From a BAE release:
BAE Systems is developing acoustic hostile fire indicator technology to protect low-flying military helicopters from small-arms fire.
Most aircraft have missile-warning and countermeasures systems but are vulnerable to small arms-fire, which has been difficult to detect. Crews often dont know they are being fired on until its too late. Combining acoustic data, noise reduction, and location algorithms with information from existing sensors, BAE Systems is developing a solution that will give helicopter crews multiple indications of hostile fire.
In live-fire tests at a private range in the United States, an HFI system measured acoustic data and accurately detected threats (see video). Further testing on military aircraft, using additional sensing technologies and techniques, will make the system more robust and improve its detection capabilities.
Officials told me the system is a "blended solution" that combines existing defensive systems -- which I assume include missile and radar warning defenses -- with a noise sensor that's been programmed to cancel out wind and rotor noise and direction finding software to point out the shot's location.
[Here's a better video of a test shoot where you can see the helicopter jinking to avoid the fire.]
I know this is possible because I went to a demo day a couple years ago on sniper detection systems for ground vehicles. There are lots of different kinds that deliver lots of different information, but the systems work well and are robust.
BAE says the shot detection system can display the location information "on existing cockpit instruments" to help a pilot take evasive maneuvers to avoid the gunfire.
Officials said this is not a current Pentagon program and the DoD has expressed some interest in the technology.