It only takes one Round in the exact right place.....and when you got lots and lots of guys emptying their mag's at you with AK's, long belts of ammo for RPD's, and other shoulder arms....from very close range...they can be very effective up to about 1,000 feet AGL.
Add into the mix some .51 Caliber Machineguns.....which are very common in that part of the World (think Technicals if you will)....they can knock big bits off your Bird up to about 5,000 feet.....as memory serves me. Up close those Tracers look like Soccer Balls going by!
The Osprey has ballistic tolerance built into it.....and the crew wears Body Armor....but there are plenty of exposed hide that could be hit. Flight helmets are good...but not bullet proof.
The ambush cited by Lone is an example of low tech, but smart folks, taking on more sophisticated forces and winning. They basically used Cell Phones and small arms to put a whacking on the Army Apaches.....ultimately capturing two Pilots after they were shot down.
Apache pilots turned back by curtain of fire