22 May 1968: Los Angeles Airways Flight 841, a Sikorsky S-61L, N303Y, was enroute from Disneyland, Anaheim, California, to Los Angeles international Airport (LAX). Captain John E. Dupies and First Officer Terry R. Herrington were in the cockpit, while Flight Attendant Donald P. Bergman was in the passenger cabin with twenty passengers. The flight was cruising on a westerly heading at 2,000 feet (610 meters) when the five main rotor blades “underwent a series of extreme over-travel excursions in their lead/lag axis.”
The five main rotor blades are identified by color markings: red, black, white, yellow and blue (clockwise as seen from above). As the black blade oscillated fore and aft, the geometry of the pitch change control rods to the blades changed, rapidly varying the blades’ pitch angles and therefore, the lift and drag they produced. This put extreme overloads on the pitch control rods and and the rod controlling the yellow blade failed. The yellow blade was no longer in control. The extreme dynamic changes in the blade’s motion was transmitted to the white blade which also went out of control, followed by the other three blades. All five blades diverged from the normal tip-path plane and began to strike each other and the helicopter’s fuselage. The yellow blade was driven out of its normal sequence between the white and blue blades and struck the fuselage at the baggage door with its top flat against the fuselage side. It broke into five sections then wrapped around the rotor mast. All blades were destroyed. The helicopter, completely out of control, fell nearly vertically to the ground. The crew radioed, “L.A., we’re crashing. Help us.”
At 5:51 p.m., Pacific Daylight Time, Flight 841 crashed on Alondra Boulevard near Minnesota Street in the city of Paramount. The aircraft was completely destroyed by the impact and post-crash fire. All 23 persons on board were killed.
The National Transportation Safety Board (NTSB) investigation determined that the probable cause of the accident was a failure of the black blade’s lead/lag hydraulic damper or a loss of effectiveness of the white blade’s damper. The reason for this failure was not determined.
Captain “Jack” Dupies was a veteran pilot with Los Angeles Airways, having worked for the airline since 1953. He had a total of 12,096 flight hours with 4,208 hours in the S-61. First Officer Herrington had a total of 872 flight hours with 589 hours in helicopters. He had joined Los Angeles Airways in January 1968.
Sikorsky S-61L N303Y, s/n 61060, was completed in June 1962. At the time of the crash, it had accumulated 11,128 total hours on the airframe. It had undergone a complete 2,400-hour overhaul approximately 6 months earlier.
The Sikorsky S-61L was a civil variant of the United States Navy HSS-2 Sea King, and was the first helicopter specifically built for airline use. The prototype, N300Y, first flew 2 November 1961. It is a large twin-engine helicopter with a single main rotor/tail rotor configuration. Although HSS-2 fuselage is designed to allow landing on water, the S-61L is not amphibious, having standard fixed landing gear rather than the sponsons of the HSS-2 (and civil S-61N). The S-61L fuselage is 4 feet, 2 inches (1.270 meters) longer than that of the HSS-2. The S-61L is 72 feet, 7 inches (22.123 meters) long and 16 feet, 10 inches (5.131 meters) high, with rotors turning.
The main rotor has five blades and a diameter of 62 feet (18.898 meters). Each blade has a chord of 1 foot, 6.25 inches (0.464 meters). The tail rotor also has five blades and a diameter of 10 feet, 4 inches (3.149 meters). They each have a chord of 7–11/32 inches (0.187 meters). At 100% r.p.m., the main rotor turns 203 r.p.m. and the tail rotor, 1,244 r.p.m. The main rotor turns counter-clockwise, as seen from above. (The advancing blade is on the helicopter’s right side.) The tail rotor turns clockwise, as seen from the left side. (The advancing blade is below the axis of rotation.)
N303Y was powered by two General Electric CT58-140-1 engines. The CT58 is an axial-flow free-turbine turboshaft engine with a 10-stage compressor section and a 3-stage turbine (2 low- and 1 high-pressure stages). The -140-1 is rated at 1,400 shaft horsepower for takeoff and 1,500 SHP for 2½ minutes, with one engine inoperative. The compressor turns 26,300 r.p.m. (100% N1) and the power turbine, 19,500 r.p.m. (100% N2). The CT58-140-1 is 1 foot, 8.2 inches (0.513 meters) in diameter, 4 feet, 11.0 inches (1.499 meters) long and weighs 350 pounds (158.8 kilograms).
The helicopter’s main transmission was rated for 2,300 horsepower, maximum.
The S-61 has a cruise speed of 166 miles per hour (267 kilometers per hour). The service ceiling is 12,500 feet (3,810 meters). The maximum takeoff weight is 20,500 pounds (9,298.6 kilograms).
Between 1958 and 1980, Sikorsky built 794 S-61 series helicopters. 13 were S-61Ls. As of September 2013, two remained in service.
© 2018, Bryan R. Swopesby