Airworthiness Directives; LAVIAARGENTINA S.A. Airplanes

As more modern aircraft and drones have become available for agricultural purposes, Piper Pawnees have largely discontinued serving in that role. Currently, agricultural spraying uses a mix of traditional fixed-wing "ag planes" (like Air Tractor, Thrush Commander), helicopters, and specialized multirotor drones like DJI Agras, XAG, Hylio.2 There are 465 currently registered Pawnees on the FAA registry. By SSA’s count, 143 Piper Pawnees are in use for glider towing. A small number of Pawnees are still manufactured by the Argentinian firm, LAVIA ARGENTINA S.A. (“LAVIASA”). That firm purchased production rights from Piper in 1988. A search of the FAA Aircraft registry disclosed only a single LAVIASA Pawnee registered in the U.S. LAVIASA’s service bulletins, in conjunction with three South American accidents discussed below, led to an AD by the Argentine aviation authority, Administración Nacional de Aviación Civil (“ANAC”). That AD has given rise to the proposed FAA AD.3 As we explain, the ANAC AD was not well thought-out, imposes damaging repair-and-replace requirements for48 leading- edge screws in the front spar, and was not supported or documented by any fatigue or stress analysis. The FAA should not adopt it. The Role of Pawnees in Glider Towing As Pawnees were phased out of agricultural spray operations, many were purchased by American soaring clubs and commercial glider operators, reconditioned, and put into use in glider towing. Based on information provided by SSA clubs and commercial glider schools, currently 143 Piper Pawnee PA-25 aircraft are in regular use for glider towing, out of the 465 currently registered with the FAA.4 It is not known how many of the other 322 Pawnees are in current use for agricultural or other purposes. SSA believes the number still in agricultural use is small given the widespread adoption of newer spray technology and equipment described above. The Pawnee has an excellent power-to-weight ratio resulting in very good climb rates when used to tow gliders. Agricultural spray equipment has been removed from these Pawnees, or the aircraft are operated with empty spray storage containers and without spray equipment. This results in typical empty weights of approximately 1,500 to 1,550 pounds, and operating weights of 2 “The most commonly used ag planes today are manufactured by Air Tractor (AT) and Thrush.” See https://agaviationadventures.com/media/lets-talk-ag-planes, and https://advexure.com/blogs/news/spray-drones-vs-traditional-crop-spraying-which-is-more-effective. 3 LAVIASA’s website shows its version of the Pawnee equipped for agricultural spray operations. See http://laviaargentina.com/aeronaves. 4 See Statement of Kenneth Sorenson, attached as Exhibit G. 4 approximately 1,900 to 1,970 pounds, depending on pilot weight. This is about 1,000 pounds below typical fully-loaded Pawnees in agricultural use. Typical glider towing operations do not involve high-G maneuvers, which significantly reduces stress. Pawnees used in glider towing are flown at a speed of 70-75 knots when towing a glider, and at 80-120 knots after glider release while returning to land. Glider-tow operations significantly reduce stress and fatigue on the aircraft, and especially on wing spars. Most Pawnees used for glider towing in the United States operate on a seasonal basis, from March or April to September or October. They are typically stored in a club’s hanger, are flown 50 to 150 hours per year, and receive high-quality maintenance. An additional 45 Piper Pawnees are used for glider towing in Australia. The Gliding Federation of Australia, Inc. (“GFA”) has published an analysis by Anthony Smith entitled PA-25 Pawnee Front Spar Fatigue Analysis. It describes the reduced fatigue stress incurred during glider towing operations as compared to agricultural spray and dusting applications. See Exhibit A. An additional GFA report, entitled PA-25 Pawnee Front Spar Stress Analysis, is attached as Exhibit B. These reports calculate and document the greatly reduced stress from glider towing. They apply well-recognized stress and fatigue analyses to provide standards for evaluating the need for inspections and the standards for remediation when issues are identified. The following graphic shows the results of a fatigue analysis of Pawnees by Australian aviation stress and fatigue expert Anthony Smith. The SSA is grateful for Mr. Smith’s assistance in preparing this letter.5 5 Mr. Smith is a former RAAF engineering officer, and a structures engineer for the RAAF. His experience with the Australian Air Force includes aircraft fatigue crack analysis and growth. He also has worked for Airbus Australia Pacific in addressing fatigue life issues in large aircraft. He serves as a volunteer with Gliding Australia. A copy of his resume is attached as Exhibit C. He is available to discuss his methodology and conclusions with the FAA. 5 The data upon which this analysis is based is described in Mr. Smith’s report, Exhibit B.6 2. The three South American Pawnee accidents that led to the ANAC AD, and to the FAA’s Proposed AD, have no relevance to U.S. Pawnee usage for glider towing The ANAC AD,7 refers to and requires compliance with two LAVIASA Service Bulletins, 25-57-09 