United Airlines Flight 718
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United Airlines Flight 718, Mainliner Vancouver, registration N6324C, was a Douglas DC-7 en route from Los Angeles's International Airport to Chicago, Newark and Philadelphia on June 30, 1956. Trans World Airlines Flight 2, Star of the Seine, registration N6902C, was a Lockheed Super Constellation en route from Los Angeles to Kansas City and New York City. The two aircraft collided in mid-air over the Grand Canyon, killing all 128 aboard both aircraft.
TWA Flight 2, carrying 64 passengers and six crew members, departed at 9:01 a.m., 31 minutes later than its scheduled departure time, and flew in controlled airspace as far as Daggett, California. It then departed controlled airspace and flew on a heading of 059 degrees magnetic in the direction of Trinidad, Colorado.
United Flight 718, with 53 passengers and five crew members aboard, departed Los Angeles at 9:04 a.m., flying at an altitude of 21,000 feet in controlled airspace as far as Palm Springs, California, then departing controlled airspace on a heading of 046 degrees magnetic in the direction of St. Joseph, Missouri.
Both aircraft estimated that they would arrive somewhere along the Painted Desert line of position at 10:31 a.m. The Painted Desert line was 175 miles long and ran from Bryce Canyon, Utah to Winslow, Arizona wholly outside controlled air space.
When Flight 2 asked air traffic control (through its company operator; in accordance with practice at the time, neither aircraft was in direct contact with ATC after takeoff) if it could fly at an altitude of 21,000 feet, it was denied that specific altitude due to traffic -- the United DC-7 -- as the two aircraft would be in conflict at a later location in controlled airspace. Flight 2 was instead cleared to fly "1,000 feet on top". This clearance gave Flight 2 the right to fly 1,000 feet above the cloud tops but also put the onus on the pilot to separate himself from other aircraft. Ironically, this "1,000 feet on top" clearance permitted the TWA flight to climb to 21,000 feet.
At 10:31 a.m., United Airlines radio operators in Salt Lake City and San Francisco both heard a garbled transmission on the company's frequency. This was the last transmission heard from either aircraft; Civil Aeronautics Board engineers later deciphered the transmission as the voice of the first officer of Flight 718 saying, "Salt Lake, ah, 718...we are going in!" The remains of both aircraft and those aboard were found the next day in the Grand Canyon near the confluence of the Colorado and Little Colorado Rivers. All 128 aboard both aircraft died on impact.
The accident was given an enormous amount of attention by the American news media. A Congressional committee was reviewing the state of air traffic at the time; one of the topics under discussion was the large number of near collisions occurring due to the antiquated air traffic control system. ATC was underfunded and undermanned and, without radar, was unable to separate traffic outside clearly demarcated airways and zones around major airports. Outside of those areas aircraft were expected to separate themselves using the "see and be seen" principle. In this instance, although the aircraft were expected over the same 175-mile-long line of position at the same time, it was impossible for ATC to positively separate them without knowing where on that line each aircraft was expected to pass over. Moreover, although the "see and be seen" principle worked at slower speeds and lower altitudes, it was an unsafe method of separation at altitudes where the atmosphere on even an apparently clear day could be hazy.
The Civil Aeronautics Board (predecessor of the NTSB) found the following as probable cause for the accident:
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- The Board determines that the probable cause of this mid-air collision was that the pilots did not see each other in time to avoid the collision. It is not possible to determine why the pilots did not see each other, but the evidence suggests that it resulted from any one or a combination of the following factors: Intervening clouds reducing time for visual separation, visual limitations due to cockpit visibility, and preoccupation with normal cockpit duties, preoccupation with matters unrelated to cockpit duties such as attempting to provide the passengers with a more scenic view of the Grand Canyon area, physiological limits to human vision reducing the time opportunity to see and avoid the other aircraft, or insufficiency of en route air traffic advisory information due to inadequacy of facilities and lack of personnel in air traffic control.
The accident underlined the limitations of the "see and be seen" principle and (along with a number of other mid-air collisions) prompted a number of developments in the United States air traffic control system. Improved ground facilities allowed aircraft to be in constant contact with ATC. The concept of narrow controlled airways gave way to the idea of larger control areas. The CAA (forerunner of the FAA) finally agreed to use radar to separate air traffic and provide advisories, often using military surplus at first. Aircraft flying at higher altitudes could no longer fly by visual rules. Traffic advisories were implemented in uncontrolled airspace. These changes, which took a number of years to implement, reduced the number of mid-air collisions and near-collisions significantly.
United still uses the flight number 718 on its Burbank-San Francisco route.
TWA used flight number 2 on its Honolulu-St. Louis route prior to merging with American Airlines.
[edit] Sources
- Air Disaster, Vol. 4: The Propeller Era, by MacArthur Job, Aerospace Publications Pty. Ltd. (Australia), 2001. ISBN 1-875671-48-X
[edit] External links
- Department of Transportation Library
- Flying Blind by Gregory Rawlins
- TWA and United collision over Grand Canyon Arizona Aircraft Archaeology
