WASHINGTON (AP) – The Federal Aviation Administration proposed levying multimillion-dollar fines against United Airlines and US Airways for safety violations, including flying a plane after mechanics stuffed shop towels into an engine.

The agency said it has proposed a $5.4 million fine against US Airways of Tempe, Arizona, for operating eight planes on a total of 1,647 flights from October 2008 to January 2009 in violation of safety directives or the company's own maintenance rules.

The agency also said it is proposing a $3.8 million fine against United of Chicago for operating one of its Boeing 737 aircraft on more than 200 flights with shop towels covering openings near where oil collects in the bottom of the engine instead of using protective caps required by the carrier's maintenance procedures.

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 hefty fines reflect the large number of flights that were allowed to carry passengers in violation of safety requirements, FAA spokeswoman Alison Duquette said. The agency doesn't know how many passengers were on the flights, she said.

FAA said US Airways failed to perform inspections required under its maintenance program related to engine work on a Boeing 757 that was flown 505 times. The airline also operated an Airbus A320 on 855 flights even though the aircraft did not meet the airline's maintenance program requirements for an engine repair, the agency said.

Among other violations, the airline also operated an Embraer 190, a regional jet, on 19 flights without complying with a safety directive that required inspections to prevent a cargo door from opening during flight, FAA said.