For more than 20 years, the Traffic Alert and Collision Avoidance System (TCAS) has provided a last line of defense. The cockpit-alert system coordinates with other TCASs to recommend evasive maneuvers. It then reassures with the words, "Clear of conflict."
TCAS has been highly successful. Studies show the risk of a mid-air collision is reduced by 90 percent with TCAS. Since the introduction of TCAS in the United States in 1989, no U.S. commercial air carrier collisions have occurred.
But TCAS needs an upgrade to accommodate new capabilities that are being introduced in the National Airspace System. With satellite-based NextGen technologies, aircraft are tracked with a higher precision than with radar and safe separation distances may be reduced. This means that we can get aircraft through busy airspace more efficiently.
"We have to do something more suited to NextGen. TCAS is not flexible enough for our evolving airspace," said Neal Suchy, program manager for Airborne Collision Avoidance System X (ACAS X), the next generation of collision avoidance.
The FAA is designing ACAS X with the ability to adapt to different kinds of aircraft and the more efficient operations made possible with NextGen technology.
In August 2013 the FAA conducted flight tests with an ACAS X prototype. There were more than 100 different encounter scenarios flown with multiple planes that were coming as close as 200 feet vertically and .3 nautical mile horizontally of each other. "The prototype demonstrated that what we’re doing is viable. It works," Suchy said.
The FAA also conducted simulations, drawn from actual TCAS encounters, which show ACAS X has the potential to dramatically decrease unnecessary alerts by one third and cut collision risk in half.
While work continues to optimize and fine-tune the system, a Federal Advisory Committee will begin to develop minimum performance standards for ACAS X. The standards are expected to be formalized in 2018 with flight evaluations to follow the next year.
I kindly recommended to everyone interested in the NextGen development of the FAA to visit this link:
You can also find here another PDF file created by EUROCONTROL about ACAS X.