If you get in any helicopter, let's say, a Littlebird, climb to 2,000 feet MSL and shut the engine off, you basically fall straight from the sky with absolutely no response from control inputs. In the real-world, there is a technique that helicopter pilots use that allows them to make a successful forced landing with an engine failure. This maneuver is called "autorotation," and I've included a Wikipedia link for more info on this down below. Obviously, you're not going to be able to successfully land every single time you have an engine failure. A successful landing depends a lot on the circumstances, such as altitude, airspeed, pilot skill, and extent of damage to the aircraft. The point is, however, that the aircraft should not fall in the manner that it does, especially with a high airspeed. Landing a helicopter with a CONTROLLED engine failure in Arma III, is near to impossible -- At least I haven't found a way to do it yet. Furthermore, lowering the collective during an engine failure seems to have no influence over the aircraft's ability to fly. In the real-world, pilots train for engine failures regularly, climbing to a sufficient altitude, lowering the collective, and then performing the aforementioned maneuver to safely land. I highly recommend taking a look at some of the flight dynamics. While arguing over aerodynamics in general is pointless, considering there is no way to EXACTLY replicate flight characteristics in a video game, I do think this is an important issue with the flight model of the game. I understand this is not a flight simulator, but I believe it's important to include, or at least somewhat simulate autorotation as it would add to the dynamics of the game. If a pilots lost engine power, he or she would have a chance, provided they knew how to perform the maneuver and was skilled enough, to potentially make a successful forced landing. As of now, a mid-air engine failure is an automatic death for the pilot and crew on board the aircraft as they have no opportunity to make a successful forced landing.
Autorotation Wiki: http://en.wikipedia.org/wiki/Autorotation_(helicopter)
Example of Autorotation: https://www.youtube.com/watch?v=DozMhsXymUQ {F19348}