cargo (for UH-60L/M) externally by sling. The Black Hawk helicopter series can perform a wide array of missions, including the tactical transport of troops, electronic warfare, and aeromedical evacuation. A VIP version known as the VH-60N is used to transport important government officials (e.g., Congress, Executive departments) with the helicopter's call sign of "Marine One" when transporting the President of the United States. In air assault operations, it can move a squad of 11 combat troops or reposition a 105 mm M119 howitzer with 30 rounds ammunition, and a four-man crew in a single lift. The Black Hawk is equipped with advanced avionics and electronics for increased survivability and capability, such as the Global Positioning System. The UH-60 can be equipped with stub wings at the top of fuselage to carry fuel tanks or various armaments. The initial stub wing system is called External Stores Support System (ESSS). It has two pylons on each wing to carry two and two tanks in total. The four fuel tanks and associated lines and valves form the external extended range fuel system (ERFS). U.S. Army UH-60s have had their ESSS modified into the crashworthy external fuel system (CEFS) configuration, replacing the older tanks with up to four total crashworthy tanks along with self-sealing fuel lines. The ESSS can also carry of armament such as rockets, missiles and gun pods. The ESSS entered service in 1986. However, it was found that the four fuel tanks obstruct the field of fire for the door guns; thus, the external tank system (ETS), carrying two fuel tanks on the stub wings, was developed. The unit cost of the H-60 models varies due to differences in specifications, equipment and quantities. For example, the unit cost of the Army's UH-60L Black Hawk is $5.9 million while the unit cost of the Air Force HH-60G Pave Hawk is $10.2 million. Operational history Australia Australia ordered fourteen S-70A-9 Black Hawks in 1986 and an additional twenty-five Black Hawks in 1987. The first US produced Black Hawk was delivered in 1987 to the Royal Australian Air Force (RAAF). de Havilland Australia produced thirty-eight Black Hawks under license from Sikorsky in Australia delivering the first in 1988 and the last in 1991. In 1989, the RAAF's fleet of Black Hawks was transferred to the Australian Army. The Black Hawks saw operational service in Cambodia, Papua New Guinea, Indonesia, East Timor and Pakistan. In