and 25-57-11. The AD states that it addresses “corrosion found in front and rear spar of left and right wings and cracks found in front spar.” (page 1). The ANAC AD was preceded by, and appears to be based upon, three South American Pawnee crashes described in ANAC Warning 292/DAG R2, dated February 26, 2024, Exhibit D. (The “Warning”). None of the aircraft involved in those crashes warrant the requirements of the ANAC AD that the FAA now proposes to apply to U.S. Piper Pawnees used for glider towing. Rather, the corrosion in the front spars involved in the South American accidents was obvious to visual examination, as pointed out in the statement of SSA’s expert Ronald Ridenour. Although the aircraft also has additional or oversize holes in the front spar, the obvious corrosion should have grounded the aircraft, according to Mr. Ridenour. In his accompanying Statement, Exhibit E, he states that the photos in the reports confirm that the accidents were due to poor inspection and maintenance practices, as reflected by the presence of visible corrosion.8 The first accident discussed in the Warning, which occurred on September 27, 2023, involved a front-spar. A photo (Fig. 1) included in the report (page 4) shows visible corrosion: 6 The chart in the text is based upon operation of the Pawnee as a towplane, which is typically conducted at less than 2,205 pounds compared to 2,900 pounds for agricultural use. Mr Smith’s report notes: • Not carrying the payload in the fuselage reduces the weight of non-flying parts significantly. • Peak limit stress at 2,900 lb (agricultural use) for B/C model is 25.966 ksi at the wing root lower spar cap at 3.8 g at 150 kts / 172 mph (VD). (See Exhibit B, page 7 for flight envelope and page 46 of stress analysis). • Peak limit stress at 2,205 lb (aerotow use) for B/C model is 20.601 ksi at the wing root lower spar cap at 114 kts / 131 mph and 27.1 ft/sec gust. (See Exhibit B, page 37 of Mr. Smith’s report for flight envelope and page 56 of his stress analysis). • Limit stress is reduced by 20.7% due to lower operating weight. 7 The ANAC AD, DA RA No. 2024-05-01 R1, is dated December 18, 2024. 8 Mr. Ridenour received a degree in Aeronautical Engineering from Purdue University. He is a long- time flight instructor and FAA designated pilot examiner. He also holds A&P authority with Inspection Authorization. He has many years of experience in flying and maintaining Piper Pawnee aircraft used in glider tow operations, as well as aviation experience as a pilot and line check airman for a major airline. Exhibit E ¶¶ 2-3. 6 Fig. 1 The second accident, in Columbia (for which no date was provided), was, according to ANAC, caused by “fatigue cracks [that] began inside the [spar] hole in areas with the presence of corrosion and plastic deformations.”. Warning, page 2. The accompany photo (Fig. 2) shows obvious corrosion identifiable by eyesight alone: Fig. 2 The third accident occurred in 2018. Again, the crash-site photos (Fig. 3) show visible corrosion that should have grounded the aircraft. See Statement of Ronald Ridenour, Exhibit E, ¶¶ 5-6. 7 Fig. 3 These photos should be compared with the interior wing photos of a 1969 Piper Pawnee in current service at the Chicago Glider Club. (Figs. 4 and 5 below.) Figure 4 shows the Pawnee wing when exposed in 2021 for installation of new fabric wing covers.9 In comparison to the South American crash aircraft the difference is startling, and shows that Pawnees subject to normal American inspections and maintenance do not remotely resemble the aircraft that gave rise the ANAC AD. Fig. 4 Note that the interior of the then fifty-two-year-old wing is corrosion free and in good condition.10 The leading-edge metal sheet is located on the left side of the photo, as is the front spar flange to which it attaches. The aircraft logbooks reflect no modifications to or rebuilding of the wing. The following photo (Fig. 5), taken in December 2025, after the wing was recovered, shows in greater detail the point at which the leading-edge metal sheet attaches to the front spar flange, viewed from inside the wing, looking at the back side of the spar surface. It illustrates the good condition of the screws and spar at that location after fifty-six years of service. Access to insert 9 Statement of Donald Kroesch, Exhibit F ¶ 5. 10 The wing is a part of a Piper Pawnee serial number is 25-4937, built in 1969. The aircraft logbooks reflect 6,400 hours of flight time, of which 3,640 were in agricultural use and 2,760 in glider towing. Statement of Donald Kroesch, Exhibit F ¶ 6. 8 the camera was gained using an access port on the bottom of the wing. Thus, this photo also confirms the available access of the back side of the spar-sheet attachment for visual inspection. Fig. 5 It is important to note the inspection protocols that initial ANAC recommendation from the three crashes discussed in its February 26, 2024 Warning differ significantly from the ANAC AD and the proposed FAA AD: It did not recommend eddy current testing, only a borescope inspection for corrosion and remedial action based on that inspection. Its Warning , Exhibit D, page 6, stated: RECOMMENDATIONS: . . . . [I]t is recommended that the owners of the aircraft to which this Warning applies, as soon as possible, . . ., carry out a detailed inspection of the wing spars, with the following premises: 1- Access is gained to the front spars, in an area covering 50cm to the right and 50cm to the left, of the P/N 61282-00 FITTING JURY STRUT (ITEM 48 OF FIGURE 5 OF THE IPC) strut attachment hardware, which is located approximately 700 mm from the wing attachment hole to the fuselage, either by uncovering this section of the wings or through inspection covers. 