Can one of you guys provide me with a save file so I can replicate the bug?
Im not that far anymore, I started new game :S I hope this guy answers!
There’s a Half-Life 2 style crate of infinite rockers at the mouth of the cave, right behind the forward-most rock. You have enough ammo, but you don’t have crap for cover.
As well, the helicopter takes an obscene amount of damage. It’ll smoke, then it’ll smoke and burn, and then I think it’ll smoke, burn and shed parts and it will STILL stay in the air until you shoot it some more. In Half-Life, I could take down a chopper with a single fully-charged Tau Cannon shot or around three or four rockets. Here, I need around 15 and the charged-up Tau Cannon shot doesn’t seem to do much more than the rockets.
Basically, think less Half-Life chopper fights and more Half-Life 2 Hunter Chopper boss fight, but without the roller mines.
I used cheats to get a gluon gun, increased damage to 1000 / shot, and the helicopter crashes insanely fast, i mean it doesn’t just die instantly: the moment i shoot it it superspeeds into the side of cliff. It might be related.
I hope they will fix this soon, because of this glitch, many people stopped playing…
I can give Save after helicopter is destroyed!
I had the same problem. Unfortunately my only save before eventually destroying it is at the end of c2a4f.
This glitch also occurred on my first playthrough. I just laughed, facepalmed, blew it up, and continued on.
Heh, I had good laugh too 
I uploaded some of my save files. The helicopter just hovers, firing the cannon from what it sounds like although no bullets are hitting me.
Uploaded my autosave and the last few saves here:
https://skydrive.live.com/redir?resid=1973992AD6E248B0!534&authkey=!ADA8_t4deI9q9k8
I had this bug on my 2nd playthrough, I was like “maybe it noticed some alien and started shooting at it, like it was shooting a tentacle in HL?” but it seems this is a bug. Also, once destroyed, the chopper falls to the “floor” of the map, and it looks like it’s lying on the air.
Yeah, first I though that its shooting some headcrabs on cliffs, but nope…
I am able to replicate this glitch.
https://ge.tt/6tdQ3cO/v/0
load this save, damage the copter a bit (2 fully charged tau cannon shots is enough) and run into the bunker to get some HP back.
after you come out, the copter will be glitched.