Why Couldn’t the Air Collision System Prevent the Washington Crash?

On the evening of January 29th, a tragic incident occurred in Washington D.C. where a commercial airplane collided with a military helicopter in mid-air, resulting in both aircrafts crashing into the Potomac River. This devastating accident claimed the lives of all 67 passengers, crew members, and military personnel on board. The question arises as to why the Traffic Collision Avoidance System (TCAS) on the aircraft did not prevent this disaster from happening.

According to Chrystal Zhang, Associate Professor of Aerospace Engineering at RMIT University in Australia, the TCAS is a safety system designed to monitor the airspace around an aircraft to prevent collisions with other equipped aircraft. These devices listen and respond to electronic signals transmitted by the system.

The TCAS operates independently from air traffic control systems with the aim of immediately alerting pilots to nearby aircraft and potential mid-air collisions. Since its development in 1974, this technology has undergone several improvements.

The first generation TCAS (TCAS I) monitors the surrounding aircraft and provides information on the position and altitude of any nearby aircraft.

In the event of a collision risk, the system generates a Traffic Advisory (TA) and notifies the pilot, who must then decide the best course of action to avoid the threat.

The advanced second generation TCAS (TCAS II) provides pilots with specific instructions on how to maneuver to avoid a collision, such as descending, climbing, turning, or adjusting speed.

These newer systems are also capable of communicating with each other to ensure coordinated recommendations to each aircraft.

In order to comply with the Chicago Convention, any commercial aircraft must be equipped with a Traffic Collision Avoidance System. However, military helicopters are not bound by the terms of the convention and reports suggest that the crashed military helicopter was not equipped with this system.

Zhang points out that regardless of whether the military helicopter involved in the crash had a collision avoidance system installed, the technology still has limitations, especially in low-altitude airspace below approximately 300 meters.

In the Washington D.C. disaster, the last recorded altitude of American Airlines flight AA5342 before the crash was about 90 meters, while the military helicopter collided with the flight at an altitude of approximately 60 meters.

The limitation of the TCAS at low altitudes is part of the system’s design because it relies on radio altimeter data, which becomes less accurate when close to the ground. This can lead to unreliable collision avoidance instructions.

Another issue is that at such low altitudes, aircraft may not have the option to further descend to avoid a collision.

According to the British Broadcasting Corporation (BBC), the airspace over Washington D.C. is heavily restricted to safeguard national security and important government buildings. Commercial flights are prohibited from flying over landmarks such as the Pentagon, the White House, and other historic sites. Experts describe it as the “most heavily controlled airspace globally.”

However, this airspace experiences high air traffic, with Reagan National Airport nearby being one of the busiest airports in the United States.

Aviation lawyer Jim Brachle, who has handled lawsuits related to Reagan National Airport, told the BBC that the airspace is crowded not only with commercial flights but also with private aircraft and helicopters shuttling frequently. These aircraft often transport high-ranking officials and politicians to sensitive locations.

Brachle stated, “The incoming and outgoing paths here are very narrow. The congestion is severe, with too many planes squeezed into a small space.”

He mentioned that a significant feature of Reagan National Airport is its proximity to the river. Additionally, there is a helicopter flyway crossing the aircraft’s final approach path at altitudes usually around 200 feet (61 meters) or lower.

He explained, “This means two different types of aircraft could enter an extremely small space with almost no safety buffer between them. If one aircraft flies slightly lower and the other slightly higher, they could end up in the same position.”