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Notable Collisions

A recurring theme that appears when examining the circumstances leading to and following the development of Mode S is the catalytic nature of collisions in forcing changes to be made. Although the government is often aware of problems within the existing system, the rate of change is often slow, or the issues are shelved until some later date. However, mid-air collisions have the effect of raising public awareness and causing a great deal of interest and pressure from the press and the public to "do something". Congress will therefore scramble to initiate measures and pass edicts to address the problem, spawning a flurry of activity and causing progress to leap forward. Unfortunately, the attention span of the public and politicians is short - weeks or months - while the time that it takes to accomplish anything in air traffic control is more on the order of years. Without the constant pressure and attention from the media, political attention turns to other matters, and progress in air traffic control again begins to slow. This continues, with the system getting progressively worse, until public attention is once again raised by the next major accident.

This cycle repeated itself in virtually all of the major events in air traffic control in the 1960s, and also in the Congress mandate that finally led to the widespread adoption of the Mode S technology, as you will see below.

Grand Canyon - June 30, 1956

Grand Canyon, 1956
picture from Plane Crash Info

On June 30, 1956, a United Airlines DC-7 flying from LA to Chicago collided over the Grand Canyon with a TWA Constellation en route from LA to Chicago. The 58 passengers on the DC-7, and the 70 passengers on the Constellation were all killed. The aircraft were flying in uncontrolled airspace, i.e. under visual flight rules without the guidance of air traffic controllers, radar, or even official flight plans. Both pilots had requested permission to fly in undesignated airspace to afford their passengers a better view, and were thus responsible for their own safety and separation, The crash was attributed to the pilots not seeing each other until it was too late.

Although the Civil Aeronautics Agency (the FAA's predecessor) denied responsibility for the accident, investigations revealed that the CAA's air traffic control system was insufficient to offer positive separation to every airplane flying across the country. Congress and other legislators, who had previously cut budgets to the CAA, were forced to recognize the severity of the air traffic control problem. They thus embarked on a massive ATC modernization plan, appropriating $250 million to the CAA to upgrade the ATC system. This money was used to purchase new radar surveillance equipment, to open new control towers, and to hire more air traffic controllers. The pieces for a good air traffic control system were potentially in place. However, the changes could not be made fast enough to satisfy the air traffic controllers, who began to resign at a very fast rate - about 30% of controllers resigned in 1957 alone. The lack of controllers only increased the workload on the other controllers, who formed the Air Traffic Control Association (ATCA) to represent controllers' demands.

A series of crashes in 1958 finally led President Eisenhower to instigate a study into the nation's aviation system. This committee recommended the design of a common air traffic control system that would serve the needs of both military and civilian aircraft, and the ultimate formation of an independent agency to take over the functions of the CAA and to oversee this system. This agency would become the Federal Aviation Agency (FAA).

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Sources: Fundamentals of Air Traffic Control, Nolan, 1999 and Plane Crash Info.

New York City - Dec. 16, 1960

NYC 1960
picture from Plane Crash Info

While the 1956 crash had occurred over an unpopulated area of the Grand Canyon, the 1960 collision of a United Airlines DC-8 and a TWA Super Constellation over New York City killed not only the 128 people on board the aircraft, but 8 people on the ground as well. Initial investigations of this crash indicated that United Airlines pilot error was the cause of the collision. The United flight had experienced partial navigation equipment failure but failed to report it to the air traffic controllers. The flight was instructed to enter a holding pattern over the Idlewild airport. At the same time the TWA Constellation was instructed by LaGuardia controllers to enter a holding pattern at the same altitude as the United flight at a safe separation distance. The United flight apparently entered its holding pattern too quickly, and overshot it, straying into the TWA's airspace and eventually colliding into it.

Undoubtedly, the United pilots were partially at blame for not adhering properly to air traffic control protocols and for overshooting their airspace. However, the investigation also revealed that the air traffic controllers lacked secondary surveillance radar and should have been able to detect the impending collisions and warn both planes in time. This incident led to the increased installation of radar equipment at busy airports.

In 1961, President Kennedy issued an order for the FAA to begin to "conduct a scientific, engineering overview of our aviation facilities and related research and development." As a result, the Project Beacon committee was formed to investigate the air traffic control system and propose a solution to the existing problems. This in turn led to the use of ATCRBS as a new air traffic control technology.

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Sources: Fundamentals of Air Traffic Control, Nolan, 1999 and Plane Crash Info.

Urbana, OH - March 9, 1967
Hendersonville, NC - July 19, 1967

In 1967, not one, but two crashes between private planes and transponder-equipped commercial aircraft occurred, pushing public opinion towards the mandating of transponders for all aircraft. The March 9, 1967 collision of a TWA DC-9 with a private Beechcraft B-55 over Urbana, OH killed 26 people when the TWA pilot failed to see and avoid the Beechcraft. Excessive speed on the part of the TWA was acknowledged as a contributing factor to the accident. Then, on July 19, 1967, a Piedmont Airlines B-727 collided with a private Cessna near Hendersonville, NC, killing 79 in the B-727 and a family of three in the Cessna. This particular accident was closely tied to inadequacies in the air traffic control system, particularly the lack of secondary surveillance radar and confusing transmissions by the air traffic control.

Both of these accidents forced a government and public reassessement of the current air traffic control system and prompted changes to be made. In this case, the most significant result of these accidents was the formation of the Air Traffic Control Advisory Committee and the Mode S technology.

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Sources: Plane Crash Info.

Grand Canyon - Jun. 18, 1986
Cerritos, CA - Aug. 31, 1986

Cerritos DC-9 crash 1986
An Aeromexico DC-9 going down over Cerritos, CA after colliding with a Piper Archer
Picture taken from Plane Crash Info.

Although Mode S development was completed by the mid 1970s, and the fully drawn out specifications delivered to the FAA in 1975, once again it took a series of collisions to drive its deployment. These crashes came in the form of a June, 1986 collision over the Grand Canyon, and an August collision over Cerritos CA. The Grand Canyon crash involved two small aircraft, a DeHavilland carrying 20 passengers, and a Helitech with five. The two aircraft collided due to the failure of both crews to see and avoid each other, though the reason was undetermined. A little over two months later, an Aeromexico DC-9 with 64 passengers collided with a private Cessna aircraft carrying a family of three over Cerritos CA. The DC-9 crashed into a neighborhood and destroyed or damaged 18 homes, killing 15 people on the ground.

Cerritos crash 1986
Crash site of DC-9/Cessna collision in Cerritos, CA
Picture taken from Plane Crash Info.

While the Grand Canyon accident brought air safety into the public eye, the Cerritos accident most clearly indicated the problems with the existing air traffic control system. The Cessna had inadvertently entered the LAX terminal control area when it collided into the DC-9. The accident was blamed on inadequate radar approach and departure equipment and procedures. This crash prompted Congressional action, beginning with a mandate establishing deadlines for the completion of the development and installation of TCAS II (Sec. 203, 1987 Airport and Airways Capacity Expansion Improvement Act, Public Law 100-223, 101 Stat. 1518). The original legislation required the FAA to implement a schedule for the certification of TCAS II by June 30, 1989, and to require the installation of TCAS II on all passenger aircraft with more than 30 seats within 30 months of the certification date. Eventually, the FAA published an incremental deployment schedule, requiring 20% compliance with TCAS II by Dec. 30, 1990, 50% by Dec. 30, 1991, and 100% by Dec. 30, 1993.

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Sources: Plane Crash Info and TCAS Hearing February 26, 1997.

The Story of Mode S: An Air Traffic Control Data Link Technology last modified: 12.06.2000