A computer breakdown caused a United Continental Holdings Inc. flight to take off earlier this year about 20,000 pounds (9,071 kilograms) heavier than pilots believed, prompting the carrier to add extra checks to ensure accurate weight calculations.
United sent pilots a weight estimate that assumed the coach section of the Boeing Co. 737-900 was empty when it was full, according to three people familiar with the incident who asked not to be named because they weren’t authorized to speak about it.
While the pilots, who didn’t catch the mistake, had difficulty getting the jetliner airborne, the plane wasn’t damaged and the flight was completed without incident, one of the people familiar with the event said. The pilots reported the trouble to a United program that encourages employees to identify safety issues, according to another person.
“Earlier this year, we experienced technology issues in capturing correct passenger counts on a small number of our flights,” Megan McCarthy, a spokeswoman for the airline, said in an e-mail yesterday.
The incident was recounted in a July 9 bulletin from United management to its pilots, said one person, who couldn’t say when the incident occurred.
United and the U.S. Federal Aviation Administration, which said the airline reported the breakdowns to it, said they’ve taken steps to prevent such incidents in the future. United is requiring its pilots to perform two additional, manual checks on weight and balance calculations before each flight, the FAA said in an e-mailed statement.
“The FAA has been monitoring the situation and is satisfied with this interim measure while the airline develops a permanent solution,” the agency said.
United has had computer and operational problems in recent months as it transitioned to new passenger-service and aircraft preventive-maintenance systems. Both were used at Continental Airlines Inc. before it merged with United parent UAL Corp. in 2010.
All airlines estimate a plane’s weight before each flight. The weight information, along with data on air temperature and other factors, allows pilots to calculate the precise speed at which they should lift the nose during takeoff.
If pilots try to take off with too much weight, it can cause a plane to scrape its tail on the ground or to skid off the runway without getting airborne, according to accident reports.
Boeing’s 737-900 models can take off weighing as much as 187,700 pounds (85,141 kilograms), according to Boeing’s website. Even fully-loaded jets frequently take off at lower weights.
The incident exposed a safety risk that has caused several accidents, including a 2004 crash in Canada that killed seven people, according to accident reports and a recent NASA study.
On March 20, 2009, an Emirates Airlines Airbus SAS A340 carrying 275 people nearly crashed in Melbourne after pilots made a 100,000-kilogram (220,460-pounds) error in pre-takeoff calculations, according to an Australian Transport Safety Bureau report.
One of the pilots accidentally entered a weight of 262,000 kilograms into a laptop used for takeoff calculations instead of the actual 362,000 kilograms, investigators found. The jet’s tail skidded on the pavement as it failed to get airborne and clipped an airport structure before climbing.
“There were no injuries, but this accident was considered to be a close escape from catastrophe,” a NASA report on the issue published in June said.
Seven people aboard an MK Airlines Ltd. Boeing 747 freighter died on Oct. 14, 2004, after making a 249,000-pound (113,000 kilograms) error before attempting to take off in Halifax, Canada, according to the NASA report.
The researchers also found that U.S. pilots had anonymously told of similar incidents to the Aviation Safety Reporting System, a NASA-run program that gathers safety reports from pilots and others.
In one such case, pilots at an unspecified carrier reported taking off 19,000 pounds (8,618 kilograms) heavier than estimated because an airline employee had listed 33 passengers instead of 133, according to the report.
“Although relatively few major accidents have yet been caused by performance data errors, our study suggests that more accidents are likely to occur unless existing measures to prevent and catch these errors are improved and new measures developed,” the report said.
The report recommended technology improvements and additional cross-checks by pilots to ensure math errors, typos and other miscalculations don’t become fatal.
Chicago-based United became the largest carrier in the world after merging with Continental two years ago. The joining of the two airlines has at times led to delays and other issues as computer systems, labor groups and aircraft fleets were blended.
United ranked last among 15 U.S. carriers in on-time arrivals for June, the latest month for which figures are available, at 70.1 percent, according to U.S. Transportation Department data. It also ranked last in May at 77.8 percent. It led in consumer complaints each month since February, according to the DOT.
“The commitment from the leadership team is to put all the resources, staffing and money we need into this,” Howard Attarian, United’s vice president of flight operations, said in an Aug. 2 interview. “The No. 1 priority of the company is to get back to reliability.”