DENVER -- Denver International Airport's largest air carrier could face possible Federal Aviation Administration fines for allegedly using duct tape to repair jets.

After discovering small holes in the wing panels of three Boeing 757 jets in March, UAL maintenance workers performed a temporary repair of the spoiler skin that was not within approved methods, techniques or practices in the current manufacturer's repair manuals, the FAA said in a news release.

The planes made 193 flights before mechanics permanently repaired the holes, according to the FAA.

"United Airlines continued to operate the aircraft when they were not in an airworthy condition," said Allen Kenitzer, a FAA spokesman in Renton, Wash.

The airline could be fined more than $800,000 for repairs, but a United spokesman said the airline doesn't agree with the finding.

"We're going to fight" the proposed fine, Joe Hopkins, a spokesman for Chicago-based United, told The Denver Post.

Hopkins said the duct-tape repair didn't harm safety or place passengers and crews at risk.

The FAA said it allows the use of tape, similar to duct tape, only for certain temporary repairs to aircraft.

This week's proposed penalty over alleged aircraft airworthiness is the second in four months. In August, the FAA proposed a $1.5 million civil penalty against United Airlines for improper maintenance of several jet engines that flew on more than 974 flights.

The FAA also proposed a $200,000 fine against against Denver-based Frontier Airlines in an unrelated investigation. The government agency said the airline violated federal regulations on aircraft weight and balance.

According to the FAA, Frontier repeatedly violated operations specifications for weight and balance on 15 Boeing 737s during a week in March.