○○○fleet⪢⪢
The United States military operates a diverse fleet of helicopters for various missions including transport, attack, reconnaissance and search-and-rescue.
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http://qindex.info/i.php?x=26318
○○○backbone⪢⪢
Though introduced 30 years after helicopters were a part of the U.S. air fleet, the CH-47 Chinook became the backbone of numerous major operations carried out by the military.
http://qindex.info/i.php?x=26416
