Airplanes do, on occasion, breach the confines of one another’s space. Usually this is a brief, tangential transgression. Almost always the mistake is caught, and safeguards are in place to minimize any hazard. Pilots are required to read back all assigned headings and altitudes, for example.
As a backup, airliners today carry onboard anticollision technology. Linked into the cockpit transponder, Traffic Collision Avoidance System (TCAS, pronounced Tea-Cass), gives pilots a graphic, on-screen representation of nearby aircraft. If certain thresholds of distance and altitudes are crossed, TCAS will issue progressively ominous oral and visual commands. If two aircraft continue flying toward each other, their units work together, vocalizing a loudly imperative “CLIMB!” instruction to one and “DESCEND!” to the other.
In 1978, a Pacific Southwest Airlines 727 collided with a Cessna while preparing to land at San Diego. In 1986, an Aeromexico DC-9 plunged into a Los Angeles suburb after hitting a Piper that had strayed, sans permission, into restricted airspace. Ten years later, a Saudi Arabian 747 was struck by a Kazakh cargo jet over India. Tragedies all, but these accidents occurred when TCAS was not yet standard equipment and when ATC protocols were not as sharp as they are today. Through technology and training, the threat of midair collisions has been greatly reduced.
But, for everything to work as it should requires the cooperation of both human and technological elements, bringing to mind the 2002 collision between a DHL freighter and a Bashkirian Airlines Tu-154 over the border between Switzerland and Germany. An ATC error had put the two planes on a conflicting course. A Swiss controller eventually noticed the conflict and issued a command for the Bashkirian crew to descend. At the same time, both airliners’ TCAS systems correctly interpreted the hazard, issuing their own instructions in the final seconds. DHL did as instructed and began to lose altitude. The Bashkirian crew, however, disregarded the TCAS order to climb and chose instead to descend, in compliance with the controller’s original request. This was a mistake. Standard procedure is that TCAS, being the last word on an impending collision, override previous ATC instructions. Had the TCAS alarm been obeyed, the jets would have been sent on safely divergent vectors. Instead, they descended directly into one another, killing 71 people.
An even worse catastrophe happened over the Amazon in 2006. A Boeing 737 collided with an Embraer executive jet. The latter managed a safe emergency landing, but the Boeing plunged into the forest, killing everybody on the plane. The investigation revealed a chain of procedural mistakes made by Brazilian controllers, compounded by evidence that the executive jet’s TCAS system may have been switched off inadvertently.
But what of dangers here in the United States, home to the world’s most crowded airspace? Isn’t our air traffic control system outmoded and much of its equipment obsolete? Aren’t improvements badly needed? To some extent, yes, and with more planes in the air than ever before, the terminal area—airspace in and around airports, where collisions are most likely to occur—has never been busier. At the same time, a need for ATC improvements does not imply that the system is rickety and rife with collision hazards. Measured year to year, the rate of airspace incursions in the United States occasionally spikes. While this can sound scary, only the rarest incursion is the sort of close call that should make people nervous. Overall our record is an excellent one, and a testament to the reliability of our ATC system, clunky and maligned as it is.