Croman Corporation

Docket Number FAA-2008-0442 Directorate Identifier 2007-SW-24-AD Croman operates a fleet of helicopters that includes four Sikorsky S-61N?s, two Sikorsky S-61A?s and three Croman SH-3H?s. Croman Corporation is a helicopter operator in Oregon conducting operations under CFR part 133, 135 and 137. Additionally, Croman holds a FAA Class III Repair Station License with additional ratings for the Sikorsky S-61Airframes, General Electric CT-58 Series gas turbine engines and a limited hydraulic rating for the repair and overhaul of Sikorsky S-61 servos. We have overhauled in excess of 30 transmissions since AD 98-26-02 was put into effect. None of the overhauls have identified fatigue cracking in the MRS flange connection. In a survey of REL Operators, FAA Repair Stations and MOT AMO?s in the United States and Canada, there has not been a cracked Main Rotor Shaft reported in 9 ? years that AD 98-26-02 has been in effect. The lack of documented failures supports keeping the existing life limit in place and does not support imposing the additional cycle limit to the MRS. Sikorsky states that since issuing AD 98-26-02 the REL mission profile parameters have changed significantly. Our Company REL mission parameters have not significantly changed in the past 31 years and I don?t believe other REL operators have either. The proposed amendment superseding AD98-26-02 increase the life limit from 2200 hours to 5000 hours and imposes an 30,000 cycle life limit. The practical effect of introducing a 30,000 cycle limit is to reduce the actual flight hour limit to as low as 1,000 hours (assuming the Sikorsky imposed 30 cycles per flight hour factor used in determining the 250 hour moving average for a shaft with an unknown number of cycles.) Sikorsky conducted an S-61 Main Rotor Shaft Fatigue Test in early 1998 and presented the results at an operator?s conference. Enclosed is some information from Sikorsky on their S61 Main Rotor Shaft Fatigue Test Program. In this test program they applied torque equal to 103% aircraft torque until the shaft cracked due to fretting wear at 200,000 cycles. This test on two main rotor shafts was completed by July 1998, 6 months before the issuing of AD 98-26-02. Why did Sikorsky wait almost 10 years to bring this ?unsafe condition? to the attention of the FAA?(comments) The Main Rotor Shaft Fatigue Test Program was flawed. For example, the test assumes that every GAG cycle will be at 103% torque. Enclosed is a power available chart from the FAA approved S61N flight manual which shows that above 20?C (68? F) and 1000? pressure altitude the helicopter can not produce 103% torque. The majority of our operations are conducted above 1,000 feet and 20?C. The second test specimen was tested at a lower and more realistic torque value of 96%, this specimen was subjected to 1.41 x 106 cycles at 96% torque then subjected to an additional 200,000 cycles at 103% torque. A second example of flawed testing, the first specimen was cracked in the dowel pin holes due to fretting wear after 200,000 cycles. If you take the 200,000 cycles, without using the mean curve or working curve, based on Sikorsky?s estimate of 30 GAG cycles an hour, the specimen shaft would have the equivalent of approximately 6,660 flight hours. The Sikorsky Service Bulletin SB61 B Gen-2 requires overhaul of the main gear box, including the main rotor shaft at a maximum interval of 1,000 hours time in service for REL operations. Therefore the specimen main rotor shaft would been inspected and reworked at each overhaul and at 200,000 cycles the overhaul would have been conducted six times. The Inspection Check from the Sikorsky Aircraft S-61L/N Overhaul Manual SA4045-83 Overhaul Manual (pg 326 & 327) requires inspection every 1,000 hours TIS of the Main Rotor Shaft. The Inspection Check also requires a magnetic particle inspection of the entire main shaft including the flange. The Repair Section of the Overhaul Manual gives detailed repair instructions (pgs 464 ? 476f) for the repair of the flange surface, dowel pin holes and mounting holes. Some of the requirements in the inspection/check for the main rotor shaft are a magnetic particle inspection, inspect mounting holes, dowel pin holes and mounting flange for fretting and wear. The repair section requires the removal of any fretting from the mounting flange and repair of the mounting holes and dowel pin holes, another magnetic particle inspection and shot peen of the reworked area. After this rework, the strength of the reworked areas are very near to new strength. Had the Test Specimen Shaft been ?overhauled? at appropriate intervals (30,000 cycles assuming 30 GAG per hour and a 1,000 hour overhaul interval) the fretting would have been removed and the dowel pin holes would have been repaired per the Sikorsky overhaul manual The likely result would have been a significant increase in the number of cycles to failure. The addition of the overlooked models to the AD is a justifiable reason for an amendment. To supercede an AD, adding a significant decrease in the life limit from 2200 hours to 1000 hours, that has performed it?s intended function for almost 10 years is not justified. Respectfully, Greg Williams Director of Maintenance Croman Corporation