The Boeing Vertol CH-46 Sea Knight is an American medium-lift tandem-rotor transport helicopter powered by twin turboshaft engines.

During June 1958, the US Army awarded a contract for the construction of ten production-standard aircraft, designated as the YHC-1A, based on the V-107; this initial order was later cut down to three YHC-1As.

During 1961, the US Marine Corps (USMC), which had been studying its requirements for a medium-lift, twin-turbine cargo/troop assault helicopter, selected Boeing Vertol's Model 107M as the basis from which to manufacture a suitable rotorcraft to meet their needs.

The Sea Knight was operated by the USMC to provide all-weather, day-or-night assault transport of combat troops, supplies and equipment until it was replaced by the MV-22 Osprey during the 2010s.

The USMC also used the helicopter for combat support, search and rescue (SAR), casualty evacuation and Tactical Recovery of Aircraft and Personnel (TRAP).

The Sea Knight also functioned as the US Navy's standard medium-lift utility helicopter prior to the type being phased out of service in favor of the MH-60S Knighthawk during the early 2000s.

During 1955, Piasecki was officially renamed as Vertol Corporation (standing for vertical take-off and landing); it was around this time that work commenced on the development of a new generation of tandem rotor helicopter.

During that year, construction of a prototype, powered by a pair of Lycoming T53 turboshaft engines, each one being capable of producing 877 shp (640 kW), commenced.

[3] In order to garner publicity for the newly developed rotorcraft, it was decided to use the prototype to conduct a series of publicised flight demonstrations during a tour across the United States and several overseas nations.

During June 1958, it was announced that the U.S. Army had awarded a contract to Vertol for the construction of ten production-standard aircraft based on the V-107, which were designated YHC-1A.

[1] During 1960, the U.S. Marine Corps evolved a requirement for a medium-lift, twin-turbine troop/cargo assault helicopter to replace the various piston-engined types that were then in widespread use with the service.

[1] The CH-46A was equipped with a pair of T58-GE8-8B turboshaft engines, each being rated at 1,250 shp (930 kW); these allowed the Sea Knight to carry up to 17 passengers or a maximum of 4,000 pounds (1,815 kg) of cargo.

This model featured various improvements, including modified rotor blades and the adoption of more powerful T58-GE-10 turboshaft engines,[1] rated at 1,400 shp (1,040 kW) each.

[6] During late 1967, the improved model was introduced to the Vietnam theater, where it supplemented the U.S. Marine Corps' existing CH-46A fleet, which had proven to be relatively unreliable and problematic in service.

This model featured fiberglass rotor blades, reinforcement measures throughout the airframe, along with the refitting of further uprated T58-GE-16 engines, capable of producing 1,870 shp (1,390 kW) each; in addition, several CH-46Es were modified to double their maximum fuel capacity.

[6] Starting in the mid-1990s, the Dynamic Component Upgrade (DCU) programmes was enacted, focusing on the implementation of strengthened drive systems and modified rotor controls.

[5] On 15 December 2006, Columbia Helicopters, Inc acquired the type certificate for the BV 107-II, and with the help of Piasecki eventually developed an original design, the Model 107-III, remanufactured from older airframes.

[13] CH-46s transported personnel, evacuated wounded, supplied forward arming and refueling points (FARP), performed vertical replenishment, search and rescue, recovered downed aircraft and crews and other tasks.

During the Vietnam War, the CH-46 was one of the prime US Marine troop transport helicopters in the theater, slotting between the smaller Bell UH-1 Iroquois and larger Sikorsky CH-53 Sea Stallion and progressively replacing the UH-34.

Within three days the accident investigators had determined that the mounting brackets of the main transmission had failed, allowing the front and rear overlapping rotors to intermesh.

The following day another CH-46A experienced a similar incident at Marble Mountain Air Facility leading to the type being grounded for all except emergency situations and cutting Marine airlift capacity in half.

The team recommended structural and systems modifications to reinforce the rear rotor mount as well as installation of an indicator to detect excessive strain on critical parts of the aircraft.

[25] According to authors Williamson Murray and Robert H Scales, the Sea Knight displayed serious reliability and maintenance problems during its deployment to Iraq, as well as "limited lift capabilities".

Prior to the complete withdrawal of U.S. forces, all seven were rendered unusable and abandoned at Kabul International Airport and are seen in many videos and pictures online.

[38][39] When the larger CH-147 Chinook was procured by the Canadian Forces in the mid-1970s, the Voyageur fleet was converted to Labrador specifications to undertake SAR missions.

[39] The Labrador was fitted with a watertight hull for marine landings, a 5,000 kilogram cargo hook and an external rescue hoist mounted over the right front door.

Nonetheless, as a search and rescue helicopter it endured heavy use and hostile weather conditions; which had begun to take their toll on the Labrador fleet by the 1990s, resulting in increasing maintenance costs and the need for prompt replacement.

However, in 1998, a CH-113 from CFB Greenwood crashed on Quebec's Gaspé Peninsula while returning from a SAR mission, resulting in the deaths of all crewmembers on board.

[43] Among the diversity of tasks was commuter service in the mid-1960s from the roof of the Pan Am skyscraper in Manhattan to JFK Airport in Queens,[44] pulling a hover barge,[45][46] and constructing transmission towers for overhead power lines.

[47] In December 2006, Columbia Helicopters purchased the type certificate of the Model 107 from Boeing, with the aim of eventually producing new-build aircraft themselves.