Sikorsky S-61

The Sikorsky S-61L and S-61N  are civil variants of the successful Sikorsky SH-3 Sea King helicopter. They are two of the most widely used airliner and oil rig support helicopters built.

Design and development
In September 1957, Sikorsky won a United States Navy development contract for an amphibious anti-submarine warfare (ASW) helicopter capable of detecting and attacking submarines. The XHSS-2 Sea King prototype flew on 11 March 1959. Production deliveries of the HSS-2 (later designated SH-3A) began in September 1961, with the initial production aircraft being powered by two 930 kW (1250shp) General Electric T58-GE-8B turboshafts.

Sikorsky was quick to develop a commercial model of the Sea King. The S-61L first flew on 2 November 1961, and was 4 ft longer than the HSS-2 in order to carry a substantial payload of freight or passengers. Initial production S-61Ls were powered by two 1350shp (1005 kW) GE CT58-140 turboshafts, the civil version of the T58. The S-61L features a modified landing gear without float stabilisers.

Los Angeles Airways was the first civil operator of the S-61, introducing them on 11 March 1962, for a purchased price of $650,000 each.

The East Pakistan Helicopter Service used three S-61s. The service linked 20 East Pakistani towns, which made it one of the most extensive commercial helicopter networks at the time.

On 7 August 1962, the S-61N made its first flight. Otherwise identical to the S-61L, this version is optimized for overwater operations, particularly oil rig support, by retaining the SH-3's floats. Both the S-61L and S-61N were subsequently updated to Mk II standard with improvements including more powerful CT58-110 engines giving better hot and high performance, vibration damping and other detail refinements.

The Payloader, a stripped down version optimized for aerial crane work, was the third civil model of the S-61. The Payloader features the fixed undercarriage of the S-61L, but with an empty weight almost 2000 lb less than the standard S-61N.

Carson Helicopters was the first company to shorten a commercial S-61. The fuselage is shortened by 50 in to increase single-engine performance and external payload.

A unique version is the S-61 Shortsky conversion of S-61Ls and S-61Ns by Helipro International. VIH Logging was the launch customer for the HeliPro Shortsky conversion, which first flew in February 1996.

One modification for the S-61 is the Carson Composite Main Rotor Blade. These blades replace the original Sikorsky metal blades, which are prone to fatigue, and permit a modified aircraft to carry an additional 2000 lb load, fly 15 kn faster and increase range 61 nmi.

The latest version is the modernized S-61T helicopter. The United States Department of State has signed a purchase agreement for up to 110 modernized S-61T aircraft for passenger and cargo transport missions in support of its worldwide operations. The first two modernized S-61 aircraft will support missions for the U.S. Embassy in Afghanistan.

Variants

 * S-61L : Non-amphibious civil transport version. It can seat up to 30 passengers
 * S-61L Mk II: Improved version of the S-61L helicopter, equipped with cargo bins.
 * : Amphibious civil transport version.
 * S-61N Mk II: Improved version of the S-61N helicopter.
 * S-61NM: An L model in an N configuration.
 * S-61T Triton: S-61 modernized upgrade by Sikorsky and Carson; Upgrades include composite main rotor blades, full airframe structural refurbishment, conversion of folding rotor head to non-folding, new modular wiring harness, and Cobham glass cockpit avionics; initial models converted were S-61N

Operators
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 * CHC Helicopter
 * Cougar Helicopters
 * Air Greenland
 * Lebanese Air Force
 * Bristow Helicopters
 * Lebanese Air Force
 * Bristow Helicopters
 * Bristow Helicopters
 * Bristow Helicopters
 * AAR Corp
 * Carson Helicopters
 * Croman Corporation
 * Helicopter Transport Services
 * United States Department of State

Former operators

 * Canadian Coast Guard
 * Helijet
 * Irish Air Corps
 * Irish Coast Guard
 * KLM Helikopters
 * Irish Coast Guard
 * KLM Helikopters
 * KLM Helikopters
 * Pakistan International Airlines
 * British Airways Helicopters
 * British International Helicopters
 * Her Majesty's Coastguard
 * Los Angeles Airways
 * New York Airways
 * San Francisco and Oakland Helicopter Airlines
 * Los Angeles Airways
 * New York Airways
 * San Francisco and Oakland Helicopter Airlines