9 2- Using a BOROSCOPE, inspect the UPPER CAPS of both spars for: a. Fasteners incorrectly positioned as indicated in BACKGROUND 7. b. Holes overlapping transversely in the beams, as can be seen in the 1st photograph of BACKGROUND 6. c. Overlapping holes, enlarging the original holes, as can be seen in the 3rd photograph of BACKGROUND 6. d. Cracks starting from one of the holes present in these areas. 3- If any of the defects indicated above are found, immediate replacement of the spar is recommended. ANAC’s AD does not explain why ANAC changed the recommended inspection from borescope to eddy current. A recent Australian “Additional Hole Design Report” (part of an AMOC approving certain added spar holes) suggests that it was a convenient, off-the-cuff and very conservative response: The AD 2024-05-01, Ref [2], introduces an eddy current inspection to be conducted at a frequency of every annual inspection or every 100 hours, whichever occurs first. It seems unlikely that the type certificate holder has adopted a fatigue safety- by-inspection scheme using a calculated critical crack growth rate based on the detectable flaw size for eddy current surface scan. It is more probable that eddy current surface scan was identified as a convenient and effective method of crack detection and the frequency of every 100 flight hours was identified as being very conservative. Exhibit H, October 19, 2025 Report, page 10. There is nothing to suggest that when ANAC added the eddy current inspection requirement it considered the alternative of using a 3 mm borescope, or gave any attention to the destructive aspects of the required screw removal and replacement, or to the resulting operator costs. 3. How SSA believes the proposed AD should be improved The proposed AD requires almost immediate inspections and repair of front and rear Pawnee wing spars, and other steps, including recurring eddy current inspections on an annual or 100-hour TIS basis, whichever occurs later. For aircraft that have had repairs to the front spar that added holes or expanded the size of existing holes the proposed AD would require automatic replacement of the spars, with no consideration of the actual effect of additional holes on spar integrity. The proposed AD requires two types of inspection. 10 Paragraph (g) requires inspection of the front and rear wing spars for cracks and corrosion in accordance with LAVISA Service Bulletin 25-57-09. For airplanes that have had wing spar alterations, this inspection must be accomplished within 10 hours-time-in service (TIS) after the effective date of the AD. For other aircraft, the inspection must be performed before exceeding 5 years TIS, or within 10 hours of the effective date of the AD, whichever occurs later. Nearly all of the 143 Pawnees used for glider towing in the United States have more than five years and more than 500 hours TIS. As a result, the 10-hour inspection requirement will apply to most Pawnees used for glider towing in the United States. The proposed AD adds that even if no corrosion is found by this inspection, if “evidence of overlapping, enlarged holes, or more than four holes [are] found and at least two of them are less than one inch away from each other,” then the inspection required by paragraph (h) must also be performed. The SSA recommends that the compliance time in paragraph (g) allowed between the effective date of the AD and the initial inspection for all Pawnees (i, ii, or iii) be increased from the proposed 10-hour TIS limit to at least 50 hours TIS. This would allow operators of glider- towing Pawnee a more reasonable time to acquire any necessary inspection equipment and install any needed inspections. Given the large number of Pawnees affected by the proposed AD and the limited number of parts suppliers, there will likely be significant delays obtaining the parts for the special inspection/access panels. Again, as noted elsewhere in this response, there is no airworthiness emergency with Piper Pawnee spars in the United States, especially for Pawnees used for glider towing. The SSA agrees with the visual nature of the inspection, as required by LAVIASA Service Bulletin 25-57-09, which did not require or recommend an eddy current inspection procedure. SSA also agrees that if cracks are discovered by the visual inspection, whether by eyesight or with the borescope, repair or replacement of components is the proper remedy. The requirement to undertake eddy current inspection was added by the Argentine aviation authority (ANAC). Significantly, it was added without explanation or supporting data analysis. SSA strongly objects to that added requirement as unnecessary, expensive and destructive of wing structure, because of the need to remove and replace 48 thread-forming sheet metal screws for each inspection. SSA’s detailed objections to required