The Sikorsky S-61L and S-61N are civil variants of the SH-3 Sea King military helicopter. It was developed and produced by the American helicopter manufacturer Sikorsky Aircraft.
The commercial version of the Sea King was developed during the late 1950s. Two versions, the land-based S-61L and the amphibious S-61N, were created. The S-61L had an enlarged cabin and dispensed with some of the amphibious features, such as its float stabilizers, for greater payload capacity. It was considered attractive to utility operators, while the amphibious S-61N appealed to off-shore operators. The first models were delivered to customers during September 1961. By the turn of the century, they had become two of the most widely used airliner and oil rig support helicopters built.[1]
Airliners were a key customer for the S-61L. Los Angeles Airways, New York Airways, and Chicago Helicopter Airways were among the first operators. However, operations in this sector proved troublesome, with profits elusive and service often subject to noise complaints and accidents.[2][3] S-61s also saw service in the search and rescue (SAR) sector in various countries. Third-party companies have often converted individual airframes by shortening the fuselage to bolster its lift capacity. Governmental organizations have procured the S-61: the United States Department of State was a prominent operator of the type into the twenty-first century.
Design and development
Background
In September 1957, Sikorsky was awarded a United States Navy development contract for an amphibious anti-submarine warfare (ASW) helicopter capable of both detecting and attacking submarines.[1] On 11 March 1959, the XHSS-2 Sea King prototype made its maiden flight. Production deliveries of the HSS-2 (later designated SH-3A) commenced during September 1961. The initial production aircraft being powered by a pair of General Electric T58-GE-8Bturboshaft engines, capable of providing up to 930 kW (1,250 shp).[4]
Sikorsky quickly decided to pursue development of a dedicated commercial model of the Sea King.[1] In fact, two prime models were produced: the land-based S-61L and the amphibious S-61N.[5] On 2 November 1961, the S-61L conducted its maiden flight; it was 4 ft 3 in (1.30 m) longer than the HSS-2 to facilitate the carriage of a substantial payload of freight or passengers. Initial production S-61Ls were powered by two 1,350 shp (1,010 kW) GE CT58-110 turboshafts, the civil version of the T58. The S-61L features a modified landing gear that eliminated the float stabilisers.[6]
On 7 August 1962, the S-61N performed its first flight.[1] Being 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 the Mk II standard, which was outfitted with more powerful CT58-140 engines that provided superior performance in hot and high conditions, along with incorporating measures to dampen vibration and various other refinements.[6]
Further development
A Coulson Aircrane S-61L dropping water during the Australia bushfire season.
Additional civil models of the S-61 would soon be developed. The Payloader, a stripped-down version optimized for aerial crane work, was the third civil model of the S-61 produced.[1] The Payloader features the fixed undercarriage of the S-61L, but with an empty weight almost 2,000 lb (910 kg) 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 (1.3 m) to increase its single-engine performance and external payload.[7] The improved lift performance lent itself to utility operations, particularly in the construction market.[8]
A unique version is the S-61 Shortsky, a conversion of S-61Ls and S-61Ns performed by Helipro International.[1] VIH Logging was the launch customer for HeliPro's Shortsky, which performed its first flight during February 1996.[9]
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 2,000 lb (907 kg) load, fly 15 kn (28 km/h) faster and increase range 61 nmi (113 km).[7]
During the 2000s, the S-61T modernisation emerged. This model includes composite main rotor blades, a modular wiring harness, and (optionally) a glass cockpit; these changes reportedly boost the helicopter's lift capability as well as increase its speed. During June 2010, the United States Department of State signed a purchase agreement for up to 110 modernized S-61Ts, which will perform both passenger and cargo transport missions in support of its worldwide operations.[10][11]
Operational history
Newsreel of an S-61N performing an emergency water landing
The first civil operator to adopt the S-61 was Los Angeles Airways, which introduced the type to service on 11 March 1962.[12] The company had reportedly bought them from Sikorsky at a unit price of $650,000 each. Sikorsky's foremost competitor for sales was Boeing Vertol with their Vertol 107 twin-rotor helicopter, which arrived on the market shortly after the S-61.[3] While regarded as one of the most successful American scheduled helicopter airlines even by the 21st century, following several accidents involving its S-61s, Los Angeles Airways ceased operations in 1971.[2]
Even prior to the receipt of approval from the Federal Aviation Administration (FAA), the S-61 had been purchased by a number of airliners, including Los Angeles Airways, New York Airways, and Chicago Helicopter Airways.[3] It was promoted as being the first U.S. helicopter designed specifically as a commercial airliner. The S-61 seated 25 passengers and had an estimated direct operating cost of 8¢ per seat mile.[3] The operating costs of civil helicopter were considered to be crucial, even prior to the S-61's introduction, as airlines had typically been unable to achieve profitable helicopter routes and became dependent on government subsidies to operate, reportedly due to the limited capacity and high operating expense of the available helicopters. It was hoped that the S-61 would be able to noticeably improve on economics compared to its predecessors.[3]
