Why an Anti-collision System might not have Prevented DCA Plane Crash

Emergency response units search the crash site of the American Airlines plane on the Potomac River near Reagan National Airport on Thursday.
11:09 JST, January 31, 2025
The crash late Wednesday of American Eagle Flight 5342 near Reagan National Airport has brought fresh attention to a technology known as a Traffic Alert and Collision Avoidance System, or TCAS, which was developed to prevent airplanes from colliding in midair after several tragic crashes.
Funded by the Federal Aviation Administration, the transponder-based system became mandatory for large airliners in the United States in 1993, and it is now common on all manner of commercial aircraft globally. Most military helicopters – such as the Black Hawk that was involved in the collision with the aircraft – aren’t equipped with TCAS but have transponders that can interact with planes.
Here’s what we know about the technology and why it might not have prevented Wednesday’s collision.
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How does TCAS work?
The system is designed to continuously detect other aircraft within a range of about 12 miles, which goes well beyond the usual safety buffer of three to five miles apart horizontally and 1,000 feet vertically. The transponders on planes automatically communicate with one another, relaying their position in the air, but pilots determine what course to take.
If two planes are converging, TCAS displays their position to flight crews and sends an alert when the system projects that they are 30 to 60 seconds from an impact, according to researchers and government reports. At that point, the pilots get an audio notification: “Traffic, traffic.” If the planes continue on a course toward a collision, TCAS will suggest a “resolution advisory” on maneuvering vertically once it projects they are 15 to 30 seconds from a crash.
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How effective is the technology?
Aviation experts say TCAS has been a powerful tool in preventing crashes. There haven’t been any midair collisions in the United States involving an aircraft equipped with a functioning anti-collision system and another with a working transponder since the TCAS technology became mandatory, according to Wesley Olson, leader of the transportation and resilience group at MIT’s Lincoln Laboratory, which has helped develop the technology over decades.
Still, TCAS isn’t completely fail-safe. In 2002, a cargo plane and a Russian passenger jet – both equipped with TCAS – collided over Überlingen, Germany, killing all 71 crew members and passengers. An investigation into the accident pointed out that an air traffic controller instructed the Russian jet to descend, while its TCAS guidance – based on feedback from the cargo plane’s transponder – directed the crew to climb. By disregarding its own TCAS, the Russian pilots “defeated the coordination logic” between the two planes, according to a paper by MIT researchers.
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Would TCAS have alerted American Eagle Flight 5342 before the collision?
Many details of the collision remain unclear, but it appears the passenger plane and Black Hawk helicopter collided at a low altitude – and TCAS isn’t designed to send alerts in those cases.
Jim Brauchle, an Air Force veteran who flew cargo planes and now represents plane crash victims as an attorney at Motley Rice, noted that helicopters are supposed to fly at an altitude at or below 200 feet along the final approach to National Airport’s Runway 33. Even if airplane and helicopter pilots are doing everything right, they “potentially only have separation of a couple hundred feet,” he said.
If the plane was flying below 500 feet above the ground, the system probably wouldn’t have sent an audio alert or a resolution advisory to pilots, according to system manuals and experts.
“It is very unlikely that the TCAS on the airliner would have provided any alerts to the flight crew,” said Olson of MIT.
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Why doesn’t TCAS send alerts at lower altitudes?
Takeoff and landing are stressful times for pilots, and TCAS has been designed to avoid sending alerts at lower altitudes to avoid distracting the crew.
“The feeling was that anything that’s making a warning-level alert, a red light flashing, would be very distracting when they should just be flying the aircraft,” Olson said.
At lower altitudes, planes also have the benefit of air traffic controllers to make sure that flight paths aren’t in conflict, Olsen said, adding: “You want pilots to focus on landing.”
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Could more advanced technology have made a difference?
The FAA is working to develop a next-generation system called ACAS X that uses machine learning and would be designed to send fewer unnecessary alerts, while the agency has also funded a program for helicopters, ACAS Xr, that would provide alerts down to 200 feet above the ground, Olsen said.
“How do we accelerate getting that into the helicopter community?” Olson said. “It’s specifically designed exactly for this case, to give helicopter pilots better situational awareness to avoid other aircraft.”
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