eddy current inspection is discussed below in the section entitled “Objection to Eddy Current Inspection Requirement.” The SSA recommends that all inspections be visual, with use of a borescope when relevant areas are not otherwise visible. Paragraph (h) of the proposed AD requires the inspection of the upper and lower spar flange of the front wing spar for cracks, evidence of overlapping, enlarged holes, or more than four holes, of which at least two are less than one inch away from another hole. This inspection is to be performed in accordance with LAVIASA Service Bulleting 25-57-11. It is to be performed at various times depending upon the age of the aircraft. Since all but one Pawnee used for glider towing in the United States are more than forty years old, the effective time for the required inspection under paragraph (h) is within 50 hours TIS of the effective date of the AD. Paragraph (h) adds that even if no cracks are found by the wing spar inspection required by paragraph (g), an eddy current inspection is, nonetheless required under paragraph (h) of the upper and lower spar flange of the front wing spar. For airplanes with front spars more than 40 years old (applicable to nearly all U.S. glider-towing Pawnees), this inspection must be done before further flight. It must 11 be repeated thereafter at intervals not to exceed 100 hours TIS or 12 months, whichever is later. If any cracks are found during the inspection, repair or replacement of the wing spar is required. The SSA recommends that the compliance time in paragraph (h) allowed between the effective date of the AD and the initial inspection for all Pawnees (i or ii) be increased from the proposed 50-hour TIS limit to at least 100 hours TIS. Given the large number of Pawnees affected by this proposed AD and the limited number of parts suppliers, there will likely be significant delays obtaining the parts for the LAVIASA-required special inspection/access panels. With regard to what actions to take if screw holes are discovered, as described in Paragraph (g), based on the fatigue analysis by Mr. Smith, the Australian expert, SSA’s recommends that unauthorized or oversize screw holes be evaluated individually for the presence of any associated cracks. If there are no cracks, the holes should not automatically be treated as a cause for spar replacement, so long as the holes meet the standards described in the section below entitled “Proposed Revised Hole Inspection Criteria.” SSA recommends improving and revising Paragraph (h) by eliminating the requirement to remove and replace, at every required inspection, all 48 thread-forming screws that hold the metal leading edge sheet to the front spar flange. The Pawnee wing design provides for 15 fabric covered ribs between the outboard end of the wing and the fuselage. Eight of the rib bays each have eight thread-forming sheet metal screws to hold the metal sheet to the spar flange. The following cross- section view of the leading edge of the Piper Pawnee wing (Fig. 6) shows the spar cap and specifies the eight attaching screws connecting the metal leading edge sheet to the spar.11 Fig. 6 SSA opposes the remove-and-replace requirement, except in one location where leading- edge removal is required because neither a 3mm borescope nor eddy current probes can access the area. Wholesale removal of screws for other inspections is unnecessary and will do more harm 11 Figure 6 is from Piper Aircraft Corp. Wing Assembly Drawing #61093 revised as of March 28, 1958. 12 than good, as more fully discussed under in the section below entitled “Objection to Eddy Current Inspection Requirement and Removal of Sheet Metal Screws.” The one area where removal of screws is unavoidable is in one rib bay of each wing where a doubler has been installed on the aft face of the front spar web immediately under the strut attachment. The doubler prevents access to the inside face of the lower spar cap flange for inspection by the 3 mm borescope or by the probes used in eddy current inspection. As a result, removal of 4 skin attachment screws on each wing spar under the strut attachment is necessary to perform the inspection in this area. SSA also recommends against requiring any eddy current inspection. Instead, it recommends substituting use of a 3 mm borescope for the 5 mm borescope specified by SB 25- 57-11 and using that borescope to conduct the inspection called for by the service bulletin, except as noted in the preceding text concerning the area of the doubler. This non-destructive borescope method is far more practical and capable of implementation, and much less expensive, than the repetitive eddy current inspection specified in the ANAC AD. SSA has committed to purchasing and providing the required 3 mm borescope in sufficient quantity to meet inspection needs of Pawnee owners engaged in glider towing. SSA recommends increasing the inspection interval from 100 hours to 1,000 hours or more for Pawnees that only engage in glider towing and are limited to 2,205 pounds MTOW. EASA introduced an AD for inspection of Pawnee spars based on the ANAC AD, except that for Pawnees under 2,205 MTW it altered the inspection interval to 1,000 hours or 4 years, whichever occurred first – as SSA recommends. Mr. Smith’s Front Spar Fatigue Analysis supports the 1,000 hour or greater inspection interval due to the reduced fatigue stress incurred during glider towing operations. See Figure 9 and accompanying discussion. 