(January 2012) Sitting in the darkened area control center in Zurich, Switzerland, the air traffic controller on duty could scarcely believe what he was seeing on his radar screen: Somehow, two airliners in his sector cruising at precisely the same flight level were just miles apart on a collision course over southern Germany. Keying his microphone, the controller hurriedly commanded one of the aircraft, a Russian jetliner, to descend. The urgency in his voice conveyed the seriousness of the situation.
In the cockpit of the Tupolev Tu-154M passenger jet en route from Moscow to Barcelona, the pilots hesitated for a moment before initiating a descent from their assigned altitude of FL 360. Seconds later, the Russian jet’s TCAS issued a traffic resolution advisory commanding a rapid climb.
Faced with contradictory instructions, one from a human and the other from an onboard computer, the Russian crew chose to obey the controller’s original order.
Thus, the chain of events leading to one of Europe’s worst-ever air disasters was irrevocably set into motion.
Unbeknownst to the Russian pilots, a DHL Boeing 757 at that same moment was headed straight for them, the result of the controller in Zurich mistakenly allowing the converging jetliners to cruise at the same altitude. In the cockpit of the DHL jet, the crew also received a TCAS warning — in their case, commanding a descent. As both airplanes steadily began losing altitude, still on a deadly collision course, the controller never intervened. Seconds ticked past on the dark night until the DHL jet, traveling at more than 500 miles per hour, sliced into the Tupolev at a nearly perfect right angle and an altitude of 34,980 feet, splitting the Russian airplane in two. The Tu-154 broke into large sections, fore and aft, the twisted metal raining down over a remote German town near the Swiss border. All on board were killed, including 57 passengers, most of them Russian schoolchildren headed to Spain for summer vacation. The badly damaged 757 managed to stay aloft for another four miles before crashing and killing its two pilots.
How could this midair collision — occurring at 10:36 p.m. local time on July 1, 2002 — possibly have been allowed to happen? In this era of advanced traffic alert and collision avoidance systems and ground-based surveillance radar, and with professional pilots and a controller on hand to continually manage aircraft trajectory, altitude and speed, how did it come to pass that two massive jetliners weighing nearly a half-million pounds combined would occupy the same point in the sky at precisely the same instant even as those safety devices and the controller on duty issued a multitude of warnings?
As is the case in most such tragedies, it was a series of unfortunate events that doomed the lives of 71 passengers and crew — and, it would turn out, also claimed the life of the Zurich controller when, two years later, the Russian father of two of the young victims murdered him in a revenge killing outside his Zurich home. Had any one of the links in the accident chain been broken, all involved might still be alive today.
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.
One such modification involved switching to a new TCAS software version known as Change 7, which fixed several known TCAS issues while also reducing the alert threshold in RVSM airspace. Yet despite the improvements Change 7 software brought, the technology still had shortcomings, leading to the creation of yet another software revision, this one known as Change 7.1. The major fix in 7.1 is the implementation of “reversal logic” and “adjust vertical speed” resolution advisories, developed (you guessed it) in response to the tragic 2002 midair over southern Germany. The upgraded software permits a TCAS to amend an original resolution advisory if it sees that an intruder aircraft is performing an unexpected maneuver, such as descending when it ought to be climbing.
It seems like such a simple thing, but in truth the algorithms that govern how TCAS views threatening aircraft and decides to deal with them are incredibly complex. They have been vastly improved from the earliest TCAS-like technology developed back in the 1950s, ’60s and ’70s. Not surprisingly, it was a series of high-profile midair collisions that prompted the development and adoption of the traffic-alerting technology in the first place. One such landmark midair occurred on June 30, 1956, when a United Airlines DC-7 collided with a TWA Super Constellation over the Grand Canyon, resulting in 128 fatalities. The accident spurred interest in a collision avoidance system then under development by Bendix Radio that worked by bouncing a UHF radio signal off the ground to other like-equipped aircraft. Using time-based calculations, the technology could determine distance and closure rate and issue a command to climb or descend depending on whether the intruder aircraft was higher or lower.
Much later, on Sept. 25, 1978, a Pacific Southwest Airlines Boeing 727 collided with a Cessna 172 over San Diego, killing all 135 aboard the PSA flight, as well as the two occupants in the Cessna and seven people on the ground when the Boeing crashed into a San Diego neighborhood. This midair led directly to the development of TCAS, which is now mandated in the United States and much of the rest of the world in aircraft with more than 10 passenger seats.