Notable accidents

 * Pakistan International Airlines Flight 17 was a Sikorsky S-61 that crashed on 2 February 1966 on a scheduled domestic flight in East Pakistan with 23 killed and one survivor.
 * On 22 May 1968, Los Angeles Airways Flight 841 crashed near Paramount, California, resulting in the loss of 23 lives. The accident aircraft, N303Y, serial number 61060, was a Sikorsky S-61L en route to Los Angeles International Airport from the Disneyland Heliport in Anaheim, California.
 * On 14 August 1968, Los Angeles Airways Flight 417 crashed in Compton, California, while en route to the Disneyland Heliport in Anaheim, California from Los Angeles International Airport, resulting in the loss of 21 lives. The accident aircraft, N300Y, serial number 61031, was the prototype of the Sikorsky S-61L.
 * On 25 October 1973, a Greenlandair S-61N, OY-HAI "Akigssek" ("Grouse") crashed about 40 km south of Nuuk, resulting in the loss of 15 lives. It was en route to Paamiut from Nuuk. The same aircraft had an emergency landing on the Kangerlussuaq fjord two years earlier, due to flameout on both engines because of ice in the intake.
 * On 10 May 1974 KLM Helikopters S-61N PH-NZC crashed en route to an oil rig in the North Sea. None of the two crew and four passengers survived. The probable cause was a failure in one of five rotor blades due to metal fatigue. The resulting imbalance caused the motor mounts to fail and caused a fire. The uncontrollable aircraft landed hard in the water, capsized and sank. Investigation indicated that the metal fatigue crack must have spread rapidly in less than four hours. The rotor blades are pressurized with nitrogen gas at 10 psi to indicate the onset of a metal fatigue failure, yet no pressure loss was indicated during the preflight inspection. As a result of the accident it was recommended to shorten inspection intervals The aircraft was recovered from the North Sea floor. It was rebuilt and currently flies as registration N87580 in the USA.
 * On 16 May 1977, New York Airways' commercial S-61-L, N619PA, suffered a static rollover onto its starboard side at the heliport on top of the Pan Am Building while boarding passengers. The accident killed four boarding passengers and one woman on the street. 17 additional passengers and the three flight crew members were uninjured. The landing gear collapse was a result of metal fatigue in the helicopter's main landing gear shock-absorbing strut assembly, which caused the helicopter to tip over without warning. The accident resulted in the permanent closure of the Pan Am Building heliport. As the heliport was closed, the wreckage was removed by disassembling it and taking the assemblies down to street level using the building's freight elevators. The airframe was taken to Cape Town, South Africa, where it was rebuilt, certified and returned to service as the first S61 used in the Ship-Service Role off the shores of the Western Cape by the company "Court Helicopter" which was later amalgamated with CHC.
 * On 16 July 1983, British Airways Helicopters' commercial S-61 G-BEON crashed in the southern Celtic Sea, in the Atlantic Ocean, while en route from Penzance to St Mary's, Isles of Scilly in thick fog. Only six of the 26 on board survived. It sparked a review of helicopter safety and was the worst civilian helicopter disaster in the UK until 1986.
 * On March 20, 1985, an Okanagan Helicopters S-61N (C-GOKZ) ditched in the Atlantic Ocean off of Owl's Head, Nova Scotia. The aircraft was en route from the MODU Sedco 709 offshore Nova Scotia to the Halifax International Airport(YHZ)when the main gearbox suffered a total loss of transmission fluid. There were 15 passengers and two crew on board. There were no injuries during the ditching, however several passengers suffered varying degrees of hypothermia. As a result of this incident, improved thermal protection and other advancements in helicopter transportation suits were instituted for offshore workers on Canada's east coast.
 * 12 July 1988 a British International Helicopters S-61N ditched into the North Sea, no injuries.
 * On 25 July 1990 a British International Helicopters Sikorsky S-61 registration 'G-BEWL' coming in from Sumburgh Airport crashed onto the Brent Spar oil storage platform as the pilots were attempting to land. The aircraft fell into the North Sea, where six of the 13 passengers and crew on board died.
 * On 8 July 2006, a Sociedad de Salvamento y Seguridad Marítima S-61N Mk.II search and rescue helicopter, crashed into the Atlantic Ocean while it was flying from Tenerife to La Palma. There were no survivors among the six people on board.
 * On 5 August 2008, two pilots and seven firefighters assigned to the Iron Complex fire in California's Shasta–Trinity National Forest, were killed when Carson Helicopters S-61N N612AZ crashed on takeoff. Of the 13 people reportedly on board, one other pilot and three firefighters survived the crash with serious or critical injuries. The NTSB determined that the probable causes were the following actions by Carson Helicopters: 1) the intentional understatement of the helicopter’s empty weight, 2) the alteration of the power available chart to exaggerate lift capability, and 3) the use of unapproved above-minimum specification torque in performance calculations that, collectively, resulted in the pilots’ relying on performance calculations that significantly overestimated load-carrying capacity and without an adequate performance margin for a successful takeoff; and insufficient oversight by the U.S. Forest Service and the Federal Aviation Administration. Contributing factors were the flight crew's failure to address the fact that the helicopter had approached its maximum performance capability on two prior departures from the accident site as they were accustomed to operating at its performance limit. Contributing to the fatalities were the immediate, intense fire due to a fuel spillage upon impact from the fuel tanks that were not crash-resistant, the separation from the floor of the cabin seats that were not crash-resistant, and the use of an inappropriate release mechanism on the seat restraints.