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Advocacy Advocacy Briefs Regulatory Brief -- Proposed airworthiness directive calls for wing removal and spar cap inspections on various model Lake airplanes -- AOPA comments

Regulatory Brief -- Proposed airworthiness directive calls for wing removal and spar cap inspections on various model Lake airplanes -- AOPA comments

December 6, 1999

Federal Aviation Administration
Central Region, Office of the Regional Counsel
Attn: Rules Docket No. 99-CE-27-AD
Room 1558, 601 E. 12 th St.
Kansas City, MO 64106

Gentlemen,

The Aircraft Owners and Pilots Association (AOPA), on behalf of over 355,000 pilots and aircraft owners, submits the following comments to NPRM 99-CE-27-AD. The NPRM proposes to implement an airworthiness directive (AD) requiring removal of the wings of various model Lake aircraft in order to inspect the wing spar caps and doublers for cracks. AOPA is concerned that the inspection provisions of the proposed AD place a substantial financial burden upon owners of affected aircraft, and may ultimately pose a greater safety risk than the airworthiness concern they were designed to detect.

Due to the complex technical nature of this issue, AOPA has relied heavily upon the research efforts of the Seaplane Pilots Association (SPA) to enlist the technical knowledge and expertise of numerous engineers, mechanics, and aircraft owners and operators intimately familiar with Lake aircraft. This research included hands-on inspections using the inspection procedure proposed in the NPRM (as outlined in REVO service bulletin B-79) and borescope inspections. This research effort is a shining example of how the real world knowledge and expertise of aircraft type clubs, owner�s organizations, mechanics, and aircraft owners and operators can produce practical and affordable solutions to legitimate airworthiness concerns. AOPA thanks SPA, and all other individuals involved, for their efforts to provide a clearer picture of the issues surrounding this proposed AD.

Research Findings:

The research conducted by the GA community has produced the following conclusions:

  1. Cracks in the wing spar cap angle do indeed occur in a significant portion of Lake LA-4-200 and Lake Model 250 aircraft.
  2. Cracks are nearly twice as common and are typically more severe in Lake Model 250 aircraft than in LA-4-200 aircraft with comparable total time in service.
  3. No inspection data is known to exist for LA-4, LA-4A, or LA-4P models.

    Note: For a complete listing of inspection data and information concerning the method in which it was gathered, see Seaplane Pilots Association�s comments to Docket No. 99-CE-27-AD dated 12-10-99.

