Attached

Avenger Aircraft and Services’ Comments Docket: FAA-2007-27862 Directorate Identifier: 2007-CE-036-AD Comments Organized by AD Section Discussion – FAA Analysis Avenger Aircraft and Services is in full support of the FAA proposed AD on the Thrush spar cap and fully understands the risk assessment approach chosen by the FAA to determine both inspection and compliance times. Avenger however would like to have the FAA further clarify the basis of the determination and the limitations. As stated, the FAA utilized a risk analysis method to determine the life limitations and on Page 20432 of the Federal Registry it states certain limitations due to the approach. However, there are a few other significant issues not specifically mentioned in the NPRM for which Avenger would like to know if they were accounted for. 1) First, due to the material characteristics of the spar cap, the fatigue life is highly susceptible to environmental conditions. As presented, there is no indication that the risk assessment took the detrimental effect of environmental conditions into account which could further reduce the life limitations. Can the FAA please verify if this was accounted for in the determination of the life limitations? 2) Secondly, there is no indication that the risk assessment took the actual flaw sizes from the fleet data into account. The life could be much shorter depending on a risk assessment which takes crack sizes into account since residual strength requirements must be met. This is particularly significant when some fleet crack sizes might have exceeded the critical size without failing due to the aircraft not exceeding limit load at that particular time. Can the FAA verify if the life limitation assessment included an evaluation of the fleet data in terms of crack size? Discussion – Long Term Continued Operational Safety Avenger agrees that ultrasonic inspections should be removed from the current AD as they are not an efficient means to find the fatigue cracks because of the configuration of the structural joint. Avenger would propose that even magnetic particle inspections should not be utilized as a valid inspection method and should also be removed from the AD. This is because the size of the flaw that can be dependably found with this inspection method is far in excess of the flaw size that would be capable of sustaining limit load, which is a FAR requirement. Based upon data in Reference NTIAC-DB-97-02, the 1 in 10 chance of missing a flaw (90% Probability of Detection) for this type of inspection is a flaw size of 0.69 inches. This inspection technique should not be permitted as it would allow damage that would be too large and could lead to catastrophic loss of the wing, especially considering the operating environment of agricultural or fire fighting usage. With regards to the bolt hole eddy current inspection procedure, any NDT procedure utilized in ensuring safety thru inspection must utilize a method which provides the highest degree of reliability and repeatability. The current standard practice for all NDT inspection procedures utilized in support of airworthiness limitations for damage tolerance certified aircraft is to require calibration standards. These calibration standards are designed to replicate the structure being inspected with simulated flaws and are used every time as a set up tool by the inspector prior to conducting the on-aircraft inspection. Utilization of these standards is the current practice by all major aircraft manufacturers and should be required for the Thrush inspections in order to ensure a 90% probability of detection. In addition, the inspectors should be fully certified Level 2 NDT personnel with bolt hole eddy current qualifications. FAA’s Determination and Requirements of the Proposed AD In this section, the FAA has recommended that “big butterfly” and lower splice plates or Thrush Custom Kit CK-AG-41 Rev. A be installed as replacement parts for their increase in strength and durability. However, a solution that was not available at the time of the authoring of the AD is currently now on the market. We would respectfully request that the following information be included. This solution is the Avenger Aircraft and Services Supplemental Type Certificate SA03654AT Avenger Extended Performance Front Spar Enhancement Kit. Avenger’s FAA STC replacement kit parts are as follows: 2 Lower Spar Caps (made from Stainless Steel, not 4000 Series Steel) 2 Front Spar Web Doublers 1 Large Butterfly (redesigned) 2 Larger Splice Blocks (redesigned) All associated hardware for installation The FAA has approved this configuration for a life limit of 40,000 hours. The installation of these parts greatly enhances the safety of the aircraft far in excess of any other solution on the market and Avenger requests that the AD be amended in this section to include a recommendation for the installation of the Avenger Extended Performance (AXP) kit as terminating action to this AD. Cost of Compliance We concur generally with the costs provided by the FAA within the AD for the Thrush Custom Kit CK-AG-41 Rev. A at approximately $64,000 per airplane (installed). We respectfully request that the following economic data be included in the Cost of Compliance section. Our FAA STC replacement kit parts are $40,000 for the parts listed above. The cost of the installation (if accomplished by Avenger) is another $25,500 for a total of $65,500. However, this installation removes the involvement with this AD because it provides an alternate part number to the ones affected by this AD, whereas the CK-AG-41 kit simply re-starts the clock on the existing AD since part numbers 20207-15 and -16 are still subject of this AD. It does not eliminate the safety issue at all (Reference AD Section Titled “FAA Analysis”). The Avenger AXP kit is designed for all aircraft operating under TCDS A4SW (with the exception of the S2D model). Therefore, all aircraft under this AD, with the exception of Model (14) in Table 1, would be covered by Avenger’s kit. Avenger recommends that this section be amended to include the costs provided above for the AXP kit as an alternative to the Thrush Custom Kit CK-AG-41 Rev. A. Part 39 – Airwothiness Directives – Compliance (Table 6) Avenger requests that the FAA explain the basis of the compliance times associated with Table 6 and the total aircraft times that are permitted to exist before action is required. Part 39 – Airwothiness Directives – Compliance (Part (k)(1)) Under Part (k)(1) of the Compliance Section, the FAA AD, the FAA accepts that if a crack is found in the spar cap during inspection, the cold work process can be utilized to ream out small cracks as defined in Ayres Corporation Service Bulletin No. SB-AG-39 dated 9/17/96. Avenger submits that this process is not thorough enough and should be amended based upon the following discussion: While coldworking (or mandrel expansion) of holes is generally accepted within the aerospace industry as a means of extending the fatigue life of affected parts, it is a process that must be undertaken with caution and prudence as to not negatively affect the part being worked. Through detailed discussions with owners and operators of this aircraft, it is Avenger’s opinion that the coldworking process accomplished as part of the Bulletin is not being conducted correctly and fatigue damage is being introduced and made more critical than if coldworking was not accomplished at all. This is because in order to utilize mandrel expansion in a safe manner, the hole in question must first be reamed to remove any corrosion or existing cracks that are too small to be detected. This “insurance cut” is required to remove any anomaly in the hole that may cause an issue during the coldworking process. Following the insurance cut, a Bolt Hole Eddy Current Inspection of the hole must be performed using the calibration standards as outlined in the Avenger comments to Long Term Continued Operational Safety in order to ensure that the hole is cleaned up properly. At present, this insurance cut is currently not being accomplished. Holes are being mandrel expanded with no initial clean up of the hole. Therefore, if a small flaw does exist in the hole, mandrel expansion will most likely increase the size of the crack through the process itself. This is supported by service history that indicates that coldworked holes are being found with cracks, some as few as 1000 hours after coldworking. Avenger recommends that this section be amended to explicitly state that prior to mandrel expansion, an insurance ream, capable of cleaning up a .03 undetected crack followed by a bolt hole eddy current inspection using a calibration standard, be accomplished prior to the mandrel expansion process.