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Software Failure Case Studies
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Air Traffic Control System Failure (September 2004 )

In September 2004, air traffic controllers in the Los Angeles area lost voice contact with 800 planes allowing 10 to fly too close together after a radio system shut down. The planes were supposed to be separated by five nautical miles laterally, or $2,000$ feet in altitude. However, the system shut down when 800 planes were in the air and forced delays for 400 fights and the cancellations of 600 more. The system had voice switching and control system, which gives controllers a touch-screen to connect with planes in flight and with controllers across the room or in distant cities.

The reason for failure was partly due to a 'design anomaly' in the way Microsoft Windows servers were integrated into the system. The servers were timed to shut down after 49.7 days of use to prevent data overload. To avoid this automatic shutdown, technicians are required to restart the system manually every 30 days. An improperly trained employee failed to reset the system, leading it to shut down without warning.

Welfare Management System Failure (July 2004)

It was a new government system in Canada, costing several hundred million dollars. It failed due to the inability to handle a simple benefit rate increase after being put into live operation. The system was not given adequate time for system and acceptance testing and never tested for its ability to handle a rate increase.

Northeast Blackout (August 2003)

It was the worst power system failure in North American history. The failure involved loss of electrical power to 50 million customers, forced shutdown of 100 power plants and economic losses estimated at $\$ 6$ billion. The bug was reportedly in one utility company's vendor-supplied power monitoring and management system. The failures occurred when multiple systems trying to access the same information at once got the equivalent of busy signals. The software should have given one system precedent. The error was found and corrected after examining millions of lines of code.

Tax System Failure (March 2002)

This system was Britain's national tax system, which failed in 2002 and resulted in more than $1,00,000$ erroneous tax overcharges. It was suggested in the error report that the integration testing of multiple parts could not be done.

Mars Polar Lander Failure (December 1999)

NASA's Mars Polar Lander was to explore a unique region of the red planet; the main focus was on climate and water. The spacecraft was outfitted with a robot arm, which was capable of digging into Mars in search of near-surface ice. It was supposed to gently set itself down near the border of Mars southern polar cap. However, it couldn't touch the surface of Mars. The communication was lost when it was 1800 meters away from the surface of Mars.

When the Lander's legs started opening for landing on Martian surface, there were vibrations which were identified by the software. This resulted in the vehicle's descent engines being cut off while it was still 40 meters above the surface, rather than on touchdown as planned. The software design failed to take into account that a touchdown signal could be detected before the Lander actually touched down. The error was in design. It should have been configured to disregard touchdown signals during the deployment of the Lander's legs.

Mars Climate Orbiter Failure (September 1999)

Mars Climate Orbiter was one of a series of missions in a long-term program of Mars exploration managed by the Jet Propulsion Laboratory for NASA's Office of Space Science, Washington, D.C. Mars Climate Orbiter was to serve as a communications relay for the Mars Polar Lander mission. However, it disappeared as it began to orbit Mars. Its cost was about $\$ 125$ million. The failure was due to an error in the transfer of information between a team in Colorado and a team in California. This information was critical to the maneuvers required to place the spacecraft in the proper Mars orbit. One team used English units (e.g., inches, feet, and pounds), whereas the other team used metric units for a key spacecraft operation.

Stock Trading Service Failure (February 1999)

This was an online US stock trading service, which failed during trading hours several times over days in February 1999 . The problem found was due to bugs in a software upgrade intended to speed online trade confirmations.

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