  4. The cracks in the wing spar cap angle appear, upon inspection, to be the result of a twisting moment applied to the wing. During braking, and perhaps during beaching and taxiing operations, an aft force is applied to the main landing gear. This force is translated to a twisting moment on the wing. The wing spar cap angle cutout radius (near the second bolt hole outboard from the wing root) is a significant concentration point of twisting stress in the wing and is thus the location where one would expect cracks to develop. Inspection of wing spar doublers that have been in contact with cracked spar cap angles reveal imprinting on the wing spar doubler that further confirms that a twisting moment is being applied to the wing at the point of failure.
  5. Based on the conclusion that the wing spar cap angle cracks are due primarily to a twisting moment applied during braking, we believe that the LA-4, LA-4A and LA-4P models are likely to show fewer and less severe cracks than the LA-4-200 and Lake Model 250. The LA-4-200 has a lower gross weight and weaker brakes than the Lake Model 250. Therefore, the wings of LA-4-200s are subjected to much lower twisting moments than the Lake Model 250. The LA-4 models have weaker brakes and lower gross weights than both the LA-4-200 and Lake Model 250. Consequently, we would expect the Lake Model 250 to have the most frequent and most severe incidence of cracking, followed by the LA-4-200. This expectation is verified by inspection data included in Appendix 1 of SPA�s comments to this docket. Extending this theory to compensate for the lack of inspection data for the LA-4, LA-4A and LA-4P, we would expect the LA-4, LA-4A, and LA-4P to have the least frequent and least severe incidence of cracking.
  6. Only one aircraft has been found to have cracks at any location other than at the spar cap angle cutout radius. That aircraft was undergoing repair for significant wing damage, and no data has been presented to show that the cracks were not the result of accident damage. The crack at the spar cap angle radius of this aircraft on the extensively damaged spar did not extend all the way to the second outboard bolt hole from the wing root. There is no apparent connection between the cracks near the sixth outboard bolt hole and the cracks at the wing spar cap angle cutout radius. Nor are we aware of any reasoning whatsoever that might explain why a crack would develop at the sixth outboard bolt hole, where there is not a significant stress concentration feature in the structure other than the bolt hole itself. Thus, it has not been shown that a single occurrence of cracking in a damaged aircraft at locations other than the spar cap angle cutout radius indicates that this condition is likely to exist or develop in other REVO models.
  7. The proposed inspection procedure appears to pose a greater threat to the structural integrity of the wing spar than the cracks that the inspection procedure is intended to detect. We believe that the FAA action requiring that the wing spar doubler (a primary structural component of the wing) be bent to its elastic limit (see Appendix 2 of SPA�s comments to this docket) will result in an unsafe condition. This inspection procedure was developed to inspect for corrosion, which the FAA has determined not to be a factor, and to inspect for cracks at locations other than the spar cap angle cutout radius, which per finding 6 above is not an issue worthy of widespread inspection.
  8. The proposed inspection procedure is difficult to perform, excessively expensive, burdensome to owners, irrelevant, unnecessary, and largely ineffective. Removing the wings is time consuming, requires special knowledge of the aircraft that fewer than half a dozen repair facilities in the United States possess, and may create a situation in which wings are damaged, destroyed, or improperly installed, thus creating an unsafe situation. The unreasonably short compliance time, combined with a very limited supply of mechanics experienced with the affected models, further exacerbates the problem. The working space generated by bending the spar doubler away from the spar cap angle (in itself a dangerous proposition) is not sufficient for an effective visual inspection, with or without dye penetrant. When SPA performed the proposed inspection with Harry Shannon, one of the most experienced Lake mechanics in the United States, the dye penetrant inspection had to be performed three times to detect a crack that we knew existed. In a normal inspection situation, the crack would not have been detected. The objective of the inspection appears to be two-fold: (1) detect and report cracks at locations other than the spar cap angle radius; and (2) detect and report cracks at the spar cap angle radius. Objective (1) is unnecessary if cracks are unlikely to occur at locations other than the spar cap angle radius, as suggested in paragraph 6 above. Objective (2) is irrelevant. Regardless of whether cracks are found at the spar cap angle radius, the repair kit must be installed. There is no question that cracks are a widespread problem, so there is no need for the FAA to acquire further data on this crack.
  9. When preparing to install the doubler kit per REVO, Inc. Service Bulletin B-79, (or perform the inspection, if the FAA determines that such inspection is warranted), we recommend that the rivet removal procedure be altered slightly. The exposed, bucked ends of the rivets are not centered over their respective underlying holes. Drilling these rivets with a #21 drill bit (the same size as the underlying hole) is likely to damage the wing spar doubler, perhaps to the extent that it must be replaced at a very high cost to the owner. A more reasonable procedure would be to shear the bucked end of the rivet, drill with a smaller #30 drill bit, and then punch the rivet out.
  10. Due to the number of sandwiched parts, rivets may not be the most appropriate hardware choice for reassembling the wing spar, as set forth in REVO, Inc. Service Bulletin B-79. Bolts would seem to be a more reasonable hardware choice, the exact specification of which should be determined by the FAA or REVO, Inc.
  11. There is currently no distinction between the wing attachment bolts to be used for reassembly of Lake Model 250 and REVO Models LA-4, LA-4A, LA-4P, and LA-4-200. Since the wing attach bolts are not at issue, owners of LA-4, LA-4A, LA-4P, and LA-4-200 should not be required to upgrade to higher strength bolts used in the Lake Model 250.

