As the investigation begins into the cause of the mid-air crash between an American Airlines regional passenger jet and a U.S. army Black Hawk helicopter, questions have been raised about the role played by the Traffic Alert and Collision Avoidance System (TCAS).
The collision, which occurred as the jet approached Ronald Reagan Washington National Airport near Washington, D.C, sent both aircraft into the Potomac River. Everyone aboard both aircraft, which included 60 passengers and four crew on the plane, and three soldiers on the helicopter, were killed.
However TCAS technology is supposed to be an effective tool in helping prevent such accidents. Here’s what we know about the system and what it could have done in this situation.
How does TCAS work?
Problems with mid-air collisions decades ago led the U.S. Federal Aviation Association (FAA) to adapt the TCAS back in 1981. TCAS is mandated worldwide on all large aircraft and on many smaller commercial flights.
Footage from a CCTV camera caught the moment of impact as a passenger plane and a U.S. Army helicopter collided. Audio from air traffic control, meanwhile, captures some of the immediate aftermath of the collision.
It’s an airborne electronic system that functions independently of the ground-based air traffic control (ATC) system, and provides collision avoidance protection for a broad spectrum of aircraft types, according to a FAA guide.
It’s really considered a last resort, but it’s incorporated with other navigational displays on the airplane and uses radio signals to survey nearby aircraft. It will then issue visual cockpit displays and audio warnings that another aircraft may be dangerously close.
“It just shows that traffic is in our location, there’s a potential collision hazard,” said Shem Malmquist, a pilot and visiting instructor of general aviation and transport aircraft at the Florida Institute of Technology.
And in certain situations, it will provide guidance for the pilots on how to avoid a collision, he said.
For example, if TCAS believes the pilot needs to pay attention to other air traffic in the area, it may say “traffic traffic,” Malmquist said.
But it could also give a “resolution advisory,” which could include the instructions “climb climb,” “descend descend,” or increase or decrease vertical speed, Malmquist explained.
“It will actually tell you what you should do,” he said. He added that the current technology only provides vertical paths of corrective action, meaning it will not guide a plane to turn.
Does TCAS always send out alerts?
While TCAS will display warnings, audio commands are automatically inhibited below a certain altitude, Malmquist said.
He said a lot of warnings on aircraft are inhibited during certain phases of a flight to prevent worse problems.
Like a fire warning, for example, during initial takeoff will be inhibited “so the pilot isn’t trying to do something when it’s safer to get airborne,” he said.
The same goes with making major manoeuvring when you’re at a low altitude, close to the ground.
“First of all, you can’t go down anyway because the ground is there, and you don’t wat to be pulling up rapidly because there is the potential of the aircraft not having the manoeuvrability capability to do that,” Malmquist said.
What role may TCAS have played in the D.C. incident?
Because of the low altitude of both aircraft when they collided, TCAS would likely not have been providing audio warning at that point, Malmquist said.
As well, the pilot would probably not have been looking at the display for warnings at that point, he said. Instead, their focus would have been looking out the window and concentrating on the runway.
“As opposed to somebody suddenly reacting and diving the airplane into the ground or pulling the airplane up … and losing control of the airplane. You don’t have any manoeuvring margin at that point,” he said.
“I don’t know that TCAS wouldn’t be helpful at all, but they inhibit the warning and alert, specifically because the risk of other problems is higher than the risk of a collision.”
