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.