- By Colleen O'Hara
- Mar 29, 1998
It was a bright, sunny morning— a good day for flying, Capt. Ross Sagun thought as he boarded the 737 and took his seat in the cockpit. Sagun anticipated a smooth trip from Las Vegas to Denver, and he and the crew prepared for departure.
Sagun received clearance from the control tower and taxied to the runway for takeoff. Flight attendants strapped themselves into their seats, and passengers settled in for the hour-and-a-half ride. Within minutes, the airplane was off the ground and cleared to climb to 7,000 feet. From the air, the hotels and casinos that once towered overhead were dwarfed by the surrounding desert and mountains.
Suddenly, at 6,700 feet, Sagun realized the flight would not be as easy as he had thought. He had just turned the airplane eastward into the sun, when an on-board warning system alerted him that a small, private aircraft was headed his way, in line for a head-on collision. On the round screen in front of him, a white diamond representing the other aircraft changed to an amber circle, and a computerized voice alerted the crew: "Traffic! Traffic!" Moments later, the circle changed to a red square, and the computer voice warned, "Conflict!" and then, "Descend! Descend!" With only seconds to spare, the 737 plunged and the single-engine plane passed just 300 feet overhead.
The warning system that saved the life of Sagun, his crew and the passengers was the Traffic Alert and Collision Avoidance System-II (TCAS-II). "TCAS is my best friend," Sagun said. "It has proven very reliable. We look at it as our last-ditch safety net. It's the last tool we have to keep the metal from touching."
Had Sagun's flight taken place in 1987 instead of 1997, the outcome might have been different. In fact, an Aug. 31, 1986, collision over Cerritos, Calif., led to the development of TCAS and its follow-on, TCAS-II. On that day, an Aeromexico DC-9 was making its descent into Los Angeles International Airport, flying at 6,500 feet in clear skies. A small Piper aircraft, carrying a family of three, slammed into the jet's tail and sent both aircraft plunging to the ground, killing all 67 people on board the two planes and 15 people on the ground.
After the Cerritos incident, Congress held hearings and passed a law requiring the Federal Aviation Administration to mandate the use of TCAS. The FAA developed the system and then required airlines to install it at their own cost. Starting in 1990, planes with 30 seats or more began installing; 100 percent compliance was required and achieved by the end of 1993. No midair collision has occurred in the United States since 1990, a safety record mainly attributable to TCAS. India, Australia and several European countries recently announced plans to require TCAS on their planes.
Stories continue to mount of how TCAS has saved air travelers' lives. In January, an American Airlines pilot said TCAS helped him avoid a disaster when his MD-80, carrying passengers from Louisville, Ky., to Dallas/Fort Worth International Airport, narrowly missed a Cessna 182 carrying four parachutists and a pilot.
The incident took place in good weather about 45 miles northeast of the Texas airport along a heavily used arrival route. The airliner was flying at about 11,000 feet. Meanwhile, the Cessna, operated by Skydive Dallas, received clearance from controllers to climb to 10,500 feet and then to 11,500. The maneuver placed the Cessna head-on with the American Airlines jet, about 4 miles away. The crew of the MD-80 received a TCAS alert and immediately descended to avoid hitting the Cessna.
The planes reportedly came within 200 feet vertically and 4.3 miles horizontally of each other. The American Airlines pilot reported that without TCAS, he would have collided with the smaller plane.
Carol Schlichtemeir, co-owner of Skydive Dallas, said changes made in October 1996 to the arrival route into Dallas/Fort Worth have increased the amount of traffic in the airspace where sky divers are allowed to jump. "The traffic comes right over top of us or sometimes 2 miles south of where we jump," she said.
As a result of the incident in January, the FAA adjusted the route around the sky-diving area. Still, Schlichtemeir said she feels better knowing TCAS is working.
Alerting Pilots to Oncoming Planes
TCAS is valuable because it shows a pilot where his or her plane is in relation to others in the air. The on-board system combines transponders, which signal an airplane's location to other airplanes in the area, and computers, which interpret data and recognize TCAS messages sent by other planes. If a plane does not have an operating transponder on board, TCAS cannot pick it up.
"In the case where you're getting an advisory against a plane that does not have TCAS, that pilot does not have any information on you and could do something totally unpredictable," said Lillian Ryals, director of communications, navigation and surveillance at The Mitre Corp's Center for Advanced Aviation System Development, which helped design TCAS. "As a result, TCAS may have to change an advisory or strengthen or even reverse it. TCAS continually tracks the other aircraft, evaluates the maneuver options, and selects the safest maneuver for the pilot."
