“Mayday!” came the frantic call from inside the tiny U.S. military attack helicopter after a rocket-propelled grenade fired by an al Qaeda-linked Iraqi lopped off its tail rotor and forced it to crash land in the desert 20 miles northwest of Baghdad. A fierce firefight soon broke out as the insurgents headed toward their prize: the downed AH-6 Little Bird chopper, and about 20 U.S. soldiers, including members of the secret Delta Force, who’d landed