SPRINGTOWN, TX, USA
N122TH
Eurocopter COLIBRI EC120B
While at cruise altitude, the helicopter's engine began to surge. The pilot initiated an autorotation to an open field. During the downwind forced landing, the skids dug into the ground, and the helicopter nosed over and came to rest on its left side. An examination of the engine revealed that the P3 air pipe, which supplies air to the fuel control, was broken close to the fitting on the combustion chamber casing. Examination of the P3 pipe revealed that its fracture was 'consecutive to the propagation of a fatigue fissure,' as a result of the pipe being 'subjected to an excessive vibratory excitation.' Engine vibration tests were conducted and two main excitation sources were identified. The two sources were the starter-generator and the breather gear assembly. The proposed solution to correct for this vibration consists of fitting an attachment clamp at mid-length of the P3 pipe, 'which moves the natural frequencies out of engine power ratings.'
On February 26, 1999, approximately 0910 central standard time, an Eurocopter Colibri EC120B helicopter, N122TH, was substantially damaged during a forced landing following engine power surges near Springtown, Texas. The helicopter was registered to and operated by the American Eurocopter Corporation of Grand Prairie, Texas. The airline transport rated pilot, sole occupant of the aircraft, sustained minor injuries. Visual meteorological conditions prevailed, and a VFR flight plan was filed for the 14 Code of Federal Regulations Part 91 business flight. The flight originated from Grand Prairie, Texas, and was en route to Amarillo, Texas. The pilot reported that while at a cruise altitude of about 500 feet agl, the engine began to surge. He initiated an autorotation downwind to an open field. He started a "decel [deceleration] approx [approximately] 70 feet, initial pitch at 20 feet and cushioned the touchdown." During the landing, the skids dug into the ground, and the helicopter nosed over and came to rest on its left side. The pilot further reported that he exited the helicopter and then shut down the engine by using the emergency fuel shutoff. According to the operator, the cockpit's wind-screen was broken out, the main rotor system sustained damage, and the left horizontal stabilizer was bent. An examination of the fuel system revealed that the 100-micron fuel filter was contaminated by black fibrous material. This material was found to be similar to the protective material found on the acceleration limiting strap brackets in the upper fuel tank. The helicopter had a total time of 22 hours. An examination of the helicopter's Turbomeca Arrius 2F turboshaft engine, serial number 34022, was conducted at Turbomeca's facility in France under the supervision of the Bureau Enquetes-Accidents (BEA) on March 24, 1999. According to the BEA report, "the turbo-engine started correctly. The flame was continuous and the governor worked as expected... When power was set by increasing the load, the NG stabilized at 81 percent (instead of reaching 100 percent as expected) while NTL was decreasing (instead of staying at 100 percent) and fuel flow was stabilized at 62 liters/hour (instead of around 150 liters/hour)..." A new fuel control unit was installed on the engine and "no significant change in the behavior of the engine was observed when the engine was re-started..." An in depth examination of the engine revealed that the P3 air pipe, which supplies air to the fuel control unit, was broken close to the fitting on the combustion chamber casing. A new P3 air pipe was installed and the "engine operated in conformity with its specifications over the power range." Examination of the P3 pipe by the Ministere De La Defense's Departement Etudes Expertises began on March 19, 1999, and concluded May 20, 1999. According to the metallurgist's report, "the rupture is consecutive to the propagation of a fatigue fissure on the right hand side of the welded joint of the flange connection on the tube side. It initiated from the internal wall of the pipe. The smelted weld zone of the two connections is large, probably due to a weld energy too high. The liquid metal bead on the internal wall of the pipe has generated a notched effect in this zone, capable of favoring the priming of the fatigue fissure. Also, the internal side of the tube has probably undergone an alteration at its end before welding which by reducing its diameter, has increasingly concentrated the constraint in the weld joint. However, this evidence alone does not explain the rupture. It seems that the pipe has been subjected to an excessive vibratory excitation." On July 7, 1999, a vibration test of the engine was conducted under the supervision of the BEA. According to the Turbomecca report, two main excitation sources were evidenced during the test. The two sources were the starter-generator and the breather gear assembly. The report proposed a solution to correct for this vibration consisting of fitting an attachment clamp at mid-length of the P3 pipe, "which moves the natural frequencies out of engine power ratings."
The improper touchdown executed by the pilot. Factors were the tail wind condition, and the manufacturer's inadequate design of the P3 pipe supports, which resulted in a fatigue fracture of the P3 pipe.
Source: NTSB Aviation Accident Database
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