New York Airways ordered an initial batch of ten S-61s to serve its helicopter routes.[3] Perhaps most prominently, it started operated flights from a heliport on the 59-story Pan Am Building, and at one point planned to perform as many as 360 helicopter flights per day.[13] Launched on 21 December 1965, the operation soon proved to be unprofitable, only carrying an average of only eight passengers, leading to the heliport's closure in 1968. While flights were resumed during February 1977, an accident three months later involving a S-61 helicopter that collapsed and flipped onto its side, killing five people, led to the heliport being closed indefinitely.[13] Two years later, New York Airlines ended helicopter operations.[14]
Between 1978 and 1986, a S-61 was used for an Airlink service between the London airports of Heathrow and Gatwick over a distance of 42 miles; it was operated jointly by British Caledonian Airways and British Airways Helicopters in partnership with the British Airports Authority (BAA).[16] While the operation proved valuable prior to the opening of the M25 motorway, its noise led to it being a point of controversy, the route overflying several densely populated areas of London. Initially, the Civil Aviation Authority banned flights between 9.15pm and 6.30am to limit its impact, but the whole service came to an end after its licence to operate was revoked by the Transport Secretary on 6 February 1986.[16] Numerous S-61s were also operated on other routes in Britain, often between the mainland and off-shore locations such as the Channel Islands, the Scilly Islands, and various oil rigs in the North Sea.[17]
Various government organisations have also adopted the S-61. During the 2010s, the United States Department of State procured in excess of 100 S-61Ts in support of its worldwide operations. These rotorcraft have been operated in both Iraq and Afghanistan, amongst other locations.[20][21]
Variants
S-61L
Non-amphibious civil transport version. It seats up to 30 passengers.[22]
S-61L Mk II
Improved version of the S-61L, cabin equipped with cargo bins.[6]
modernized upgrade performed by Sikorsky and Carson Helicopters. Upgrades include composite main rotor blades, full airframe structural refurbishment, conversion of folding rotor head to non-folding, new modular wiring harness, and Cobham-supplied glass cockpit avionics. Initial models converted were S-61N.[11]
AS-61N-1 Silver
License-built model of the S-61N by Agusta, with a shortened cabin.
N300Y, the Los Angeles Airways prototype of the Sikorsky S-61L Helicopter, lifting off from the Disneyland Heliport
1960s
On 2 February 1966, Flight 17, operated by a Sikorsky S-61 helicopter registration AP-AOC, crashed on a scheduled domestic flight in Faridpur Division, East Pakistan after the main gearbox failed, killing 23 of the 24 passengers and crew on board.
On 10 December 1966, operated by a Sikorsky S-61 helicopter registration AP-AOA, crashed on a scheduled domestic flight in DhakaEast Pakistan.[56]
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.[58]
On 10 May 1974 a KLM Helikopters S-61, registration N PH-NZC, crashed en route to an oil rig in the North Sea. None of the six people on board (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 (69 kPa) 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.[59] The aircraft was recovered from the North Sea floor. It was sold to Carson Helicopter in the USA and re-registered as N87580.[60]
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.[61] The landing gear collapse resulted from 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.[13] 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 for the Ship-Service Role off the shores of the Western Cape by the company "Court Helicopter" which was later amalgamated with CHC.[62]
On 20 March 1985, an Okanagan Helicopters S-61N (C-GOKZ) ditched in the Atlantic Ocean off Owls 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.
On 25 July 1990, a British International Helicopters Sikorsky S-61 registration 'G-BEWL' 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.[63]
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.[64][65][66]
^Apostolo, G. "Sikorsky S-61".The Illustrated Encyclopedia of Helicopters. Bonanza Books, 1984. ISBN0-517-43935-2.
^ abcSchneider, Daniel B. "F.Y.I.", 25 July 1999. Accessed 30 September 2007. "Q. Back in the 1960s and 70s, helicopters bound for Kennedy International Airport used to take off from a deck atop the old Pan Am Building. Why was the service halted? A. As many as 360 helicopter flights a day were planned by New York Airways after the 59-story Pan Am building was completed in 1963, but a bitter public outcry delayed the first few flights until December 21, 1965 ... The operation proved unprofitable since the helicopters carried an average of only eight passengers, and the heliport, which had cost $1 million to build, closed in 1968 ... After another round of hearings – and renewed protests – flights resumed in February 1977. Three months later, the landing gear on one of the Sikorsky S-61 helicopters collapsed while passengers were boarding, flipping it on its side and sending a 20-foot rotor blade skidding across the roof and over the west parapet wall ... Within hours, the heliport was closed indefinitely."
^"USFA Fatality Notice". United States Fire Administration. 6 August 2008. Archived from the original on 17 September 2008. Retrieved 11 August 2008.
^"FAA/NTSB Investigations". Los Angeles Injury Law Firm (See post #4 titled "Nine Firefighters Believed Dead After Helicopter Crash in California"). 6 August 2008. Archived from the original on 24 June 2007. Retrieved 3 February 2009.
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