We've got a problem. Our choppers are crashing for reasons we've not understood yet.Degtyarev
Stingray squad: find the source of the charges which hit the two helicopters is a main mission in S.T.A.L.K.E.R.: Call of Pripyat.
You acquire the Stingray squad: find the source of the charges which hit the two helicopters mission by completing both Stingray 4: investigate the crash site and Stingray 5: investigate the crash site. This mission completes on its own near the end of the game when Strelok arrives at the Laundromat.