Washington (CNN) -- United Airlines is facing a $584,375 fine after a federal inspection showed that pilots and flight attendants were far more likely to be excused from the airline's random drug and alcohol tests than ground-based employees were.
The Federal Aviation Administration also said Friday it found at least 13 instances in which employees were transferred to "safety sensitive" jobs -- typically meaning jobs as flight crews or mechanics -- before drug and alcohol tests were completed. Of the 13 transferred employees, six performed maintenance on United's aircraft before the airline received negative drug test results, the FAA said.
All of the alleged infractions occurred during the summer of 2009, according to the FAA, and the FAA did not indicate whether the alleged lapses led to any problems with aircraft or flights.
United has 30 days to respond to the agency's letter proposing the fine.
"Drug testing is both a critical and a required safety measure that all operators must follow," FAA Administrator Randy Babbitt said in a release.
The airline issued a statement saying, "Safety is United's top priority. We are reviewing the letter and will cooperate fully with the FAA to resolve their concerns."
According to the FAA, which inspected records at five United hubs, United used different methods to randomly select flight crew personnel and ground personnel for random testing. Pilots and flight attendants were significantly more likely to be excused from tests, the FAA said.
In 2008, for instance, 25 percent of pilots and 44 percent of flight attendants were excused from taking drug tests, while only 5 percent of ground personnel were excused, the FAA said.
That same year, about 16 percent of pilots and 12 percent of flight attendants were excused from alcohol tests, compared with 4 percent of ground personnel, the agency said.
The inspection showed the methods used by United did not ensure that all of its safety-sensitive employees stood an equal chance of being selected for random drug and alcohol testing, the FAA said. According to the agency's regulations, all employees must have an equal chance of being tested when random selections are made.
United's protocols included a multiple-step process that, on a monthly basis, selected employees for testing by first randomly selecting flight numbers for flights arriving at United's six hubs on specific dates.
The FAA said it has twice warned United the company's random test selection methods did not give each eligible flight crew member an equal chance of being selected for the tests.