4. Objections to Eddy Current Inspection Requirement SB No. 25-57-11 requires the visual inspection of the front spar leading edge attach points for integrity and for visual cracks, the presence of additional holes in the spar cap flange (that is, holes in addition to the original four holes per each of the spar bays called for by the Piper design). It also calls for inspection for oversize screws that were used in place of the #4 thread-forming screw. SSA agrees with the required visual inspections for all these elements using the 3 mm borescope, but does not agree with the requirement for screw removal, as discussed below. SSA objects to the requirement of an eddy current inspection for each of these separate reasons: (i) The inspection protocol requires repeated removing and replacing approximately 48 sheet metal screws at each inspection, which is likely to generate damage to perfectly good spars, while visual inspection by borescope will detect corrosion and cracks before they have grown to dangerous size. (ii) Visual inspection, including by 3 mm borescope, is much less costly and much more practical because it does not require transporting qualified Level II or Level III eddy current technicians hundreds of miles to perform the inspection. 13 (iii) Eddy current inspections will be difficult since SB 25-57-11 calls for eddy current tests over much of the length of the front spar. It will be difficult and time consuming due to the need to locate and relocate the probes at many sites over the course of a single examination. (iv) The eddy current inspection protocol requires adding six new inspection panels on each wing, costing an additional $1,600 for the panel inspection kit, plus considerable labor to install. In addition, the eddy current inspection probes are unable to access the doubler area on each wing in one rib bay, as discussed above. We address each issue in turn. (i) The inspection is likely to cause damage to perfectly good spars. The reoccurring inspections require removing and replacing 48 thread-forming screws every 100 hours or annually, whichever occurs last. This mandates a repetitive, destructive inspection process that will threaten spar integrity by stripping, cross-threading, creation of burrs, or other conditions that will eventually result in the required replacement of the currently compliant spars, due to the destructive nature of the inspections. This conclusion is supported by every A&P and IA with whom SSA has consulted, including Mr. Ridenour and Mr. Kroesch. See Statements of Ridenour, Exhibit E ¶¶ 7-8, and Kroesch, Exhibit F ¶¶ 8-11. Both have repaired and refinished Pawnee wings (as shown in Figs. 4 and 5 above) and each is familiar with their construction. Non-destructive visual inspection by borescope examination should be compared to the repeated remove-and-replace procedure required for eddy current inspection of the spar. In his Statement attached as Exhibit F, Mr. Kroesch states that the removal of sheet metal screws that have been in place for five or six decades will be difficult and time consuming. It is likely that some will break, requiring that they be drilled out and likely destroying the existing thread in the spar. Repeated removal and replacement, he states, is likely to result in damage to the spar holes when visual inspection by borescope of both sides of the spar-sheet interface is possible, as shown by Figures 8 and 9 below. See Kroesch Statement ¶¶ 8-10. Figures 7 and 8 below show images from inspection of a Pawnee wing taken from inside the front spar by inserting a 3 mm borescope in the channel allowing access to the interior. They illustrate the access that is possible with the 3 mm borescope without the destructive removal and reinstallation of the spar-flange attachment screws, as required by SB 25-57-11. Their detail and clarity confirm the ability to detect any cracks that might compromise the structural integrity of the aircraft – including cracks that are far smaller than the size required to generate a spar failure. The size of such cracks is discussed below. If the FAA agrees with SSA’s recommendation to substitute 3 mm borescope inspection for eddy current inspection, SSA has committed to purchasing a sufficient number of 3 mm borescopes to be loaned to technicians performing visual inspections on Pawnees used for glider towing. 14 Fig. 7 15 Fig. 8 The visible detail shown in Figures 7 and 8 is clearly sufficient to display cracks, were any present. Kroesch Statement, Exhibit F, ¶ 12; Ridenour Statement, Exhibit E, ¶10. In that regard, it is important to note the calculated length of a crack that would be needed to cause spar failure. Mr. Smith has provided the following information to summarize his detailed fatigue and crack- propagation computations (Fig. 9). 