The United States has a voluntary metric policy; because the metric system is not required, many businesses operate in both U.S. customary and metric units. Simultaneously using two measurement systems, however, often causes unit errors; in some cases, these errors have been extraordinarily costly, while others have caused fatalities. Some of these disastrous errors have been formally investigated and published as in the following instances:
The National Aeronautics and Space Administration conducted a mission in 1998 to further explore Mars. In this mission, NASA and Lockheed Martin, its industrial partner, utilized both U.S. customary units and metric measures. NASA reported in the root cause investigation that a subcontractor did not properly account for changing the non-SI measurements in its software; consequently, the Mars Climate Orbiter crashed because it did not attain the proper orbit. This failed project, totaling $327.6 million in 1998, was unable to obtain intended data. NASA continues to struggle with dual measures, attributing challenges to U.S. aerospace sector reluctance to adopt the SI. The ongoing challenge was observed again in the 2010 internal inquiry related to the Constellation Project measurement practices.
In 1983, a Boeing 767 flight from Montreal to Winnipeg required an emergency landing because the plane was not properly fueled. Bob Pearson, the pilot of this flight, prevented disaster by safely landing at an airfield in Gimli. The staff of Air Canada recently began using the metric system instead of the customary system. Faulty unit conversions, compounded by other errors, caused the aircraft to be fueled with about 10,000 kg of fuel instead of the required 22,300 kg. Because of the pilot’s skillful landing, passengers arrived with minimal injury; however, this unit mix-up could have cost the lives of those on-board.
In 1999, a Korean Air MD-11 crashed after takeoff from Shanghai on its way to Seoul. The flight crew was instructed to ascend to 1,500 meters. During ascent, the crew believed it had misinterpreted the request, believing the requested altitude to be 1,500 feet. This assumption was made because aviation altitudes are provided in feet throughout most of the world. During the flight correction, the crew lost control of the craft, resulting in a crash. This crash caused the loss of 8 lives and injured 37 others.
In 2007, the United States House of Representatives sought to offset its carbon emissions. To achieve this, the House purchased carbon offsets from the Chicago Climate Exchange. From the Green Capitol Initiative Report, it was determined that 24,000 short tons of offset would be purchased; however, the chief administrative officer purchased 30,000 metric tons of offset. This amount was about 9,075 short tons more than the report’s determined amount. This excess offset cost the House an estimated $24,447.
In 2003, Tokyo Disneyland’s Space Mountain roller-coaster derailed because of a part specification error. In 1995, the design drawings were converted to metric and marginally adjusted. Both the original customary drawings and the metric drawings were maintained. When parts were ordered for the Space Mountain coaster, the original drawings were accidentally used; the ordered axles were 44.14 mm instead of the updated 45 mm. This error caused the axle to excessively vibrate and eventually break. Although individuals were not injured in this incident, Tokyo Disneyland suffered immense reputational damage.
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