Next Steps for the T-6 Fleet
Submitted by Rick Siegfried
The FAA has collected information on compliance with the wing attach angle Emergency AD from the T-6/SNJ/Harvard fleet. The results of that data have been published in the October issue of NATA Skylines. We are certainly pleased that as a result of the inspections required by AD2005-12-51 aircraft with attach angle damage issues were identified prior to another incident or accident.
The Emergency AD will be revised to a final permanent version in the next 6 to 8 months. We will have a chance to comment on the proposed permanent AD through the FAA’s Notice of Proposed Rulemaking process. It is in our best interest to have factual data prepared prior to the comment period for the proposed AD to support our desire to extend the wing attach angle inspection interval. Our goal is to extend the inspection interval of the current AD to a value that reflects the data found in the fleet during the inspections required by the Emergency AD. Mr. Fred Guerin of the Los Angeles Aircraft Certification Office (ACO) is the lead FAA representative in this process. The FAA has other offices (i.e. Small Aircraft Directorate) that will support Mr. Guerin as he works to revise the Emergency AD to a final version. NATA and EAA Warbirds have been communicating with Mr. Guerin regarding the type and scope of engineering studies we could present as acceptable data to support extending the inspection interval.
The upper attach angles have nowhere near the load carry requirement of the lower attach angles. The FAA currently has enough data to safely extend the inspection interval on the upper attach angles but the data they have on the lower angles is insufficient. A Designated Engineering Representative (DER) has been contacted to conduct a damage tolerance analysis on the lower attach angles. This test will be conducted in coordination with the FAA’s Los Angeles ACO office. We have also been it contact with Boeing Aircraft. Boeing is the current type certificate holder for the T-6/SNJ. We are requesting the original material and specifications for the wing attach angles. This data will be helpful to the DER’s and the FAA in determining proper inspection intervals. The NATA and EAA Warbirds of America believe it is critical to have this analysis completed before the comment period begins on the proposed permanent AD. The funds requested by NATA in the October Skylines will be used for this engineering analysis.
Attach Angle Bolt Torques
Submitted by Rick Siegfried
We have received several comments on the torque values specified in AD 2005-12-51 and the Service Instruction from the SAAF. The torque values in the AD specify 60-65 in/lbs for AN4 (1/4") bolts and 100-105 in/lbs for AN5 (5/16") bolts. Many people have contacted the FAA and us with the opinion that this torque value is insufficient. One point that needs to be emphasized is the published torque values do not include the rotational drag of the elastic stop nuts (AN365). Standard maintenance practice dictates that mechanics add this value to the specified torque. I did a random sample of new and used AN365-4, and -5 nuts. The torque required to turn AN4 (1/4") nuts varied between 15-19 in/lbs. The torque required on AN5 (5/16") nuts varied between 18-22 in/lbs. This value must be added to the torque value specified in the AD.
I also reviewed the North American Erection and Maintenance Instructions T.O. 1T-6C-2
and Navy AN 01-60F-2 Manuals. Assembly of the outer wing to the center section refers only to using the torque values appropriate for the size bolt. Refer to Section IV, Charts and Tables for recommended torque values. This chart lists AN4 (1/4-28) nuts torqued to 50-70 in/lbs and AN 5 (5/16-24) nuts to 100-140 in/lbs. The rotational drag torque value for the respective nuts must be added to these values. The inspection guide went on to specify that at every Major inspection (90 hours time-in-service or Annually) the wings, horizontal stabilizer, and vertical stabilizer attaching bolts are visually inspected for looseness, and attachment fittings for cracks and corrosion. The Special Inspection Requirements in Section V every 8th Major (240 hours time-in-service or every 4 years) recommends checking wing attaching bolts and wing panel to center section tension-type bolts only, for looseness. (Spot check approximately 20 percent of bolts on each wing, using torque wrench to detect obvious looseness; remove and inspect all attaching bolts for wear, cracks, and corrosion if one quarter or more of the spot checked are loose; re-torque all bolts spot checked and/or removed, to specified torque values.)
The above information suggests it may be wise for all T-6/SNJ/Harvard owners and operators recheck the bolt torques 30 to 60 hours after completion of the AD. This will insure proper wing to center section bolt torques. I also caution every owner to assure that the nuts do not contact the bolt shoulder and result in an under torque on the bolt. Each torqued nut should have no more than two treads protruding to verify correct bolt length. Please feel free to contact anyone of us with questions or suggestions you may have regarding any issues dealing with T-6 Attach Angles or any other maintenance items you have found with the fine aircraft left in our care.
Paul Redlich firstname.lastname@example.org
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