Big Timber, MT, USA
N1502
McDonnell Douglas 369E
While operating at an altitude of about 40 feet and 40 knots airspeed on a low level coyote predation flight the pilot of the (Hughes) McDonnell Douglas 369E helicopter heard a loud bang and observed both the "Engine Out" and "Auto Relight" warning lights illuminate. The pilot reduced collective and immediately transitioned to a forced landing onto uneven terrain during which the helicopter touched down hard and the right skid separated. Post-crash examination of the Rolls Royce (Allison) 250-C20B turboshaft engine revealed that the compressor coupling adapter had fractured circumferentially resulting in the disconnection of the compressor from the turbine assembly. Further examination revealed that the Rolls Royce (Allison) manufactured compressor adapter coupling had been installed approximately 1,005 hours earlier and that the separation had been the result of multiple fatigue fractures in the coupling associated with severe fretting on the adapter's outer diameter where it inserts into the spur adapter gearshaft.
HISTORY OF FLIGHT On June 25, 2003, approximately 0900 mountain daylight time, a (Hughes) McDonnell Douglas 369E helicopter, N1502, recently acquired by the US Department of Agriculture (USDA), operated by the USDA APHIS Wildlife Services, and being flown by a commercial pilot sustained substantial damage during a hard landing following a total loss of power in level cruise flight approximately six nautical miles northeast of Big Timber, Montana. The pilot and passenger received minor injuries. Visual meteorological conditions prevailed and the aircraft was operating under a USDA flight following procedure at the time. The flight, which was public use, was to be operated as a low level coyote predation operation. The pilot reported that while operating at an altitude of about 40 feet and 40 knots airspeed in level flight (refer to CHART HCD-I) he heard a loud bang, which he described as considerably louder than a compressor stall. The engine immediately lost power and the pilot observed both the "Engine Out" and "Auto Relight" warning lights illuminate. The pilot reported "...I attempted to reduce collective as much as my altitude allowed, and placed the aircraft in decelerative mode. I pulled initial pitch, leveled the aircraft, and struck the ground fairly hard. We bounced but were still level and I used remaining collective pitch to cushion impact but we struck the ground a second time at no more than 5 mph and rolled to the right side. My right landing skid had detached prior to striking the ground the second time, making an upright landing impossible...." PERSONNEL INFORMATION The pilot in command reported a total of 9,450 hours of flight experience of which 6,100 hours were in the Hughes/McDonnell-Douglas 369E helicopter. He also reported that all but 50 hours in the make/model was logged as pilot in command time. AIRCRAFT INFORMATION According to the helicopter's Hobbs meter, the time at the accident was recorded as 1660.0 hours (refer to graphic image I). The last airframe/engine inspection, signed off as a 300 hour/100 hour/annual, was conducted on February 20, 2003, at a total (engine and airframe) time of 7,847.4 hours and a Hobbs time of 1646.7 hours (refer to Attachments MRP-I and MRA-I). The total engine and airframe time at the accident would have been 7,860.7 hours. The helicopter was equipped with a Rolls Royce (Allison) 250-C20B turboshaft engine, serial number CAE 836294. According to records maintained by Rolls Royce, the compressor-coupling adapter (part number: 23039791E-1, serial number: 71205) was manufactured by Allison on December 15, 1999, and was shipped out as a spare part four days later. Refer to Diagrams I and II, which show the location within the engine of the compressor coupling adaptor and a close-up view of the compressor/adaptor/spur adaptor gearshaft with respect to one another. According to the airframe/engine maintenance records the last compressor overhaul was on June 1, 2000, at a total time of 6,855.0 hours. The compressor assembly buildup card also contained an entry referencing the compressor-coupling adapter (part number: 23039791E-1, serial number: 71205) at this same total time (refer to Attachments AR-I and OR-I). There were no further log entries indicating a compressor overhaul between this date and the accident. WRECKAGE AND IMPACT INFORMATION The aircraft crashed on hilly, grass covered, terrain approximately six nautical miles northeast of Big Timber, Montana. The accident site coordinates were reported as 45 degrees 55.12 minutes north latitude and 109 degrees 52.27 minutes west longitude (refer to graphic images 2 and 3). SURVIVAL ASPECTS Both the pilot and passenger's shoulder restraint system failed during the crash sequence. Both occupants exited the rotorcraft with their dual shoulder harness straps hanging free around their necks and post crash examination revealed that each belt's retention bar had exited the rectangular tongue, which the retention bar passes through and is retained by. There was no evidence of mechanical failure or fracture of either occupants retention bar or tongue. The cover plate for each belt however, had separated from the rectangular tongue unit on each restraint during the impact (refer to attached Survival Factors Factual Report and Metallurgical Report No.s 03-115). TESTS AND RESEARCH The engine was removed from the airframe and shipped to Rolls Royce, Indianapolis, Indiana, for further disassembly and examination under the Board's oversight. The initial examination revealed that the forward compressor rotated freely, however the N1/N2 gear train would not rotate. Disassembly of the engine revealed that the compressor coupling adapter had fractured circumferentially with the forward end of the compressor coupling remaining lodged within the compressor impeller and the aft end of the compressor coupling remaining with the spur adapter gearshaft. The compressor rotor assembly and the spur adapter gearshaft along with the associated compressor coupling adapter sections were shipped to the NTSB Materials Laboratory for further examination. The NTSB Materials Laboratory examination revealed that the forward separated section of the compressor coupling adapter was vibro peened with part number 23039791E-1 and serial number 71205 and the part was identified as having been manufactured by Rolls Royce. Examination and testing of the two adapter sections revealed that the adaptor was of alloy steel that met the Rolls Royce Engineering Material Specifications, the hardness was also within specifications, and the part had been properly heat treated. Further metallurgical examination revealed that an approximate 0.1 inch wide band aft of the fracture line on the pilot (outside) diameter of the compressor coupling adapter displayed severe fretting*. Magnified optical and scanning electron microscope examinations of the fracture surfaces of the compressor coupling adapter revealed features in the undamaged areas consistent with fatigue progression. Several fatigue origins were noted on the outside surface of the pilot diameter and the origins were noted within the previously described area of fretting (refer to Metallurgical Report No. 03-095). *"Fretting wear: Surface damage to a metal part resulting from micro-welding due to slight movement in a nearly stationary joint. Also called fretting corrosion." From "Understanding How Components Fail", 2nd Edition, Donald J. Wulpi.
Fretting on the outside diameter of the compressor coupling adapter which resulted in fatigue and subsequent fracture of the adapter, and ultimately the disconnection of the compressor from the turbine assembly. Contributing factors were low rotor RPM, uneven terrain and collapse of the right skid on landing.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
