The most egregious of the saftey violations included flying a plane after mechanics stuffed shop towels into an engine to stop an oil leak.
United operated a Boeing 737 that returned to Denver in April 2008 after shutting down an engine due to low oil pressure indications, FAA said. During teardown of the engine a week later, United mechanics found that two shop towels, instead of required protective caps, had been used to cover openings in the oil sump area when maintenance was done in December 2007, FAA said.
"As a result of United's failure to follow its maintenance procedures ... it flew the aircraft on more than 200 revenue flights when it was not in an airworthy condition," FAA said in a statement. United's maintenance procedures specifically require use of protective caps or covers on all components that could be adversely affected by entry of foreign materials, the agency said.
United spokesman Megan McCarthy said the airline "immediately reported the incident and our findings to the FAA. United Airlines has the highest standards for safety and we are fully confident we took appropriate and necessary measures to ensure those standards are met."
The agency said it is proposing a $3.8 million fine against United of Chicago.
Under FAA rules, the airlines have 30 days to present mitigating evidence before the agency can impose the fines. It's not unusual for fines to be reduced as the result of negotiations.
The largest fine proposed by FAA was $10.2 million in March 2008 against Southwest Airlines for operating nearly 60,000 flights in 2006 and 2007 of planes that had missed required examinations for structural cracks and for flying them 1,451 times after being notified of the missed inspections. That fine was settled for $7.5 million in March 2009. The highest fine levied by FAA was nearly $9 million against Eastern Airlines, but it was never collected because the airline went bankrupt.
Last year, FAA ordered American Airlines to pay a $7.1 million fine for flying two jets 58 times without making repairs after an FAA inspector and American's own mechanics found problems with their autopilot systems, among other violations.