"It did not take long, however, for the VC and NVA to realise that the Huey was
particularly susceptible to small-arms fire. A high percentage of the 3,300
helicopters lost in the war were brought down in this way, and flying into hot LZs
(landing zones) was a terrifying business. More than 1,000 pilots and 1,100 crew
members lost their lives in Vietnam: a high price to pay for the ease of movement
the helicopter provided."
Source: http://conflictarchaeology1.files.wordpress.com/2011/12/the-huey-helicopter-and-the-vietnam-war.pdf