The first critical mistake, of course, was made by the controller, who erred by allowing the airplanes to continue flying at the same altitude on a collision course. The next involved a second Zurich controller on duty, who left his station before the accident to rest, in violation of European air traffic rules. The Russian pilots, who ignored their TCAS resolution advisory in deference to the controller, bear the blame for the next critical error (pilots are trained to give priority to TCAS warnings over controller instructions). This mistake was exacerbated by the fact that the main surveillance radar for the sector was undergoing maintenance at the time, forcing the controller to rely on a slower, secondary radar. As a result, he never noticed that the DHL jet was also descending, and so he never alerted either crew to alter course.
Simulations performed after the midair have shown that the jets likely would have missed each other had the pilots of both airplanes simply obeyed their own onboard TCAS II systems. This advanced traffic-sensing technology, introduced in the 1990s, can communicate with TCAS II units in other nearby aircraft, using computing algorithms to quickly agree on the proper course of action for two or more airplanes to avoid a collision. In fact, modern TCAS technology works well enough that midair collisions involving large, transport-category airplanes are rare almost to the point of being nonexistent. It should require a mind-boggling conflagration of errors and missteps for two TCAS II-equipped commercial airliners to have a collision. That’s not to say it will never happen again, but the risk is infinitesimally smaller today than it was in the Wild West days before the adoption of the technology.
Yet many pilots flying general aviation airplanes today seem perfectly content to do so without any type of traffic-alerting technology on board, permitting see-and-avoid tactics and controller-issued warnings to suffice for the most part. A growing number of GA pilots, of course, benefit from lower-sophistication TAS (traffic alert systems), TIS (traffic information services) or even PCAS (portable collision alert system) gear, which is certainly better than having no traffic-alerting technology at all. And as we know, it won’t be too many years before all U.S.-registered aircraft are equipped with an advanced traffic-sensing technology thanks to the upcoming ADS-B (automatic dependent surveillance – broadcast) mandate, which takes effect in 2020.
TCAS Basics
Before we dive into a discussion of traffic-alerting technology, let’s take a hypothetical flight from New York to Los Angeles. Say we’re flying along in a Gulfstream G450, the autopilot sustaining us aloft at FL 360, when we notice a Boeing 737 some distance ahead that appears to our eyes to be rocketing directly toward us, as though it’s flying exactly at our altitude. As we continue toward the target at a closure rate of more than a thousand miles an hour, the 737 gradually seems to rise before passing harmlessly overhead, precisely on the inverse of our track. That the 737 seemed to be “at our altitude” before miraculously “climbing” was mere illusion: The curvature of Earth belied the 737’s true altitude a scant 1,000 feet above ours.
Until fairly recently, ATC maintained vertical separation of 2,000 feet for opposing traffic over the continental United States. The introduction of reduced vertical separation minimums in the airspace between FL 290 and FL 410 in much of the world has cut the margin in half while effectively doubling the amount of traffic that can fit in a given block of airspace. The first time a flight crew sees a distant airplane hurtling toward them just 1,000 feet above their assigned altitude can be disconcerting — but RVSM has been in force long enough by now (since the late 1990s over the North Atlantic and generally the mid-2000s elsewhere) that the tighter vertical separation has become routine. The reduced spacing, however, has required a whole host of equipment upgrades and crew training to guarantee safety.
