Discussion
What Events Have Caused This AD?
The FAA received reports of three occurrences of cracks found on the left hand upper longeron and upper diagonal support tubes where they intersect on the left hand side of the fuselage frame just forward of the vertical fin front spar attachment point on Air Tractor Model AT-602 airplanes. The crack starts at the forward edge of the weld where the tubes come together. We initially determined that the cracks resulted from high vertical tail loads during repeated hard turns. The cracks were found by the pilot and/or ground crew when they noticed excessive movement in the empennage due to the loss of torsional rigidity.
Air Tractor started installing extended reinforcement gussets on AT-402 and AT-802 series airplanes at the factory to alleviate the crack condition from occurring. The extended reinforcement gussets were intended to transfer the
loads away from the joint. However, further cracking has been reported on 3 more AT-602 airplanes, as well as 1 AT-402 series, and 3 AT-802 series airplanes. One of the AT-802 airplanes had the extended reinforcement gusset installed during factory production. Air Tractor discovered that the factory installed extended reinforcement gusset, which runs further forward than the original gusset, is also cracking at the forward end of the extended gusset.
What Is the Potential Impact if FAA Took No Action?
This condition, if not corrected, could cause the fuselage to fail. Such failure could result in loss of control of the airplane.
Has FAA Taken Any Action to This Point?
We issued a proposal to amend part 39 of the Federal Aviation Regulations (14 CFR part 39) to include an AD that would apply to certain Air Tractor Model AT-402, AT-402A, AT-402B, AT-602, AT-802, and AT-802A airplanes. This proposal was published in the Federal Register as a supplemental notice of proposed rulemaking (NPRM) on June 28, 2002 (67 FR 43568). The supplemental NPRM proposed to require you to repetitively inspect the upper longeron and upper diagonal tube on the left hand side of the aft fuselage structure for cracks and contact the manufacturer for a repair scheme if cracks are found.
Was the Public Invited to Comment?
The FAA encouraged interested persons to participate in the making of this amendment. We did not receive any comments on the proposed rule or on our determination of the cost to the public.
FAA's Determination
What Is FAA's Final Determination on This Issue?
After careful review of all available information related to the subject presented above, we have determined that air safety and the public interest require the adoption of the rule as proposed except for minor editorial corrections. We have determined that these minor corrections:
--provide the intent that was proposed in the NPRM for correcting the unsafe condition; and
--do not add any additional burden upon the public than was already proposed in the NPRM.
Is There a Modification I Can Incorporate Instead of Repetitively Inspecting the Left Hand Upper Longeron and Upper Diagonal Tube of the Fuselage Frame for Cracks?
The FAA has determined that long-term continued operational safety would be better assured by design changes that remove the source of the problem rather than by repetitive inspections or other special procedures. With this in mind, FAA will continue to work with Air Tractor in performing further tests to determine the cause of the cracking and to provide a corrective action, terminating the need for repetitive inspections.
Why Are Air Tractor AT-500 Series Airplanes Not Included in This AD?
The Air Tractor AT-500 series airplanes have a similar design in the upper longeron in the aft fuselage structure. However, we have not received any reports of damage in this area on those airplanes. The only reports of damage are those previously referenced on the AT-402 series airplanes, Model AT-602 airplanes, and AT-802 series airplanes.
Air Tractor is currently researching this subject on the AT-500 series airplanes. Based on this research and if justified, we may propose additional rulemaking on this subject for these other airplanes.
Cost Impact
How Many Airplanes Does This AD Impact?
We estimate that this AD affects 248 airplanes in the U.S. registry.
What Is the Cost Impact of This AD on Owners/Operators of the Affected Airplanes?
We estimate the following costs to accomplish the inspection(s):
Labor cost
Parts cost
Total cost per airplane
Total cost on U.S. operators
1 workhour x $60 = $60
No parts required
$60.
$60 X 248 = $14,880.
We have no method of determining the number of repairs or replacements each owner/operator would incur over the life of each of the affected airplanes based on the results of the proposed inspections. We have no way of determining the number of airplanes that may need such repair. The extent of damage may vary on each airplane.
Regulatory Impact
Does This AD Impact Various Entities?
The regulations adopted herein will not have a substantial direct effect on the States, on the relationship between the national government and the States, or on the distribution of power and responsibilities among the various levels of government. Therefore, it is determined that this final rule does not have federalism implications under Executive Order 13132.
Does This AD Involve a Significant Rule or Regulatory Action?
For the reasons discussed above, I certify that this action (1) is not a "significant regulatory action" under Executive Order 12866; (2) is not a "significant rule" under DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979); and (3) will not have a significant economic impact, positive or negative, on a substantial number of small entities under the criteria of the Regulatory Flexibility Act. A copy of the final evaluation prepared for this action is contained in the Rules Docket. A copy of it may be obtained by contacting the Rules Docket at the location provided under the caption ADDRESSES.
List of Subjects in 14 CFR Part 39
Air transportation, Aircraft, Aviation safety, Incorporation by reference, Safety.
Adoption of the Amendment
Accordingly, under the authority delegated to me by the Administrator, the Federal Aviation Administration amends part 39 of the Federal Aviation Regulations (14 CFR part 39) as follows:
PART 39--AIRWORTHINESS DIRECTIVES
1. The authority citation for part 39 continues to read as follows:
Authority: 49 U.S.C. 106(g), 40113, 44701.
39.13 [Amended]
2. FAA amends 39.13 by adding a new AD to read as follows: