CALDERWOOD, TN, USA
N4580Y
Bell 204-B
The main drive failed at the transmission end resulting in loss of transmission drive. The pilot performed a autorotation landing in trees and the helicopter rolled inverted and descended through the trees coming to rest on a river bank. Metallurgical examination showed the main drive shaft teeth had separated at the transmission end due to wear from frictional overheating which is a result of loss of lubrication. The helicopter manufacturer instructions require that the main drive shaft be inspected by a mechanic for loss of lubrication and signs of overheating during a daily inspection before the first flight of each day. The operator did not have records to show that the daily inspection had been performed prior to the accident flight.
On April 16, 1997, about 1550 eastern daylight time, a Bell 204-B, N4580Y, registered to South Coastal Helicopters, Inc. and operated by the U.S. Forest Service and the State of Tennessee, as a Title 14 CFR Part 91 public-use flight, crashed following loss of transmission drive near Calderwood, Tennessee. Visual meteorological conditions prevailed at the time and a company visual flight rules flight plan was filed. The helicopter was destroyed and the commercial-rated pilot was not injured. The flight originated from Copperhill, Tennessee, the same day, about 1500. The pilot stated his company had a long term contract with the U.S. Forest Service. He had been in stationed in Tennessee with the helicopter for about 45 days. He was dispatched by the U.S. Forest Service dispatcher at Cleveland, Tennessee, to assist the State of Tennessee, Division of Forestry, with putting out a forest fire near Calderwood. After arrival on scene he dropped several buckets of water. While making another run, after filling his bucket, which was on a 100-foot line, he climbed to about 120 feet above the water. He suddenly heard a loud bang and the engine appeared to lose power. He entered autorotation and landed in trees on a hill near the river bank. After entering trees the helicopter rolled inverted and descended to the ground. He exited the helicopter and a postcrash fire erupted and consumed the helicopter. Postcrash examination of the helicopter by an FAA inspector and a representative of Bell Helicopter showed the main engine to transmission drive shaft was separated at the coupling on the transmission end. No other evidence of failure or malfunction was found with the helicopter structure, flight controls, or systems. Examination of the engine assembly, after recovery from the crash site, showed no evidence of failure or malfunction of the engine assembly. The engine accessories were destroyed by the post crash fire and could not be tested. Metallurgical examination of the separated main drive shaft was performed under FAA supervision at Bell Helicopter Engineering Laboratories, Fort Worth, Texas. The examination showed the male coupling teeth were separated from the remainder of the coupling on the transmission end of the shaft. Examination of the recovered teeth showed each tooth "peeled back from the drive-side to the coast-side." The teeth had had been worn due to frictional overheating, resulting in failure. Metallurgists reported that the normal cause of overheating are loss of lubrication, the improper lubricant, or severe mis-alignment. (See attached Bell Helicopter Report). The Bell Helicopter Maintenance and Overhaul Instructions for the Bell Model 204B helicopter requires that a "Daily Inspection" be performed by a mechanic before the first flight of each day. Item 26 of the Daily Inspection requires that the mechanic inspect the "main driveshaft boots or seals and surrounding area for evidence of grease leakage. Inspect temperature indicators TEMP-PLATES, installed on driveshaft couplings for evidence of overheating indication, deterioration, debonding or excessive discoloration of the epoxy overcoating." The helicopter operator did not supply records to NTSB after the accident showing that the Daily Inspection had been complied with on April 16, 1997. (See attached pages from the Bell Helicopter Instructions). The Bell Helicopter pilot preflight checklist does not require that the pilot inspect the main drive shaft prior to flight. The pilot is only required to inspect the engine and transmission cowling for security. (See attached pages from the pilot checklist). Aircraft logbook records showed the main drive shaft was installed after overhaul on N4580Y on May 1, 1990, at aircraft total time 3,937 hours. On December 1, 1994, at aircraft total time 5,800 hours the main drive shaft was overhauled and reinstalled on the helicopter. On January 13, 1997, at aircraft total time 6,565 hours, the main drive shaft was inspected and the lubrication repacked. At the time of the accident the aircraft total time was about 6,645 total hours. (See attached pages from logbook records). A representative of the U.S. Forest Service stated shortly after the accident that the flight was a public-use flight by the U.S. Forest Service. Another representative of the U.S. Forest Service reported to NTSB about 25 hours after the accident that the U.S. Forest Service was not the operator of the helicopter, but that the State of Tennessee was the operator. Representatives from the State of Tennessee stated they were called and offered the services of the helicopter by the U.S. Forest Service dispatcher at Cleveland. They accepted the offer under a cooperative agreement with the U.S. Forest Service. They did not expect to pay for the helicopter, which is part of the cooperative agreement. The pilot stated his company would be paid for the flight by the U.S Forest Service. A postaccident drug screen performed on samples submitted by the pilot was negative.
The inadequate maintenance inspection of the helicopter resulting in loss of lubrication from the main drive shaft going undetected until failure of the drive shaft.
Source: NTSB Aviation Accident Database
Aviation Accidents App
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