Sagun, who chairs the Air Line Pilots Association's air traffic control committee, said TCAS saved the day on another flight of his about three years ago. The 737 he was piloting departed Denver for San Jose and was cleared by the tower to climb to 9,000 feet and then to 10,000 feet. However, the ascent headed his plane straight toward another 737. "TCAS gave us a resolution advisory and suggested we climb, which we did," he said. But TCAS determined the separation between the planes was lessening and warned Sagun to increase his rate of climb. "We had a near miss of 300 feet," he said. "TCAS definitely saved the day on that one. It was very close."
In this case, air traffic controllers admitted they had made a mistake, Sagun said. "It's a human system, and humans make mistakes."
Before TCAS, pilots relied on a see-and-avoid strategy, but "eyes have a limited capability to see planes, especially with a head-on target. And that's been the cause of a few accidents," Sagun said. "I don't know any pilot[s] who would say they don't like TCAS."
The FAA's database is loaded with TCAS saves. For example, in June last year, an American Airlines DC-9 flying in good weather at 10,000 feet and headed for Las Vegas observed on TCAS another aircraft at the same altitude about 2 miles away. By the time the airport tower issued a warning, the American Airlines plane was already climbing and turning right thanks to a TCAS advisory. The DC-9 safely avoided what was later identified as a twin-engine aircraft.
Pilots agree that TCAS is an essential piece of equipment, almost as good as having another pair of eyes in the cockpit. "TCAS is a long-awaited and finally realized system that enhances the safety of air carriers," said Capt. Albert "Spike" Nunn, a pilot who has been with American Airlines for 32 years. "It gives you a warm and fuzzy feeling. From a safety aspect, it's excellent."
Nunn said American was one of the first airlines to install TCAS on its entire fleet. The airline said it has spent more than $200 million on TCAS gear and training so far. But the cost has been worth it, Nunn said, because TCAS gives pilots a bigger view of the airspace around them. "For example, during an approach for landing, if you're going 180 knots [about 207 mph] and closing in on another aircraft, you may have to slow down to maintain proper spacing for landing," he said. "TCAS will give you that visual."
Overcoming Growing Pains
The idea for TCAS dates to 1956, when two planes collided over the Grand Canyon, and airlines realized they needed a collision avoidance system. TCAS is an outgrowth of a system developed in the 1970s that was called the Beacon Collision Avoidance System and used transponders located on airplanes. The system was replaced by the family of TCAS systems in 1981.
The success of TCAS is attributed to an unusual level of cooperation between the FAA and the airlines. FAA funded most of the development and design of the system. Mitre developed the system logic, and the FAA Tech Center and a team of contractors handled software verification and validation and performed operational tests. MIT Lincoln Laboratory developed air-to-air surveillance.
TCAS, like most complex systems, had its growing pains. "When TCAS was deployed, there were a lot of nuisance alerts, and pilots and controllers were losing confidence in the system," said Andy Zeitlin, principal engineer for avionics at Mitre CAASD. "We changed the system to fix that, and the results were dramatic." Version 7 of TCAS, which reduces the number of false alarms, was approved in December and will begin appearing as early as the end of this year.
"TCAS is a good example of [what good can be done] when we work together in the interest of safety," said Albert Prest, vice president of operations at the Air Transport Association.
"TCAS is an industry program that would not have occurred without FAA leadership," said Capt. Robert Buley, a pilot and manager of flight technology development at Northwest Airlines.
"If it were not for a congressional mandate, we would still be designing this. Had we not had the mandate, there would have been more collisions we would have had to live with," Buley said.
Buley has been involved with TCAS in one form or another since the 1970s. He is chairman of the RTCA standards committee that led the TCAS standards-and-design effort. All the time devoted to TCAS has been time well-spent, he said. "I will be walking down an airport concourse and a pilot will stop me and shake my hand and say, 'Thanks for TCAS,' " he said. "I even get calls from overseas."
There's no question TCAS has saved lives. Since its introduction, the number of reported near collisions has dropped, said Larry Niven, the FAA's TCAS program manager. "There is a correlation between the date TCAS was introduced and now and the drop in near midair collisions. It's certainly an indication that TCAS has had a positive effect."
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System Logic: Developed by Mitre Corp.
Air-to-Air Surveillance Technology: Massachusetts Institute of Technology's Lincoln Laboratory
Operational Evaluation: ARINC
Software Verification/Validation: TASC, Coleman Research and Rannoch Corp.
TCAS Manufacturers: Allied Signal, Honeywell Inc. and Rockwell Collins