The United States Marine Corps has been flying the CH-46 Sea knight helicopter, commonly known as the "Phrog," since 1962. The Phrog is a descendant of Boeing's first turbine-powered helicopter--the Boeing 107 -- which first appeared in 1958, and which is still flying various missions throughout the world. In 1961, the USMC picked the CH-46 as its new medium-lift troop transport, replacing the ageing UK-34. Over 600 Phrogs were delivered to the USMC between 1962 and 1971. By the early 1990s fewer than 250 airframes were still flying in the Corps. 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. 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.