Sikorsky S61-L accident at New Jersey, 18 Apr 1979 (N618PA)

Accident narrative

About 1823 e.s.t. on April 18, 1979, New York Airways Flight 972, a Sikorsky S61-L helicopter (N618PA), crashed at Newark International Airport in New Jersey while attempting an emergency landing. Of the 18 persons on board, three passengers were killed, 10 passengers and three crewmembers were injured seriously, and two passengers received minor or no injuries.

**The flight**

Flight 972 was a scheduled trip from Newark International Airport to La Guardia Airport in New York. The helicopter departed Gate 21 at 1820 with 15 passengers and three crewmembers. The first officer was flying the aircraft from the left seat; both flight crew members were qualified as captains. The captain had about 12,000 flight hours, including about 2,500 hours in S-61 helicopters, and the first officer had 14,500 hours, with about 5,000 hours in the S-61.

The flight was cleared for takeoff, to climb to 1,400 feet, and to depart eastward. Visual meteorological conditions prevailed, with scattered clouds at 5,506 feet, 30 miles visibility, and winds from 010 degrees at 15 knots.

**The emergency**

At 1822:56, as the helicopter climbed to 1,200 feet about 1 mile east of the airport, the crew heard a loud bang similar to an explosion, followed by severe vibrations in the flight controls. At 1823:04, the first officer transmitted, "Mayday, mayday, mayday, New York Five is landing on the runway." The captain transmitted shortly after that they were going to make an emergency landing at Newark. The flight attendant cautioned the passengers to ensure their seatbelts were fastened tightly and later instructed them to remove their eyeglasses.

The flight crew stated that they knew immediately there was a problem with the tail rotor. The first officer stated his first instinct was to enter autorotation and land immediately. However, he elected not to autorotate because the maneuver would have increased rotor rpm and rate of descent, which he believed would intensify the severe vibration and make the aircraft uncontrollable. Instead, the crew elected to reduce rpm, slow the airspeed to about 60 knots, and attempt to land straight ahead on the airport. The local controller cleared them to set it down anywhere except runway 29.

Witnesses saw the helicopter approaching the airport at an altitude of 500 feet, swaying from side to side with the tail moving erratically. As the helicopter descended through about 150 feet, witnesses saw an object—described variously as a suitcase, a body, or a mailbag—fall from the left side of the aircraft. The first officer stated that at 150 feet, the vibrations ceased. Simultaneously, the helicopter's flightpath and attitude changed abruptly. It began to turn, and its nose went down 35 to 60 degrees. The first officer lost all directional control, and the aircraft would not respond to rudder pressure.

At 1823:42, the captain stated, "No tail rotor," and closed the fuel levers for both engines. The first officer attempted to flare the helicopter by applying back pressure to the cyclic and pulling up on the collective, which raised the nose slightly. The helicopter struck the ground in a nosedown attitude on the right side of the fuselage, bounced once, and fell on its left side. It came to rest on the west edge of runway 22L. There was no fire.

**What the investigation found**

The main wreckage was relatively intact from the tail cone aft, but the bottom of the forward fuselage was crushed upward. The tail rotor gearbox and the attached tail rotor assembly were found about 410 feet west of the main wreckage. A 35-inch outboard section of the black tail rotor blade was found 5,100 feet south of the crash site. The main rotor blades, main rotor head assembly, flight controls, and power train showed no evidence of preimpact failure.

Metallurgical examination of the black tail rotor blade revealed a progressive-type fatigue failure. A fatigue crack had propagated across 90 percent of the blade's leading edge spar and about 2 inches of the bonded aluminum outer skin. The fatigue began in the spar before it started in the skin. The spar fatigue originated at the outboard corner of the aft face of the spar, where the radius was measured below engineering specifications. However, because similar radii on the other four blades were also below specifications, the Board was not able to determine exactly what initiated the fatigue failure.

The Board found that the fatigue crack in the spar developed over a period significantly longer than 2 hours, while the skin crack developed over at least 2 flight-hours. The blade had been visually inspected twice on the day of the accident, including 45 minutes before the crash. The Board noted that because the spar was completely enclosed in an aluminum envelope, a fatigue crack in the spar could not be seen during a visual inspection. The skin crack was likely present during the last inspection but was less than 2 inches long and tight, making it difficult to detect. The Board concluded that the visual inspection procedures in effect were not adequate to detect a skin fatigue crack that developed from a spar crack.

The investigation found that the loss of the 35-inch section of the black blade generated unbalanced forces in the rotor system. This caused three of the four tail gearbox attachment lugs to fail under static overload. The left aft bolt of the gearbox mounting showed damaged threads, indicating it had partially backed out. The Board could not determine if this occurred before or as a result of the blade failure, but concluded that a loose tail rotor gearbox was not the cause of the fatigue initiation.

The failure of the attachment flanges permitted the tail rotor and gearbox assemblies to separate from the helicopter at about 150 feet, causing the abrupt nosedown pitch and complete loss of directional control. The Board concluded that the flight crew's decision to make a controlled emergency landing at autorotational speed was an appropriate decision, noting that autorotation might have caused the gearbox to separate sooner.

The accident was partially survivable. The Board found that vertical deceleration forces, estimated at 15 g's in the forward cabin, exceeded the 4-g minimum certification standard for the FAA-approved seats. The impact forces caused the fuselage sidewalls to flex, leading to the failure and separation of sidewall tiedowns and seats, which contributed to the fatalities and serious injuries. Four occupants reported taking a brace position before impact and received less severe injuries. The company had no procedure requiring the flight attendant to instruct passengers to assume a brace position.

**Probable cause**

The National Transportation Safety Board determined that the probable cause of this accident was "the separation of the tail rotor assembly and gearbox from the aircraft at an altitude which made further controlled flight impossible. The rotor assembly and gearbox separated because of severe vibrations in the rotor assembly which were induced by the loss of a tail rotor blade due to fatigue failure."

The Board further determined: "Contributing to the severity of the passengers' injuries were the seat failures which occurred when the deceleration forces exceeded the relatively low design strength of the FAA-approved seats, and the lack of guidance on a passenger brace position for emergency landings."