There are currently two versions of TCAS, known as TCAS I and TCAS II. Required on aircraft with between 10 and 30 passenger seats, TCAS I is a less sophisticated version of the technology that sends out continuous signals to interrogate Mode C transponders aboard nearby aircraft. The TCAS receiver then calculates approximate bearing and relative altitude of aircraft within the selected range, usually out to a distance of about 40 nautical miles. Color-coded traffic symbology shown on a cockpit display indicates aircraft that pose a threat versus those that do not. TCAS I issues a traffic advisory (TA), calling out “Traffic! Traffic!” in cases when they do. When a pilot flying a TCAS I-equipped aircraft receives a TA in VFR conditions, his or her job is to visually identify the intruder and climb or descend as necessary. In IMC, the pilot notifies ATC for assistance in resolving the conflict.
TCAS II is a more sophisticated technology, and as a result it is also much more costly. Required on airplanes equipped with more than 30 passenger seats or with maximum takeoff weights higher than 33,000 pounds, TCAS II operates similarly to TCAS I, but in addition it can issue resolution advisories (RAs) to the pilots. It does this by first determining whether the intruder aircraft is climbing, descending or flying straight and level, and then advising the pilots to execute the proper evasive maneuver. RAs can include commands to climb, descend, maintain vertical speed, adjust vertical speed and increase or reduce rate of climb or descent.
Traffic Tech for GA
Recognizing a need for traffic awareness technology for the general aviation market, avionics makers and the FAA worked together in the late 1990s to create a new class of safety avionics known as the traffic alerting system. Priced lower than TCAS I, a TAS typically consists of a low-power, remote-mounted traffic receiver, antennas mounted on the top and bottom of the fuselage (or sometimes only on the top or bottom) and a cockpit display.
The lowest priced TAS products typically can detect traffic out to a distance of about six nautical miles, while at the top end, these systems can track targets out beyond 20 nautical miles. In general, the faster your airplane, the longer-range TAS you should consider buying. At the low end of the TAS market are the Avidyne TAS 600 ($8,490 list price) and Garmin GTS 800 ($9,995 list). Both are superb choices for light piston airplanes and helicopters, with the TAS 600 able to track targets to a range of seven nautical miles in all directions and the GTS 800 to a range of 12 nautical miles ahead of the aircraft and fewer behind. For higher-performance piston singles, twins and turboprops, Bendix/King sells the KTA 870 ($14,995 list), Garmin the GTS 820 ($19,995), L-3 the Sky899 SkyWatch ($20,990), and Avidyne the TAS 620 ($20,990), with several other models filling gaps between.
TAS products display traffic using standard TCAS symbology, the major difference being no RAs are issued by such systems. The same is true for fliers on a tighter budget who opt for TIS-compatible Mode S transponders capable of displaying traffic targets broadcast by certain ground radar stations. Unfortunately, TIS coverage is restricted to a little more than 100 radar sites around the country, a number of which are scheduled for decommissioning as the ADS-B mandate approaches. Still, the FAA plans to keep scores of these sites operational for the next decade or more, meaning there is value in TIS technology if you often fly in areas covered by the service.
The ADS-B Mandate
Clearly, you can see ADS-B is the common denominator in any discussion you’re likely to have about purchasing a new transponder over the next several years. In May 2010, after thousands of hours of flight trials, the FAA formalized its mandate for ADS-B compliance as the “backbone” of the Next Generation Air Transportation System (NextGen). The “dependent” part of ADS-B simply means that everybody operating in the ATC environment will ultimately depend on everybody else to broadcast their identity, position, track, airspeed and so forth via ADS-B “Out” technology. Put plainly, traffic surveillance is going interactive, with NextGen air navigation transitioning from ground-based radar systems to a space-based, satellite-driven aircraft tracking system. Those radars will remain online in case of emergency and for use by the Transportation Security Administration, but for the most part satnav will be the norm in the future.