16 Fig. 9 As stated in Figure 9, a spar crack would need to be 0.8752 inches in length to generate spar failure in an aircraft limited to 2.205 pounds, increasing to 0.9355 inches for aircraft with wing tanks, also limited to 2,205 pounds. The 3 mm borescope provides more than adequate means to identify cracks requiring replacement of the wing spar, well before the cracks reach a size large enough to pose a significant risk of spar failure. As noted in Figure 9, Mr. Smith’s crack length calculations are based on a maximum aircraft weight of 2,205 pounds. The SSA would support the issuance of an AD that adopts SSA’s proposed borescope inspections and also states that such procedures would only apply to Piper PA-25 Pawnee aircraft operating at no greater than 2,205 pounds maximum takeoff weight. That would require a change in the allowable MTOW. It would not adversely impact Pawnees used for glider towing. (ii) Visual inspections are much less costly and difficult to arrange than eddy current inspections. Currently, mechanic or repair station hourly rates range from approximately $100 to $200, not $85 as stated in the NPRM. The estimated cost of the eddy current inspection is not addressed in the NPRM. Contacts with possible American providers of this service have generated estimates of between $1,200 and $4,500, plus travel expenses, depending upon the location and availability of providers. The proposed AD states that such inspections may be recurring every 100 hours or 12 months, whichever occurs later. In determining the real-world cost of such inspections, the cost to prepare the wing for inspection must be included. The inspection requires accessing the spars through inspection holes on the underside of the wing and removing 48 screws temporarily, in order to remove the leading-edge tab from the spars. By contrast, a visual inspection for cracks can be accomplished by inspecting the front spar by use of the 3 mm borescope. The presence of a Level II or III eddy current specialist is not required. These specialists are often difficult to find and schedule; they are likely to be located far from the aircraft to be inspected. 17 Visual inspections can be done by local A&P mechanics, at hourly rates far below those for Level II and Level III eddy current technicians. SSA clubs have received the following expense estimates for eddy current inspections of Pawnees: Scott Gifford (Hood River, OR) reported that one eddy current technician’s quote (including travel time) was $3,600 per aircraft. That cost is to perform the actual eddy current test, not to gain access to the spar (removing the screws and wedging the leading-edge skin away). Another possible provider told Mr. Gifford that it lacked the required equipment for such a small market and deemed it uneconomical to purchase the equipment. Chris Klix (Puget Sound Soaring Assoc) reported receiving an estimate for an eddy current inspection on its PA25-260 D model of $4,500, another estimate of $1,167, and a third of $3,220. Rick Hoffmann (Saint Louis Soaring Association) reported receiving estimates of $1,500-$2,000, plus travel expenses. (iii). Eddy current inspections will be difficult. The ANAC AD calls for eddy current tests over much of the front spar. Accomplishing such inspections will be difficult and time consuming because the technician who operates the equipment is not an aircraft mechanic and will, therefore, need the services of a qualified mechanic to provide access to the test sites within the wings. The upper and lower spar flange areas must be tested. AD, page 5. Technicians must be qualified at Level II or III. AD, page 6. Specified probes and equipment must be used. AD, page 6. The leading edge of the spar caps must be free and open; screws must be removed. AD, page 7. The area to be tested must be cleaned of dirt, grease, oil or other contaminants. AD, page 7. Two scans at each location are required “over the entire length and between adjacent ribs”. AD, page 9. “The actual scanning of the areas to be inspected. . . . should cover 80 cm (31.49 in) . . .” AD, page 12. In summary, the preceding discussion explains why the ANAC AD eddy current inspection procedures do not make sense for American Pawnees in glider tow operation. Nor does the inspection intervals the proposed AD would impose. Defining inspection intervals in years is illogical because airplanes do not suffer stress or fatigue loads due to the passage of time sitting in hangers. Time in service hours is the relevant measure. All operating U.S. Pawnees are already receiving 100 hour or annual inspections to comply with FAA regulations. If the FAA adopts a new AD to address Pawnee wing spars, SSA requests that it be phased into the existing 100 hour or annual inspection schedule, to be performed when the next of those inspections would normally occur. Only a visual inspection using the 3 mm borescope should be required, except for the area of the spar doubler, where a borescope may not be needed. If the aircraft passes that inspection, a repeat borescope inspection for spar integrity should be required every 1,000 hours TIS, or a greater number of hours set by the FAA’s AD. 18 5. Proposed Revised Hole Inspection Criteria SSA urges revision of the requirement of the proposed AD that the presence of non- conforming or extra screws attaching the leading-edge sheet to the spar flange automatically requires spar replacement. Based on the fatigue analysis by the Anthony Smith, the Australian expert, unauthorized or oversize screw holes should be evaluated based on size and location, not automatically