Sikorsky S-61L accident at Paramount, California, 22 May 1968 (N303Y)

Accident narrative

On May 22, 1968, about 1751 Pacific daylight time, Los Angeles Airways Flight 841, a Sikorsky S-61L helicopter (N303Y), crashed and burned at Paramount, California. All 23 persons aboard were fatally injured, and the aircraft was destroyed by impact and fire.

### The flight

Flight 841 was a scheduled passenger flight from the Anaheim Heliport, which serves Disneyland, to Los Angeles International Airport. The crew consisted of Captain John E. Dupies, who had 12,096 total flying hours with 4,208 in the S-61; Copilot Terry R. Herrington, who had 872 flying hours with 589 in helicopters; and a flight attendant.

The aircraft had arrived in Anaheim as Flight 441. During a three-minute turnaround, the pilots remained in the cockpit and the No. 2 engine was shut down while 20 passengers boarded. Flight 841 departed at 1740. Its gross weight and center of gravity were well within limitations. Weather conditions were clear with visibility of more than 15 miles in full daylight.

The crew made routine radio reports to Fullerton Tower at 1742 and to company flight control at 1747. About 1750, the pilots of an inbound company helicopter, Flight 742, saw Flight 841 pass approximately half a mile away in the opposite direction. They reported Flight 841 was at an altitude of about 2,000 feet on a normal westerly heading and appeared completely normal.

Within 30 to 60 seconds later, a pilot aboard Flight 742 and company flight control heard a radio transmission: "LA, we're crashing, help us." Flight 841 could not be contacted thereafter.

Witnesses observed that during its final two to two-and-a-half miles of flight, the helicopter descended from about 2,000 feet to between 600 and 800 feet above the ground. The aircraft slowed, and there seemed to be erratic action of the main rotor blades. The helicopter then turned left. One witness described a violent yaw to the left of nearly 90 degrees, at which point he saw one main rotor blade extremely out of track on the high side. Sharp sounds followed, and witnesses saw the main rotor blades striking the front and back of the fuselage. Parts separated from the aircraft, which immediately fell in a near-vertical trajectory and crashed.

### What the investigation found

The main body of the aircraft struck the ground nosedown on its left side. Other major portions, including the aft fuselage, tail rotor pylon, and pieces of main rotor blades, fell along a wreckage path 1,100 feet wide and 2,100 feet long to the east, indicating they had separated in flight.

The Sikorsky S-61L main rotor system has five identical blades, color-coded red, black, white, yellow, and blue. Examination revealed that the red, black, white, and blue blades had struck and penetrated the cockpit and aft fuselage in the order of blade rotation.

The yellow blade did not strike the cockpit or aft fuselage. Its pieces were found farthest back along the wreckage path, 2,100 feet from the main wreckage. Damage, scratches, and paint transfers showed that the yellow blade had hit flat against the aircraft's right-side baggage door, and its remaining inboard part was then dragged around the front of the cockpit and rotor head.

The yellow blade's pitch change control rod was found detached at its lower trunnion end, where it normally secures to the rotating swashplate. The swashplate's trailing attachment ear was broken off.

Through extensive failure analysis and testing, the Board determined the sequence of the structural failure. The sequence began with the black, yellow, and blue blades, followed by the red and white blades, undergoing a series of extreme overtravel excursions in their lead/lag axis. These excursions were so severe that in some instances blades overlapped.

During these extreme excursions, the yellow blade overtraveled in the lead direction, and its pitch change control horn likely jammed under the white blade's horizontal hinge pin. This subjected the yellow blade's pitch change control rod to downward and rearward loading many times its design operating strength, causing the overload detachment of the rod. Once detached, the yellow blade became uncontrollable in its pitch and flap axes and struck the baggage door. The resulting extreme imbalance in the rotor head deflected the other four blades downward into the aircraft structure, destroying the helicopter's structural integrity.

The Board ruled out engine failure, flight control system malfunctions, and foreign object damage. Both engines were at or near idle at impact, which the Board believed was most probably the pilots' reaction to a critical situation to reduce the crash fire hazard. Anomalies found in the auxiliary servo control valves were determined to be the result of post-crash fire and reassembly errors during the investigation.

The Board concluded that the extreme lead and lag excursions were caused by a loss of main rotor blade damper integrity. This loss resulted from either a failure of the black main rotor blade damper or a loss of effective damping action by the white main rotor blade damper. The black damper's piston threaded shaft, clevis end, trunnion, and part of its horizontal hinge pin were missing and never recovered, preventing a complete analysis of its failure. Alternatively, a quick disconnect on the white blade's damper fluid reservoir was found disconnected; while tests showed insignificant leakage, the possibility remained that it could have leaked enough fluid in flight to lose damping action. The aircraft manufacturer believed a loss of damper effectiveness in the white blade was more probable, but the Board found the evidence insufficient to definitively isolate one cause over the other.

### Probable cause

The Safety Board determined that the probable cause of this accident was "the loss of main rotor blade damper integrity due to either a failure of the black blade damper or a loss of effective damping action by the white blade damper. This resulted in uncontrolled excursions of the main rotor blades in their lead/lag axis, an overload detachment of the yellow main rotor blade pitch change control rod and destruction of the structural integrity of the aircraft by blade strikes. The precise reason for either of the possibilities for the loss of damper integrity is undetermined."