ASN Wikibase Occurrence # 342654
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| Date: | Friday 26 July 1991 |
| Time: | |
| Type: |  Bell 206L-1 |
| Owner/operator: | |
| Registration: | |
| MSN: | |
| Fatalities: | Fatalities: 1 / Occupants: 1 |
| Aircraft damage: | Destroyed |
| Category: | Accident |
| Location: | north of Alpine Lake, CA -
 United States of America
|
| Phase: | Take off |
| Nature: | External load operation |
| Departure airport: | |
| Confidence Rating: |  Information is only available from news, social media or unofficial sources |
At approximately 1500 PDT on July 26, 1991, a Bell 206 LI departed from the Bald Mountain Helitack Base enroute to a small holdover lightning fire in California. This fire was located north of Alpine Lake in the Pacific Southwest Region, Stanislaus National Forest. On board the helicopter were the pilot, helitack crewmembers, and initial attack response equipment. After landing the crew in a meadow near the fire, the pilot had the water bucket hooked on and commenced dropping water from a nearby lake onto the fire.
Approximately one and a half hours after arriving at the fire, the pilot had made 12 bucket drops. The pilot was then told to longline two pillow tanks of water to the fire. The pilot left the water bucket at the meadow helispot, picked up a helitack crewmember, and returned to the helibase for the needed equipment and to refuel. On the way, the pilot was asked what length longline he would need and he indicated the 100-foot line would be enough.
After refueling and loading two 100-gallon pillow tanks, a 100-foot longline, and the remote hook, nets and associated equipment, the pilot and the crewmember returned to the valley. The equipment was unloaded and readied for filling and pickup. The pilot flew back to the meadow helispot near the fire, picked up two more helitack crewmembers, and returned them to the helibase. On this trip, the pilot circled the fire and the person on the ground indicated where he wanted the tanks placed: a small opening enlarged by falling a large lodge pole pine. The opening was quite close to the fire line on the north side and the intent was to hook a hose to the pillow tanks and gravity-feed water to the fire. The pilot radioed the valley that the 100- foot longline would be okay.
The tanks were filled from an engine at the valley and helitack crewmembers prepared the longline load.
While most of the engine crew had been shuttled to the fire by another helicopter, a Foreman had remained with the engine. The Foreman operated the engine and monitored the tank gauge to insure that only 75 or 80 gallons were put into each tank, as directed. Helitack crewmembers verified that the pillow tank was about six or seven inches high when filled, which, according to their training, would correspond to 75 gallons.
At liftoff, the helicopter gross weight was within established limits for the density altitude at the load landing spot. The load calculation for this flight was completed after the helicopter took off and was not reviewed or signed by the pilot.
The pilot took off and lifted out the first pillow tank about 1700 PDT. The pickup and departure appeared normal. He circled over the meadow helispot. His approach was apparently slow and his descent fairly steep as he brought the load in over the lower trees on the northeast side of the opening. During the flight the pilot had commented by radio, “It’s really squirrelly up here today.” And, on short final, “Don’t get under this load.” The person on the ground, preparing to direct the setting down of the pillow tank, did not hear the first comment, but answered, “okay” to the second.
As the helicopter came to a hover, the load hit the ground somewhat harder than normal and a few feet north of the spot previously indicated to the pilot. The pilot disconnected the remote hook electrically and the helicopter rose slightly and moved forward a few feet. The helicopter skids were about 16 feet higher than the top of a 90-foot tree on the pilot’s right at the point where the load touched the ground. The plane of the main rotor overlapped the treetop by approximately two feet. There was a 106-foot snag-top tree about 40 feet in front of the helicopter.
While still in a hover, the helicopter began to wobble and tilt, first to the right and then to the left, and then began to descend vertically. The witnesses observed that the engine and/or rotor noise changed noticeably.
The pilot keyed his mike and in a distressed voice called out, “Get out of the way!”
The helicopter continued to descend and the main rotor hit the 90-foot tree about four feet from the top.
Further strikes occurred progressively down the tree. As the helicopter descended to about 50 feet up the tree and turned to the right, the main rotor began disintegrating and cut off the tail boom
At approximately 40 feet, the helicopter pitched nose-down and free fell to the ground on top of the pillow tank and longline. The impact was relatively hard. The helicopter ended upright and at least 45 degrees nose down. The main rotor blades were at a high pitch or angle of attack during all of the tree strikes. Two crewmembers from the engine were on scene immediately. They noticed the helicopter engine was still running at flight idle and a moderate amount of fuel was leaking out. The cabin floor was pushed upward by the force of impact. The pilot had been fatally injured
The investigation team stated that the flight profile of the final approach to land the load in the opening was conducive to the helicopter entering into a main rotor vortex ring state that was aggravated by a shift in direction or down draft in the wind. This could have caused a descent beyond the pilot’s capability to arrest in the vertical space available to perform an escape maneuver. They deemed the mishap was caused by the strike of the main rotor blade near the top of a tree.
Sources:
https://www.researchgate.net/publication/2840945_United_States_Department_of_Agriculture_Forest_Service
Revision history:
| Date/time | Contributor | Updates |
|---|---|---|
| 31-Jul-2023 12:04 | harro | Added |
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