CUSICK, WA, USA
N311KA
Kaman K-1200
While lifting approximately 5,000 pounds of logs from the drop area, the aero-logging helicopter experienced a malfunction of the free-wheeling sprag clutch assembly, and a resultant torsional separation of the engine drive adapter. Although the pilot attempted an emergency descent to a down-slope clearing, the aircraft impacted trees and dropped onto the terrain below.
On July 11, 2000, approximately 1230 Pacific daylight time, a Kaman K-1200 helicopter, N311KA, collided with trees about 10 miles north of Cusick, Washington, after experiencing a loss of main rotor rpm during aero-logging operations. The commercial pilot, who was the sole occupant, received serious injuries, and the aircraft, which was owned and operated by Superior Helicopters, sustained substantial damage. The 14 CFR Part 133 long-line aero-logging operation was being conducted in visual meteorological conditions. No flight plan had been filed, and there was no report of an ELT activation. According to the pilot, who was moving approximately 5,000 pounds of logs, he recorded their total weight while in a hover over the drop site. He then lifted the helicopter straight up to a height where the logs would clear the tree line. Just as the load was clearing the trees, the helicopter "shuddered," its engine "fluctuated" three times, and the pilot heard a "loud crack." The main rotor RPM immediately started to decrease, and the pilot turned toward a clearing about 100 feet downhill from his position. Just prior to reaching the clearing, the helicopter impacted a tree and fell to the terrain below. During the investigation it was determined that 11 of the 33 sprags in the Free-wheeling Sprag Clutch Assembly (K974110-005) had rolled beyond the "full torque" position, and 24 of the 33 sprag retainer bars had separated from the sprag retainer cage. In addition, the surface of both the sprag assembly center input shaft (K974047-005) and the sprag assembly spiral bevel input pinion (K974013-005) contained areas of severe mechanical wear, smearing, heat flowing, and distortion of the surface metal. It was also determined that the engine adapter shaft had separated in a manner consistent with a predominantly torsional overstress failure. An FAA-monitored inspection and test run of the Lycoming T5317A-1 turboshaft engine (No. LE-81016) was conducted on July 26, 2000. During the test, the engine was operated at ground idle, flight idle, maximum continuous power, and takeoff power. In addition, the engine was made to perform "snap accelerations" from flight idle to takeoff power and from ground idle to takeoff power. During all of these tests, the engine operated satisfactorily under all conditions, and no anomalies or abnormal conditions that would have contributed to a loss of main rotor RPM were identified. In addition, the TA-7 Fuel Regulator and the PTG-5-1 Governor were functionally tested and subjected to a teardown inspection. No functional anomalies or irregularities that would have contributed to a sudden loss of rotor RPM were noted. A partial disassembly of the fuel regulator revealed no foreign objects or contamination.
The termination of power to the main rotor system as a result of the malfunction of the free-wheeling sprag clutch assembly, followed by the torsional overload separation of the engine adapter shaft. Factors include trees in the area where the emergency descent was attempted.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