Recommendations:

For the reasons cited above, AOPA recommends that the FAA modify the proposed AD to include the following provisions:

  1. Require that the REVO, Inc. B-79 Kit be installed on all Lake Model 250�s and LA-4-200�s in accordance with the schedule below:
    1. For the Lake Model 250, within 100 hours time in service or twelve calendar months from the effective date of the Airworthiness Directive, whichever comes first. AOPA maintains that the time in service of affected aircraft is irrelevant. While the available data certainly indicates that cracks exist, these cracks have existed for years without causing a single structural failure. There are few shops in the United States that regularly engage in service on REVO Lake aircraft. The proposed compliance schedule of this AD will inevitably result in extremely high workloads at the few maintenance facilities that specialize in Lake aircraft. Ultimately, this will lead to extensive periods of aircraft downtime for affected aircraft owners. Allowing owners an extra 50 hours time in service will prevent unnecessary grounding of these actively utilized aircraft.
    2. For all LA-4-200 models, within 300 hours time in service or 36 calendar months from the effective date of the Airworthiness Directive, whichever comes first. This recommendation is based on the available data, operational history, and practical considerations. Inspection data shows that LA-4-200�s display half as many cracks as Lake Model 250�s. Further, cracks in the LA-4-200�s are not as well developed as cracks in the Lake Model 250�s. Although LA-4-200�s have been in service much longer than Lake Model 250�s, they do not have a history of wing failure of which we are aware. Thus, there is not an immediate threat to the operational safety of LA-4-200.

    Additionally, there are several times as many LA-4-200 models on the US registry as Lake Model 250. To require that these aircraft be repaired in the same time frame as the Lake Model 250 would completely overwhelm the handful of qualified shops, inevitably leading to extensive periods of aircraft downtime for affected aircraft owners.

  2. Remove the LA-4, LA-4A and LA-4P models from the final AD

    There is simply no hard data to support the conclusion that an unsafe condition exists in these models. Extrapolated inspection data suggests that while a few cracks may exist, these cracks are not likely to be well developed. Consequently, it is highly unlikely that these cracks will compromise the structural integrity or airworthiness of these comparatively lower weight, lower horsepower aircraft. For these reasons, AOPA recommends that these aircraft be removed from the AD.

  3. Eliminate the entire inspection procedure as proposed in the NPRM. For the reasons set forth in section 8 above, the inspection procedure is irrelevant, burdensome, and is likely to create an unsafe condition in and of itself.
  4. Require that any wing removed from any LA-4, LA-4A, LA-4P, LA-4-200, or Lake Model 250 be inspected for cracks in the wing spar cap angle and wing spar doubler from the wing root to the first rib, or to the inboard fuel tank for aircraft thus equipped, prior to being reinstalled. Require that the results of such inspections be reported to the FAA. Carefully consider whether the information to be reported can be reported accurately. For example, information about utilization may be roughly accurate for the present owner, but probably does not accurately represent prior owners� utilization of an aircraft. Require reporting of the damage history and the reason for wing removal for each wing inspected.
  5. Incorporate findings 10, 11, and 12 (above) into the final Airworthiness Directive. These relatively minor changes have the potential to significantly improve safety while reducing the cost of the repairs.

Conclusion:

AOPA maintains that the proposed inspection provisions of this AD will cause extensive inspection/repair backlogs, extended aircraft downtime, and may ultimately cause more harm than good. Further, AOPA maintains that the lower horsepower, lower gross weight LA-4, LA-4A, and LA-4P model aircraft are not likely to develop cracks that would compromise airframe integrity. Accordingly, AOPA recommends that these aircraft be excluded from the provisions of this AD (excluding the provisions proposed in recommendations section 4 above).

For the reasons cited above, AOPA recommends that the FAA eliminate the proposed inspection requirements of this AD and implement the revised AD compliance schedule recommended above.

Thank you for your time and consideration in this matter. AOPA stands ready to assist the FAA in reconsidering the proposed provisions of this AD.

Respectfully,

Dennis E. Roberts
Vice President/Executive Director
Government and Technical Affairs