The crash of an Army helicopter in Iraq on Friday, coupled with two others that were struck by ground fire in the last two weeks, reflects a growing threat by insurgents taking aim at American aircraft, military officials said. Twenty-two Americans died in the three incidents. Even beyond these aircraft — one of which had already landed when it was struck — there has been a sharp increase in recent weeks in attacks against other American helicopters, whose defenses are limited, the officials said. The defenses, which can include radar-detection systems and chaff and flare dispensers, are designed to guard against missiles, the officials said, but not rocket-propelled grenades. If the Black Hawk that went down near Tikrit on Friday, killing all six soldiers aboard, was struck by ground fire, the likely source would have been a grenade. Full Story