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| Date: | Friday 14 August 1998 |
| Time: | 17:55 LT |
| Type: |  Bell 206B II |
| Owner/operator: | Alaska State Troopers |
| Registration: | N70711 |
| MSN: | 1331 |
| Year of manufacture: | 1974 |
| Total airframe hrs: | 14159 hours |
| Engine model: | Allison 250-C20 |
| Fatalities: | Fatalities: 0 / Occupants: 2 |
| Other fatalities: | 0 |
| Aircraft damage: | Substantial |
| Category: | Accident |
| Location: | Chickaloon, AK -
 United States of America
|
| Phase: | Standing |
| Nature: | Unknown |
| Departure airport: | Willow, AK (UUO) |
| Investigating agency: | NTSB |
| Confidence Rating: |  Accident investigation report completed and information captured |
The commercial helicopter pilot said that he had completed a normal, westerly approach, and subsequent landing atop a 4,000 feet msl mountain. The pilot stated that as he reduced the throttle to the flight idle position, in preparation for engine shut down, a severe airframe vibration occurred. A postaccident inspection revealed that the helicopter's main rotor transmission had been torn from the lower isolation mount assembly, and the helicopter's transmission deck sustained substantial damage. The main rotor drag pin assembly, main rotor driveshaft, forward coupling assembly, and associated fasteners, were sent to the NTSB Materials Laboratory for inspection. The inspection discovered no evidence of a progressive fracture mechanism, and all fracture faces displayed features typical of overstress separation. The main rotor transmission isolation mount was retained by the NTSB, and sent to the manufacture for evaluation, and testing. An FAA airframe certification engineer attended the evaluation process, and reported that the isolation mount operated slightly below acceptable stiffness limits. He added that since the isolation mount sustained damage during the accident, an accurate stiffness value was unattainable.
Probable Cause: A fracture of the main rotor drag pin assembly.
Accident investigation:
 |
|
Sources:
NTSB ANC98TA125
History of this aircraft
Other occurrences involving this aircraft
| 17 January 1982 | N203AL | Air Logistics | 0 | Centerville, AL | |
sub |
Location
Revision history:
| Date/time | Contributor | Updates |
|---|---|---|
| 10-Mar-2024 17:11 | ASN Update Bot | Added |
Corrections or additions? ... Edit this accident description
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