And that’s a good thing, really. The benefit of NextGen surveillance is that it’s more precise and widely available than radar. Currently, it takes 12 seconds for a long-range radar station to complete one sweep versus about five seconds for a terminal-area radar. That’s a long time, and as a result, controllers must maintain wider separation among aircraft for safety’s sake. ADS-B technology updates about once a second, allowing closer separation. Hence, NextGen will offer growth capability to accommodate future air traffic demands, which will be high.
Who will need ADS-B equipment? The short answer is, just about everybody who flies in controlled airspace. Even if you don’t fly IFR, your ability to obtain VFR traffic advisories and transition through most ATC-controlled airspace will still require that you have ADS-B “Out” capability on board. Basically, wherever you need a transponder now, you’ll need ADS-B in the future.
The minimum equipment you’ll need to have on board before the 2020 ADS-B mandate is a Mode S ADS-B transponder and an approved WAAS GPS navigation source providing required position, vector, altitude and velocity data. The FAA has given U.S. aircraft operators two options for satisfying the ADS-B “Out” requirement. The first is the 1090 MHz “extended squitter” (or ES) broadcast link available on certain Mode S transponders. The second is the dedicated 978 MHz universal access transceiver, or UAT. If you fly at or above FL 180 or in Canada and the Caribbean, you’ll have to go with the 1090 MHz ES link. Aircraft flying below FL 180 can opt to use either the 1090 MHz ES or the UAT broadcast link. Both options will meet the ADS-B “Out” requirement — and when equipped with an ADS-B receiver as well as a transmitter, both systems can also provide ADS-B “In” functionality for display of traffic data in the cockpit. However, only UAT equipment has the ability to offer FIS-B service providing free datalink weather and other flight information (such as real-time TFRs and notams). That’s a shame, because it will mean equipping with multiple technologies to gain the full scope of ADS-B benefits. Interestingly, Rockwell Collins says it has no plans to include UAT technology in its TCAS line.
So how much will ADS-B compliance cost? That’s a hard question to answer because competition for your ADS-B dollars has yet to heat up in a significant way. Still, while 2020 might seem like a long way in the future, do the math and it suddenly becomes clear that we in the United States will need to equip about 25,000 airplanes per year over the next nine years.
“That’s assuming people start upgrading now, which they won’t,” said Bill Stone, avionics product manager for Garmin. “They’ll wait as long as they can. If suddenly we have 100,000 aircraft per year to upgrade, that’s pretty much an impossible situation” for avionics installers.
Yet preparing for ADS-B now is pretty pointless for aircraft owners because we’re on the verge of an explosion of new products, including hybrid traffic surveillance systems that will combine ADS-B with TAS and TCAS, not to mention ADS-B apps that will transform flying in ways we can only imagine. You can bet that once aircraft owners wake up to the fact that the mandate is nearly upon us, competition among avionics makers will grow. So, if you’ve done nothing yet to prepare for the ADS-B mandate, don’t worry; if you’ve still done nothing seven or eight years from now, that’s another story.
ADS-B Benefits
One of the pioneers in ADS-B technology, especially in the air transport market, is ACSS, a joint company owned by L-3 and Thales of France. The Phoenix-based company has developed an app suite known as Safe-Route that harnesses ADS-B technology to save airlines significant fuel and time — and could serve as a blueprint for technologies that might benefit GA. Among the intriguing ADS-B applications ACSS is working on, one called Merging and Spacing allows pilots to fly closer in-trail procedures by permitting the pilots to monitor their distance from aircraft ahead on a cockpit display. Fitting more airplanes in a busy terminal area can mean reduced delays and fuel savings resulting from more direct routings. For UPS, which has evaluated Merging and Spacing at its busy cargo hub in Louisville, Kentucky, the technology has been shown to increase the number of aircraft that are able to land at busy times, boosting the company’s bottom line.
On the ground, ACSS’ SAMM (surface area movement management) technology uses software algorithms to alert pilots of potential runway incursions, yet another ADS-B benefit few pilots are even aware is possible. The application gives flight crews an airport surface map and tracks the movements of their aircraft and other ground and airborne traffic in the terminal area, alerting the crew when potential conflicts arise by displaying threats on an EFB display.
It’s the kind of technology you could easily envision running on an iPad. But will we ever get there?
“Absolutely,” said Cyro Stone, director of ADS-B products for ACSS. “We’re creating this application for the airlines, but bringing technologies like SAMM down to general aviation is already on our future road map. It’s only a matter of time.”
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