Reply #14: Here's one reference to the vulnerability

[htuttle]

However, apparently it was the oil tank, not the fuel tank, that I'd read about:

Due to its heavy armor, the Hind is nearly impervious to guerrilla small arms unless the guerrillas can fire down at the helicopters using weapons positioned high on the sides of mountains.10 The Hind has only three known vulnerable points: the turbine intakes, the tail rotor assembly, and an oil tank inexplicably but conveniently located beneath the red star on the fuselage.(11)

http://www.airpower.maxwell.af.mil/airchronicles/aureview/1985/jan-feb/nelson.html

Point is, it's little details like this that anti-occupation forces begin to learn as the occupation goes on.