The Sikorsky S-58DT is a medium-weight multi-role
utility twin pac coupled turbo shaft powered helicopter accommodating up
to 16 passengers and 2 crew. The Sikorsky S-58T is a development of the
single piston-engined Sikorsky S-58 also known as the CH-34. On 30
January 1957, the Sikorsky HSS-1F Seabat flew first. In fact was this
the first tuboshaft-engined Sikorsky S-58. The Sikorsky HSS-1F was a
flying testbed for the General Electric T58 turboshaft engine. The
airframe was modified to accept two GE Model XT58T engines. However the
T58 turboshaft engine gave the Sikorsky S-58 a significant
power-to-weight advantage ratio, just a single turboshaft engined S-58
was converted at that time and the Sikorsky S-58 in production remained
piston-engined. However, the Sikorsky CH-34 was also built and developed
under license in the United Kingdom by Westland Aircraft from 1958. This
company introduced Westland Wessex. The Westland Wessex was derived from
the Sikorsky S-58, a free-turbine supplanting the original piston
engine. The Wessex prototype flew first on 20 June 1958 and the model
entered service in 1961 with the Royal Navy. In 1970, Sikorsky announced
it was developing a turboprop conversion package for the S58. With the
turboprop coversion package, the lifetime of the Sikorsky CH-34
airframes that were withdrawn from use with the military operators
could be lengthed. Turbine conversion with Pratt & Whitney PT6 Twin-Pac,
comprising two PT6 engines and combining gearbox; improved performance
includes greater speed and lifting power, and better hot-and-high
operation. The first S-58T conversion flew on 19 August 1970. The
turboprop-engined S58T was initially fitted with a Pratt & Whitney PT6T3
twinpack turboshaft. Later aircraft were fitted with a Pratt & Whitney
PT6T6 twinpack turboshaft. In April 1971, Sikorsky received FAA approval
for the S-58T Twin Pac-powered turbine conversion for S-58 airframes.
Sikorsky set up a production line to convert customer S58s, offered kits
for S58 operators to perform the conversion and purchased used S58s,
like most of the Sikorsky H-34 Choctaw helicopters that were withdrawn
from use 1972-74 in Germany. The converted H-34 helicopters were offered
for sale as Sikorsky S-58T. One hundred and forty-six conversions, or
conversion kits, were produced until the rights were sold to California
Helicopter International in 1981.
Sikorsky S-58DT s/n 58-1492 was ordered as a Sikorsky HUS-1 Seahorse
under US Navy and US Marine Corps BuNo 149390. In October 1962, the
HUS-1 was redesignated UH-34D and BuNo 149390 entered
service with the USMC as a Sikorsky UH-34D. The Sikorsky UH-34D Seahorse
was powered by a Wright R-1820-84B radial air-cooled engine and able to
carry up to 12 passengers. After Sikorsky UH-34D 149390 was withdrawn
from use by the US Marine Corps, the helicopter was transfered to
Chicago Helicopter Airways and registered N65CH. Since, the aircraft was
rebuilt as a S-58DT powered by the Pratt & Whitney Aircraft of Canada
PT6T-6 Twin Pac coupled turbo shaft. This conversion permitted the
S-58DT to carry up to 16 passengers at a maximum speed some 16 mph (26
km/h) higher than the S-58s 122 mph (196 km/h) over slightly longer
ranges, with the additional safety of a twin-engine power plant. After
the conversion, the helicopter was registered PH-NZE in the Netherlands
with KLM Noordzee Helikopters BV, Schiphol-Oost, on 1 May 1975. KLM Noordzee Helikopters
flew this helicopter out of Schiphol-Oost and the municipal heliport in Den Helder for
serving the oil and gas rigs in the North Sea. On 6 September 1977, the
registered operator name changed to KLM Helikopters BV. In 1980, the
helicopter was sold and registration PH-NZE was cancelled on 12 February
1980. The same year, the Sikorsky S-58DT was registered PK-OBN with
Airfast Services Indonesia P.T., Singapore-Seletar. Airfast operated a
number of Sikorsky S-58T helicopters until they were replaced by Bell
212 and 412 helicopters. After being withdrawn from service as the
last S-58T in service with Airfast, registration PK-OBN was cancelled.
On 8 February 2006, the helicopter was registered N826MW in the USA as a Sikorsky S-58T with
Midwest Helicopter Airways Inc., Willowbrook, Illinois. Registration N826MW was cancelled on 21 January 2010.
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