Sunday, Sep 7, 2008
Almost every air crash seems to involves not one, but two or more unusual breakdowns of technology and/or procedure. Investigators' final reports are full of uncanny coincidences that, seemingly against all odds, came to be in the moments before an accident. To ask, "Well, what are the chances of that?" is to invite some depressing retrospective amazement.
In the case of the July 1 crash, the coincidences included an understaffed control sector, a disabled alarm system and even a malfunctioning phone line. But to understand the incident and all it will inevitably embroil, we should first take a look at the workings of the air traffic control system.
In the case of my near-miss in 1986, we were flying under visual flight rules (VFR), using nothing more elaborate than what a flight instructor might call "see and avoid" technique. Keeping clear of other airplanes, in the frequently down-home, low-tech world of general aviation, is not a lot different, conceptually, from what's exercised on the highway. But airline flying is a another story. In this realm, flights operate under "positive control." Simply put, this means a flight is not only in the careful care of the flight crew, but also under the competent watching eyes of air traffic control (ATC) as well.
ATC is hardly just the control tower -- that tall concrete structure rising above the terminals with its spinning radar and cluster of antennae, at the top of which you imagine a group of men standing in a windowed room saying things like, "cleared for takeoff." ATC is a collective term for a host of control facilities. Towers are responsible only for ground movements and planes within the immediate area and below certain altitudes. Tower staff clear flights to taxi, takeoff and land. But away from the airport, flights are watched over by other, often remotely located staff.
Let's follow a flight from start to finish. In the meantime, keep in mind that airplanes use something known as a transponder (two or more are usually on board) to detail whereabouts, speed and altitude back to ATC radar. At some airports, radar also can be used to follow planes along foggy taxiways. Though it happens infrequently, a flight can operate gate-to-gate without a single controller ever once having the plane in actual, physical sight.
Departing from New York, bound for San Francisco, a flight will obtain weather and flight-plan clearances either directly over the radio or, more frequently, via computer. (Any takeoff delays also can be relayed this way.) When ready to taxi, the crew will call for "pushback" clearance and will next receive taxi instructions. Getting to the runway can involve a half-dozen radio frequency changes and various conversations with the likes of "clearance delivery," "gate control," "ground control" and possibly others. These may or may not be physically located in the tower, but are always somewhere at the airport. Finally a flight will be cleared for takeoff by the control tower itself.
Only moments after liftoff we are handed over to "departure control," which follows the plane on a radar screen issuing various turns, altitudes and so forth as the aircraft is sequenced into the higher altitude route structure. During a single interaction with departure control, a flight can progress through several different sectors, each maintained by a separate controller.
Once at higher altitude we are guided by a series of air route traffic control centers (ARTCC), commonly called "Center" by crews and controllers. Boston Center, New York Center, etc. Centers maintain huge swaths of sky, rendering them somewhat irrelevant to their namesake identifiers, and all the airliners within. Again, these are broken down into various sectors, each in the hands of individual controllers. Often, ARTCC facilities are located in secure buildings far from an airport. The Boston Center, for instance, in charge of airspace extending all the way to the Canadian Maritimes, resides in a building in Nashua, New Hampshire.
Flying from Center to Center across the country, the crew might make upwards of 10 or more radio frequency changes, in touch with a different person each time. Pilots may be asked to change altitudes or take a slightly different route from what the flight plan originally called for. While controllers do not tell pilots how to get from city to city, amendments to altitudes and routings are very common.
Descending into San Francisco, the above routine happens more or less in reverse. The flight is given over to "approach control," then to the control tower for landing clearance, followed by another rigmarole with ground personnel before docking safely (and, of course, on time) at the gate.
Following the midair crash over Europe, one Associated Press report stated, "There was only one controller in the Zurich tower at the time," setting off all sorts of anachronistic imagery. Yet, as just illustrated, the control tower at Zurich's international airport had nothing to do with the accident. The flights were in the upper, en-route portions of airspace. (Terminology changes somewhat when we leave the U.S., but the basic ATC concepts remain the same.) In truth, only one controller was monitoring that specific sector of sky.
That the jets were left on a collision course is unsettling, as is the disabling of the Swiss controllers' crash warning system and the missing second controller. But myths and hysteria over what some perceive to be an old-fashioned and dangerously overburdened system need first be reckoned with. The image of a lone man sitting in a tower with a microphone is foolishly misleading.
Independent from the air-to-ground link with ATC, airliners today also carry onboard technology to protect them from collisions. Linked into the transponder systems, TCAS (traffic collision avoidance system, pronounced "tea-cass" in an almost folksy slang) gives pilots a graphic representation of nearby aircraft. TCAS will issue progressively ominous oral and visual commands (to climb or descend) once certain thresholds of distance and altitudes are crossed. If two aircraft are erroneously flying toward each other, their TCAS units work together, issuing "CLIMB!" instructions to one, and "DESCEND!" to the other.
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