The Boeing CH-46 Sea Knight helicopter has served the US Navy and Marine Corps faithfully since the early 1960s. The Sea Knight was originally designated the H-49. This venerable aircraft's primary mission areas in the Navy (as the H-46D) include Combat Logistics Support and Vertical Replenishment (VERTREP), Search and Rescue, and Special Operations. As a Marine Corps platform, the H-46E is used primarily during cargo and troop transport. The unique tandem-rotor design of the Sea Knight permits increased agility and superior handling qualities in strong relative winds from all directions, allowing, in particular, rapid direction changes during low airspeed maneuvering. This capability has resulted in the safe, efficient and graceful transfer of many millions of tons of cargo and many thousands of passengers over the years.

Readily identified among current Navy and Marine Corps helicopters are the H-46 series Sea Knights, with their tandem rotor configuration setting them apart from the single rotor design of other Navy/Marine helos. Tandem rotors have been a feature of all production helos built by Boeing/Vertol, and its original predecessor company, Piasecki.

The H-46 “Sea Knight” helicopter is one of the largest helicopters in the US Navy inventory. The “Sea Knight” is a twin-turbine powered, dual-piloted, tandem rotor helicopter designed by the Boeing Company Vertol Division. The aircraft is 16 feet 8 inches tall. There are six rotor blades on the aircraft, each measuring 25 feet 6 inches. With blades spread, the aircraft is 84 feet 4 inches long. The average weight of the H-46 is 18,000 pounds, with a maximum lift capability of 6,000 pounds. It can carry 25 combat-loaded troops, or can be outfitted to carry medical evacuation litters in case of disaster. It has the fuel endurance to stay airborne for approximately two hours, or up to three hours with an extra internal tank.

The helicopter has the ability to land and taxi in the water in case of emergency, and is able to stay afloat for up to two hours in two-foot seas. Because of its tandem rotor design, the “Sea Knight” is an extremely versatile aircraft. It is able to excel in various flight maneuvers, such as rearward and sideward flight, while other helicopters are extremely limited. This makes the helicopter ideal for its primary Navy mission of vertical replenishment.

In 1958, Vertol completed a company sponsored prototype of a new helicopter design, powered by two Lycoming T-53 turbine engines for potential military or civilian use. The 107 first flew in April, exhibiting most of the basic configuration characteristics to be found in all of its 107-series successors. The Army ordered three YHC-1As which were developed as GE-T-58-powered military evaluation vehicles under a Bureau of Aeronautics contract. First flying in August 1959, the YHC-1As were followed by an improved commercial/export model, the 107-11.

During 1960, the Marines evolved a requirement for a twin-turbine troop/cargo assault helicopter to replace the piston engine types then in use. Following a design competition, Boeing/Vertol was selected to build its model 107M as the HRB-1, early in 1961. It retained the general configuration of its predecessors, including the aft sponsons carrying the fixed main gear, a fixed nose gear and built-in emergency flotation provisions so it could land and take off from the water in light seas. Special features included power-operated blade folding, integral cargo handling provisions, a rear loading ramp that could be left open in flight, personnel recovery and rescue equipment, and provisions for hoisting 10,000 pounds externally. These and other features marked a significant step forward in helicopter capability in the time period.

First flight in August 1962 was followed by a change in designation to CH-46A, development flight testing, (including the first NPE in January 1963), and BIS trials beginning in March 1964. Fleet introduction of CH-46As with the Marines and UH-46As with the Navy took place in November 1964. The latter were modified for use in the vertical replenishment role.

The CH-46 Sea Knight was first procured in 1964 to meet the medium-lift requirements of the Marine Corps in Viet Nam with a program buy of 600 aircraft. The aircraft has served the Marine Corps in all combat and peacetime environments. However, normal airframe operational and attrition rates have taken the assets to the point where a medium lift replacement is required. The safety and capability upgrades are interim measures to allow continued safe and effective operation of the Sea Knight fleet until a suitable replacement is fielded.

Production continued in subsequent years, along with modifications to improve some of the H-46's characteristics. With service in Southeast Asia came installation of guns and armor. Increased power requirements were met by installation of higher powered T-58-GE-10s in the CH/UH-46D models, which also featured new cambered (droop snoot) rotor blades. The final CH-46E, with further increased power, was preceded by the last production version, the CH-46F, before production was completed with delivery of the 524th H-46 in February 1971.

The early A models now serve as search and rescue HH-46As. CH-46s equip Marine Reserve squadrons, and conversion of earlier aircraft to the new CH-46E version was completed with fiberglass blades slated added to its other improvements.

The mission of the CH-46E Sea Knight helicopter in a Marine Medium Helicopter (HMM) squadron is to provide all-weather, day/night, night vision goggle (NVG) assault transport of combat troops, supplies, and equipment during amphibious and subsequent operations ashore. Troop assault is the primary function and the movement of supplies and equipment is secondary. Additional tasks are: combat and assault support for evacuation operations and other maritime special operations; over-water search and rescue augmentation; support for mobile forward refueling and rearming points; aeromedical evacuation of casualties from the field to suitable medical facilities.

The current H-46 Sea Knight Block Upgrade provides for installation of increased fuel capacity stub wings and an emergency helicopter flotation system. The H-46 Dynamic Component Upgrade provides for safety, engineering and electronic improvements. Modifications to improve the Sea Knight continued during 1998. The installation of a new rotor head and upgraded transmission has improved flight and rotor controls, and eliminated current rotor head inspections. Sixty percent of the H-46 fleet had been modified by 1998, and the program completed fleet installations in FY 2000. Two additional upgrades to the H-46 include installation of the ARC-210 radio and night vision goggle heads up display, concurrently being installed with the integrated communication navigation control system modification. The communication navigation control system installations were 60 percent incorporated into fleet aircraft by 1998. These modification programs completed by the end of FY 2000. Additionally, program managers oversaw a blade-balancing modification to the H-46 during 1998. This modification reduced aircraft vibrations and increased reliability of the airframe and rotating subsystem components.

The CH-60 Fleet Combat Support Helicopter will complement and eventually replace the Navy's aging fleet of H-46 helicopters. As a result of the advanced airframe life of the H-46 fleet, the Navy's logistics helicopter force is experiencing a near-term inventory shortfall.

The Navy Air Systems Command ordered the grounding of all CH-46 helicopters on 18 August 2002 as a precaution after discovery of a crack in a rotor component of a Sea Knight at the Cherry Point Marine Corps Air Station in North Carolina. A similar problem was found a few days later in a CH-46 deployed aboard the amphibious assault ship Belleau Wood in the Persian Gulf area. Inspection of all 291 CH-46 Sea Knight helicopters in the Navy and Marine Corps found only one with a flaw of the kind that triggered the temporary grounding of the fleet, and the full fleet was returned to service.

Source: http://www.globalsecurity.org/military/systems/aircraft/ch-46.htm