The U.S. Federal Aviation Administration said United Parcel Service should pay a $4 million civil penalty for not complying with certain rules on maintaining and operating its aircraft.
The agency is alleging that UPS failed to follow procedures when repairing four of its aircraft used on more than 400 flights between October 2008 and June 2009. The planes involved were two DC-8 and two MD-11 aircraft.
A spokesman for UPS said that "there was never a safety issue" and called the penalty "unwarranted and unreasonable."
"We believe we were compliant with FAA rules and will vigorously defend our position," UPS spokesman Mike Mangeot said in a statement.
U.S. Transportation Secretary Ray LaHood said in a statement that "air carriers must comply with federal regulations to ensure aircraft are maintained to the highest level of safety."
Michael Huerta, the FAA administrator, said the aircraft should stay grounded until UPS makes the proper repairs. UPS has 30 days to respond to the agency.