require spar replacement if the holes do not warrant that result under computed standards. Mr. Smith’s analysis shows that extra holes are acceptable provide that (i) they have an adequate angle and spacing from other holes – namely are located at least .25 inches (center to center) to the left or right of a vertical line through each hole; (ii) do not contact and damage the inside of the edge of the cap, and (iii) do not damage the flange bulb. The Australian aviation authority (CASA) has approved individual using these standards for Pawnee wings. See Exhibits H and I The following drawing (Fig. 11), provided by Mr. Smith, shows this standard visually. Fig. 11 Mr. Smith explains that the stress concentration between two holes in a plate under tension (i.e., the spar) is defined by the distance and angle between the holes. In the configuration above the stress concentration between the two holes is the same as the stress concentration between the hole closest to the flange edge and the edge of the flange. As a result, the fatigue life of either stress concentration will be identical. Any pair of holes at the same horizontal distance but closer vertically will have a shallower angle and be more benign in terms of stress and fatigue generation. Any pair of holes at the same vertical separation but further apart horizontally will also have a shallower angle as well as being further apart and be more benign. 19 Mr. Smith’s stress analysis also establishes that replacement of the original #4 sheet metal screws with the next size larger (which is a #6 screw) does not adversely impact spar strength and failure rates. As designed and built, the ribs are attached to the spar flange by screws on both the front and rear faces of the upper and lower spar flange. At each rib location, the front-face screw is at the same span-wise location as the rear-face screw, such that the cross-sectional area of spar flange material lost due to the screw holes is equal to two #4 screw thread diameters. The lost area for each 0.112 inch diameter #4 screw hole is 0.00985 inch2 The cross-sectional area lost at each span-wise location equals 0.0197 inch2 for the two holes. A single #6 screw diameter of 0.138 inch results in a lost cross-sectional area of 0.0150 inch2. Thus, replacing a single #4 screw with a single #6 screw results in a loss of cross-sectional area that is only 76% of the area lost by two #4 screws at the same span-wise location. Thus, there is no engineering basis for rejecting a spar because a single #4 screw used to attach the leading edge to the spar flange has been replaced by a #6 screw. . 6. The proposed AD would impose devastating costs on small soaring clubs and commercial glider schools that rely on glider tows for training As noted above, Piper Pawnees are overwhelmingly the aircraft of choice for glider towing in the United States. They represent 74% of all glider tow aircraft, according to a recent SSA survey of soaring clubs and commercial operations. Additional relevant information regarding the U.S. fleet of glider tow-only Pawnees is included in the Sorenson Statement, Exhibit G. The NPRM significantly underestimates the cost of compliance with the proposed AD, both for conducting required eddy current inspections (as discussed above) and for remediation if wing spar replacement is required. As noted, the hourly rate for repair station personnel as stated by the NPRM of $85 is far below U.S. levels today. Actual rates range from $100 to $165 per hour, with East and West Coast rates well above that range. The eddy current inspection costs consist of two categories: One-time costs to install six inspection panels per wing, and repeated inspection costs thereafter. The one-time cost to purchase the inspection panels and reenforcing materials from Laviasa is currently $1,600 per aircraft. The estimated time to install the materials is 16 hours (according to the estimate of Messrs. Kroesch and Ridenour in their Statements). Applying a $125 hour rate, results in labor costs of $2,000 to install the panels, plus the cost of the inspection panel kits. The resulting one-time total cost for the installation of the panels is estimated at $3,600. Next to be considered is the cost of performing each individual inspection: A. Eddy Current Technician and Equipment ($1,200-$4,500) - assume $2,000 B. Travel and hotel to the aircraft to be tested – assume $1,000 C. Cost to open the wing, remove screws and provide access for the eddy current equipment – will depend on how many screws come out without difficulty. At 10 hours per wing (20 x $125) $2,500 Total: $5,500 20 This is a reasonable estimate of the cost of a single inspection. Some technician costs and expenses will be greater and some less. Clearly the cost to open the wing and remove 48 screws that have been in place for five or six decades is hard to predict and may vary by airplane. But it is obvious that the task is not simple or quick. At a minimum, careful lubrication and use of penetrating oil on each screw will be necessary to avoid breaking screws or the spar to which they are attached. The inspection cost of $5,500 is for a single annual or 100-hour spar inspection. For a single Pawnee operated 100 hours per year, the cost of inspection would be $55 per hour – far in excess of all the other hourly operating costs (insurance, engine reserve, fuel). Imposing that level or expense when a much less expensive alternative is available makes no sense. It illustrates the need for another inspection method and schedule. If the wing spars must be replaced, the costs will also be far greater than the estimates contained in the NPRM. An Australian repair station reported labor hours of 110 hours per wing in the jig to change the spars and carry out some basic rib repairs. An additional 120 hours were required per wing for fabric and paint. Two days labor were required for the wing’s removal and installation/rigging before the aircraft was returned to service. The actual cost of obtaining replacement spars (if available) from LAVIASA or another supplier, Univair, is currently $6,500 for one set (one wing’s) front and rear spars. Thus, the cost to replace the spars on a single Pawnee is as follows: A. Cost of two set of front and rear spars, as of November 2025: $13,000 B. Labor cost of 330 hours x $125 per hour $41,250 Total $54,250 That estimate does not include shipping costs or consider price increases as demand mounts. Nor does it take account of delays due to parts availability limitations, or lost revenue while the aircraft is out of operation, at least for several months, and possibly longer. SSA soaring clubs are modest-sized organizations. Membership numbers range upward from 20 members. A few larger clubs have 80-150 members. None exceed 200 members. All are operated on a non-profit basis. Most are exempt from federal taxation under Section 501(c)(3) of the federal tax laws. Most SSA soaring clubs will be unable to meet the financial burden of complying with the AD if wing spar replacement is required, without curtailing operations or selling other aircraft or gliders used for training. Commercial operators of Pawnees tend to be small enterprises, also unable to afford to comply with the inspection and spar replacement costs. Nor are there readily available substitute aircraft at reasonable prices. If there were, they would already be in use. As of the date of drafting this letter, an on-line search found only one LAVIASA substitute Pawnee, a 2024 “Puelche II,” listed for sale. The asking price was $379,125. But the on-line posting stated that the aircraft was no longer available.12 12 See https://www.aviatorsmarket.com/detail/aircraft-for-sale/2024-piper-pa-25-260-puelche- ii/7385 (last viewed on December 11, 2025). 21 SSA does not support sacrificing safety to avoid expensive inspections when necessary. But before an expensive, repetitive inspection regime is ordered for a fleet of aircraft with no U.S. accident record due to Piper wing spar failures, SSA urges the FAA to rethink the proposed procedures and frequency requirements and adopt those that are equally effective and less costly. 7. The Proposed AD fails to address a few Pawnees with metal wings One other issue should be noted: The proposed AD does not take account of, or otherwise address, STC SA501SW, which allowed replacement of the Piper fabric-covered wings with an all-metal wing commonly referred to as a Hutcherson Wing. In the all-metal wing there is no “leading edge” attached to the spar. The ribs and wing-skin stand-offs are riveted and not screwed to the spar cap. There have been no problems with screw holes associated with wing-skin or leading-edge attachment to the spar. The proposed eddy current inspection procedure would require completely unriveting the spar from the wing, which serves no purpose as there is no direct attachment of the leading edge to the spar cap flange. A separate letter of comment by Kenneth Sorenson has been submitted concerning application of the proposed AD to the Hutcherson wing. The SSA agrees with that letter, which explains that clarification is needed regarding the applicability of any proposed Piper-wing AD to the Hutcherson wing. CONCLUSION The ANAC AD appears to be an Argentine answer to inadequate South American inspection and maintenance practices. The photos in Figures 1 - 3 clearly evidence such inadequate practices for the aircraft that crashed, which generated the ANAC AD. The U.S. Piper-built Pawnee PA-25 fleet has been operating for many decades without a single reported wing spar failure. That fact supports a different approach to inspection requirements for U.S. Pawnees used for glider towing. The FAA should avoid rote adoption of the ANAC AD where there is no need for it, where it goes far beyond the current certificate holder’s own inspection standards, and where the consequences for American soaring would be dire. The SSA recommends the following: • Limit Pawnees used only in glider towing operations to 2,205 pounds MTOW. • Provide for visual inspections using a 3mm borescope for corrosion, additional or oversize holes, and cracks, instead of eddy current inspections. • Only require spar replacement for non-original screw numbers or size where necessary, applying standards described above. • Increase the allowed compliance time between the effective date of the proposed AD and the initial inspection for all glider-towing Pawnees from the proposed limits of 10 and 50 hours TIS for paragraph (g) and (h), respectively, to 50 and 100 hours TIS, respectively.