Why TCAS air traffic safety system didn't prevent Washington plane crash - expert
The Traffic Alert and Collision Avoidance System is meant to act as the last defence against mid-air collisions but has its limitations.
Authorities believe everyone on board a passenger plane and a US Army helicopter that collided in mid-air before crashing into a river in Washington DC on Wednesday have been killed.
An American Airlines flight, carrying a total of 64 people, collided with Black Hawk helicopter in Washington DC that was carrying three soldiers on a training mission. All 67 people are thought to have been killed in the crash.
Emergency services are battling freezing conditions to retrieve the bodies. Several members of the figure skating community were believed to be on board the flight, including two world-champion Russian figure skaters were on the American Airlines plane.
An investigation is under way to identify the causes of the crash, but experts are already trying to pinpoint what happened with the American Airlines flight's TCAS system.
Here's what we know about the TCAS system, and possible reasons why it didn't prevent the Washington plane crash.
What is a TCAS system?
A TCAS system, which stands for "traffic alert and collision avoidance system", is meant to act as the last defence against mid-air collisions in passenger planes.
It is a type of airborne collision avoidance system that is legally stipulated by the International Civil Aviation Organisation to be fitted to all aircraft above a certain weight — such as the American Eagle Flight No 5342 regional jetliner.
"TCAS systems are last-resort safety net systems designed to ensure that aircraft do not collide mid-air. The TCAS system on board an aircraft interrogates the transponders of nearby aircraft," Peter Neenan, partner, aviation at UK leading litigation-only law firm Stewarts, told Yahoo News.
Following pilot response, the system "provides the altitude and range", and then issues any needed alerts to the pilots.
"The UH-60 Black Hawk is fitted as standard with the necessary transponder for the TCAS system to work. They are mandatory in the US for commercial aviation like the American Airlines aircraft," he added.
Why didn't the TCAS system work?
In this instance, there is a clear limitation to TCAS. While the system may have been fully functional, the plane being low to the ground meant that the system's highest alert — known as a resolution advisory, or RA — was not triggered.
"Resolution advisories are inhibited during approach to land when the aircraft is below 1,000 feet (+/- 100 ft) above ground level," Neenan explained. "It is likely that the system was partially inhibited during the approach."
It is likely therefore – given the altitude of impact – that the RA function was inhibited, he said. Traffic advisories would still be issued but they are a lower alert.
Neenan added that "the system is usually a reliable system".
When will investigators find out what was responsible?
The National Transportation Safety Board, a US government agency that investigates civil aviation accidents, will be instructed to investigate the accident.
As it is "a long and detailed investigation", Neenan explained, and could take up to two years to complete.
"It is too early to speculate on why the aircraft hit each other, but they will certainly look at the role of air traffic control, TCAS and the crew for the aircraft," he added.
What will this mean for aviation safety in the future?
While research into collision avoidance systems like TCAS has been ongoing since the 1950s, crashes like the one on Washington still happen.
Following the 2002 Uberlingen mid-air collision in southern Germany, where all of the passengers and crew aboard both planes were killed, the TCAS system was overhauled with significant changes made.
An investigation had found that the collision had been caused in part by ambiguities in the procedures regarding the use of the system, which saw one pilot following air traffic control commands while one was following TCAS command.
This included improved pilot training, clearer instructions regarding TCAS commands, and updates to the system itself, particularly addressing ambiguous "adjust vertical speed" commands which were replaced with more explicit instructions like "level-off" to ensure pilots properly respond to collision avoidance alerts.
While these features were brought in to prevent similar accidents in the future, it is clear the update is not foolproof.
In light of this, Neenan thinks "it is likely" that there will be changes made to TCAS in the future,
'"As part of the investigation, changes could be made to TCAS, air traffic control or any other function," he added.
