RON ROVTAR PHOTOGRAPHY/ALAMY STOCK PHOTO
pilots had flown helicopters for 44 years, including 20 years operating the S-64 and 20 years fighting fires in Australia. On the accident flight, he was serving as second-in-command (SIC), the pilots having switched positions at the end of every two-hour duty cycle. The pilot-in-command (PIC) had 18 years of helicopter experi- ence, 4 of them in the S-64, and the crew chief’s 34 years as a helicopter engineer included 26 “maintaining and developing the S-64.”
The Mission At about 10:00 am, N173AC, nicknamed “Christine,” left Melbourne’s Essendon Fields Airport (YMEN) for the Latrobe Regional Airport (YLTV) 125 kilometers (km) (67.5 nautical miles [nm]) to the east. The helicopter had been dispatched to drop water on a bushfire on the west side of Thomson Dam near the town of Aberfeldy. The fire team’s aerial attack supervisor had already
identified a dip site, subject to the flight crew’s approval, 7 km (3.8 nm) west of the fire front at an elevation of 3,480 feet above mean sea level. The site, on the west side of the Wood Creek Dam, was at the bottom of a steep, narrow valley. The crew judged it to be confined but within their operational limits. These included a mini- mum obstacle clearance of half a main rotor diameter; while hovering to refill its tank, the Aircrane would have one rotor diameter’s clearance from the dam and two from the trees on the opposite bank.
The Flight After breaking for lunch, the crew was detailed to an area toward the north end of the fire. On each pass, they approached the dip site on a west- erly heading from the south, making a descending right turn into a high hover to align the ship next to the dam before descending vertically to the surface. In addition to obstacle clearance, Erickson’s
external-load operating procedures required bank angles of no more than 30 degrees, a landing attitude of no more than 10 degrees nose-high, and a descent rate of less than 800 feet per minute (fpm) within 200 feet of the surface. Airspeed was left to the pilot’s discretion.
Satellite data showed that on the first series of flights, the Aircrane crossed the tree line at the southern edge of the dip zone at an average speed of 30 knots, descending at an average 630 fpm along a final approach seg- ment that measured 300 to 400 meters.
2020 Q3 ROTOR 57
The initial drop sites were almost due east of the
Wood Creek Dam, allowing each circuit to follow a near-rectangular ground track. As the afternoon pro- gressed, the drop sites edged incrementally farther north. The crew completed two two-hour shifts without incident, with the two pilots exchanging seats and responsibilities (PIC vs. SIC) at the end of each. “After a number of water drops” (the ATSB’s report
wasn’t more specific), Christine’s crew was reassigned to fight another flame front farther north and west, east-northeast of the dip site and slightly closer. The change required the helicopter to approach the dip site on a southwesterly heading, tightening the turn to final from roughly 90 degrees to at least 135 and shortening the final approach segment. A steeper descent profile, with a sharper flare to arrest it, was required as a result. On the accident approach, a steep nose-high attitude prompted the SIC to warn his colleague to clear the trees to ensure tail rotor clearance before descending further. Once past the trees, the pilot tightened the flare. A witness characterized this approach as a rapid nose- high descent. Satellite data indicated that the helicopter banked about 30 degrees during its right turn to the lake; nearing the water, its descent rate increased from 650 to 750 fpm. The crew told investigators that neither the bank angle nor the rate of descent seemed excessive, though both were “at the higher end of their normal range.” Once clear of the tree line, however, “the aircraft generated no lift and fell into the dip site, colliding with water” tailfirst.
The accident aircraft takes on water from Wonderland Lake to fight a mountain wildfire near Boulder, Colorado, USA, in 2010.
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