM 2024-2143
NPRM 2143 (Docket FAA-2024-2143)
tinyurl.com/NPRM-2143
Public Comments on NPRM 2143
https://tinyurl.com/NPRM-2143-Comments
Piper Service Bulletins
Piper SB 1345A (Main Wing Spar Inspection)
Piper SB 1372 (Main Wing Spar Inspection)
Piper SB 1412 (Main Wing Spar Inspection)
Further Reading
AD 2020-26-16
www.piperflyer.com/media/k2/attachments/2020-26-16.pdf
