Panaca, NV, USA
N48020
Bell 47G-3B-1
The helicopter experienced a loss of engine power and landed hard during an autorotation, severing the tail boom. The number 6 cylinder's intake valve separated, with the valve's head wedged in the intake valve seat bore. Removal of the rocker box cover on that cylinder revealed that the spring retainer was broken into numerous pieces. The piston head showed over 15 impressions consistent in dimension to that of the valve head. After examining the number 6 cylinder, a metallurgist determined that the valve failed in between the intake valve head and the valve stem.
On December 1, 2002, about 1310 Pacific standard time, a Bell 47G-3B-1, N48020, experienced a loss of engine power and landed hard during an autorotation near Panaca, Nevada. The Bureau of Land Management was operating the helicopter under the provisions of 14 CFR Part 91. The commercial pilot, the sole occupant, was not injured; the helicopter sustained substantial damage. Visual meteorological conditions prevailed, and a company flight plan had been filed. The local public use positioning flight originated from Caliente, Nevada, about 1200. The pilot was positioning the helicopter to perform a horse herding operation for the Bureau of Land Management at the time of the accident. Two witnesses reported seeing two grey smoke emissions from the engine prior to a loss of engine power and hard landing, severing the tail boom. In a written statement, the mechanic who regularly preformed the maintenance on the helicopter, reported that earlier in the year the helicopter underwent a 1,000-hour engine inspection, which included an engine overhaul; it was returned to service on April 30, 2002. The cylinder assemblies used for the overhaul were purchased from Engine Components, Inc., in San Antonio, Texas. The maintenance tag attached on the cylinders attested that "the repair made to the unit listed on reverse side was made and inspected in accordance with current regulations of the FAA [Federal Aviation Administration] and the part is Airworthy and approved for return to service." The mechanic further stated that he conducted an engine examination after the accident under the supervision of the FAA. Upon removal of the induction tubes and the induction manifold, the mechanic discovered internal discoloration similar to that for the exhaust tubes. He removed the number 6 cylinder and noted that the intake valve was broken, with the valve's head wedged in the intake valve seat bore. Removal of the rocker box cover on that cylinder revealed that the spring retainer was broken into numerous pieces. The piston head showed over 15 impressions consistent in dimensions to that of the valve head. The mechanic added that at the time of the accident, the cylinder had accumulated 10 hours since overhaul and the engine had accumulated 110.3 hours since the last overhaul. The cylinder was sent to Engine Components, Inc., for further examination under the auspice of the FAA. The mechanic's complete written account is appended to this report in the public docket. Engine Components, Inc., completed a technical report in summation of their findings. In pertinent part, the director of quality and metallurgy stated that the retainer failed due to overload. He opined that during engine operation the valve fractured first, resulting in the valve stem impacting the retainer and keys leading to their facture and deformation. He noted that the broken valve was a new part supplied by Lycoming during the cylinder overhaul. The complete report is appended to the report in the public docket.
separation of the no. 6 cylinder intake valve head from the valve stem, which resulted in a loss of engine power. The mechanism responsible for the valve failure is unknown.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
