An emerging danger for jetliners and other planes are newer, smaller aircraft that pilots can't see coming.

Drones.

Boeing is trying to develop a system to detect those drones and provide some advice to a pilot on what to do to avoid a collision.

Preventing mid-air collisions has been a goal of aviation from the beginning. Visual flight rules require pilots to see and avoid each other.

Early radar could show airplanes on a screen to an air traffic controller. Later, airplanes would identify themselves to airport traffic controllers and to other airplanes by squawking a numerical code.

For several decades, airliners have the additional protection of the Traffic Collision Avoidance System (TCAS). It operates at several levels. At higher levels, a computer box in one plane can use algorithms. Those calculate the trajectory of its plane and the trajectory of another plane on a collision course. TCAS boxes on both planes can talk to each other and give directions to pilots to avoid conflict. For example, one box could tell a pilot to descend and the other to ascend.

WATCH: How Boeing is helping pilots avoid drones

But in a world where the number of planes is expected to double over the next 20 years, comes drones that are not broadcasting that they’re there. Smaller drones are required to stay five miles away from airports and below 400 feet. But for years now we’ve heard reports from airline pilots encountering drones, usually on approach to an airport.

Some drones fly over the top of airliners at several thousand feet.

Boeing is testing technology to spot these drones before a collision and warn pilots. It's called "Detect and Advise." If a camera on board the plane picks up a drone or other unidentified flying object, an indicator might display on a cockpit screen showing its location, likely accompanied by some sort of audio alert. It could also advise some kind of action. And perhaps like TCAS, it could recommend some sort of evasive action by the pilot.

The testing is still in the very early stages. For now, the data just goes into a computer box which is forwarded to Boeing researchers in Australia who are trying to develop the system.