C-17 Globemaster III
The C-17 is the newest airlift aircraft to enter the Air Force's inventory. The C-17 is a four engine turbofan aircraft capable of airlifting large payloads over intercontinental ranges without refueling. Its design is intended to allow delivery of outsize combat cargo and equipment directly into austere airfields.The C-17 will deliver passengers and cargo over intercontinental distances, provide theater and strategic airlift in both airland and airdrop modes, and augment aeromedical evacuation and special operations missions. The C-17's biggest contribution to the present airlift system will be long range direct delivery. The C-17 is capable of rapid strategic delivery of troops and all types of cargo to main operating bases or directly to forward bases in the deployment area. The aircraft is also able to perform theater airlift missions when required.
The C-17 Globemaster III is the third aircraft to use this designation. Two previous versions of this venerable system have seen service, from the Berlin Airlift (the C-74 Globemaster) to Southeast Asia (the C-124 Globemaster II).
Significant features of the multi-engine C-17 include: supercritical wing design and winglets to reduce drag, and provide increased fuel efficiency and range; receiver in-flight refueling capability; externally blown flap configuration, direct lift control spoilers and high impact landing gear system, which contribute to the aircraft's capability to operate into and out of small austere airfields; forward and upward thrust reverser system that provides backup capability, reduces the aircraft ramp space requirements, and minimizes the interference of dust, debris, and noise on ground personnel activities; cargo door, ramp design and cargo restraint systems that are operable by a single loadmaster and that permit immediate equipment offload without special handling equipment; two-man cockpit with Cathode Ray Tube (CRT) displays that reduce complexity and improve reliability; maximum use of Built-In Test (BIT) features to reduce maintenance and troubleshooting times; and walk-in avionics bay below the flight deck that improves accessibility.
The C-17's system specifications impose a demanding set of reliability and maintainability requirements. These requirements include an aircraft mission completion success probability of 93 percent, only 18.6 aircraft maintenance manhours per flying hour, and full and partial mission capable rates of 74.7 and 82.5 percent respectively for a mature fleet with 100,000 flying hours.
The C-17 measures approximately 174 feet long with a 170-foot wingspan. The aircraft is powered by four fully reversible Pratt & Whitney F117-PW-100 engines (the commercial PW2040 series version is currently used on the Boeing 757). Each engine produces 40,440 pounds of thrust, located on pylons ahead of and below the wing leading edge. The engines are equipped with directed-flow thrust reversers capable of deployment in flight. On the ground, a fully loaded aircraft, using engine reversers, can back up a two percent slope. The thrust reversers direct the flow of air upward and forward to avoid ingestion of dust and debris. Combined with propulsive lift, the advanced thrust reversers enables short landings. The thrust reversers are an integral part of the C-17 nacelle. When thrust reversal is initiated, both fan and core exhausts are redirected. Thrust is directed forward and upward through exposed louvers for maximum reverse thrust. During ground operations, the thrust reversers can be deployed with engines idling, directing engine blast away from personnel working cargo.
Technologically, the heart of the C17 is its propulsive lift system, which uses engine exhaust to augment lift generation. By directing engine exhaust onto large flaps extended into the exhaust stream, the C17 is capable of flying steep approaches at remarkably slow landing speeds. This equates to the aircraft's ability to land pay loads as large as 160,000 pounds on runways as short as 3000 feet.
The "externally blown flap" or "powered-lift system" enables the airplane to make slow, steep approaches with heavy cargo loads. The steep approach helps pilots make precision landings with the aircraft, touching down precisely in the spot desired on limited runway surfaces. This was accomplished by diverting engine exhaust downward, giving the wing more lift. In the flap system, the engine exhaust from pod-mounted engines impinges directly on conventional slotted flaps and is deflected downward to augment the wing lift. This allows aircraft with blown flaps to operate at roughly twice the lift coefficient of that of conventional jet transport aircraft.
Like other military transports, the C-17 uses a "supercritical" wing. These are advanced airfoil designs that enhance the range, cruising speed and fuel efficiency of jet aircraft by producing weaker shock waves that create less drag and permit high efficiency.
In the mid-1970s, NASA Langley developed the winglet concept through wind tunnel research. Winglets are small, winglike vertical surfaces at each wing-tip of an aircraft that enable the airplane to fly with greater efficiency. They curve flow at the wingtip to produce a forward force on the airplane, similar to the sail on a sail boat. The concept was first demonstrated in-flight on a corporate Gates Model 28 Longhorn series Learjet, and further tested on a large DC-10 aircraft as part of the NASA Aircraft Energy Efficiency (ACEE) Program. Winglets were installed on a KC-135A tanker on loan from the Air Force and flight tested at NASA Dryden in 1979 and 1980. Eventually, winglets were applied to the C-17.
Sixteen-thousand pounds of composite materials have been applied to the aircraft. Several of the major control surface and secondary structural components of the C-17 are made of composites. The most direct contribution to C-17 applications was the development of the DC-10 graphite-epoxy upper aft rudders. These rudders have accumulated more than 500,000 flight hours since they were introduced into regular airline service in 1976. The high-time rudder alone has flown for 75,000 hours. The control surfaces of the C-17 follow the same multi-rib configuration as the DC-10 rudders.
The aircraft is operated by a crew of three (pilot, copilot and loadmaster). Cargo is loaded onto the C-17 through a large aft door that accommodates military vehicles and palletized cargo. The C-17 can carry virtually all of the Army's air-transportable, outsized combat equipment. The C-17 is also able to airdrop paratroopers and cargo.
Maximum payload capacity of the C-17 is 170,900 pounds, and its maximum gross takeoff weight is 585,000 pounds. With a payload of 130,000 pounds and an initial cruise altitude of 28,000 feet, the C-17 has an unrefueled range of approximately 5,200 nautical miles. Its cruise speed is approximately 450 knots (.77 Mach).
The design of this aircraft lets it operate on small, austere airfields. The C-17 can take off and land on runways as short as 3,000 feet and as narrow as 90 feet wide. Even on such narrow runways, the C-17 can turn around by using its backing capability while performing a three-point star turn.
The aircraft is operated by the Air Mobility Command with initial operations at Charleston AFB, SC, with the 437th Airlift Wing and the 315th Airlift Wing (Air Force Reserve).
The Air Force announced 23 January 2001 that McGuire Air Force Base, NJ, had been selected as the preferred site for 13 additional C-17 Globemaster III airlift aircraft. Alternative bases for the additional C-17s are Dover AFB, DE, and Charleston AFB, SC. Results of airfield and facility surveys at all Air Mobility Command East Coast bases highlighted the three potential locations. East Coast basing is preferred with this increment of C-17s because of the strategic proximity of the base in relation to the military forces they would deploy, most of which are in the Eastern United States.
The Air Force has a requirement for additional strategic airlift aircraft to help meet mobility requirements. Thirteen C-17s have been allocated to AMC to replace retiring C-141 Starlifter airlift capability assigned to support special operations missions. An additional C-17 aircraft has been allocated to the Air Education and Training Command for training purposes. It will be based at Altus AFB, Okla. The additional C-17s will be assigned to a single active-duty unit collocated with an associate Air Force Reserve unit, with the first aircraft tentatively scheduled for delivery in July 2004.
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