Just as

United Airlines

has begun improving its image -- settling contract talks with its

pilots, issuing televised apologies and bonus frequent flyer miles to passengers and boasting of a dramatic decline in cancelled and delayed flights -- its poor past performance has caught up with it again.

On Thursday, the

Department of Transportation

released figures showing that less than half of all United Airlines flights arrived on time in July, making it the worst on-time performer among the nation's major carriers for the third month in a row.

Plagued by crew shortages, traffic control back-ups and unusually bad weather at some of its hub airports, the nation's largest airline was able to get less than 42% of its flights to the gate on time in July, the most recent month for which data is available.

"We certainly acknowledge that our performance was rather poor," said United spokesman Chris Brathwaite.

But Brathwaite hastened to add that the carrier's on-time rate has vastly improved in the past few weeks. So far this month, three-quarters of its flights have arrived within 14 minutes of the scheduled time (the industry's standard for an "on-time" arrival). On Friday, just 15% of its flights had been delayed by midafternoon.

Still, passengers were not the only ones upset by the unusually high number of flight disruptions this summer.

Flight Attendants Stand Up

United's 25,000 union flight attendants have filed for protest permits at the airline's 19 "base" airports around the world. This week they began protesting at the airports in San Francisco and Denver, with plans to expand the demonstrations to the remaining airports as soon as the permits are approved at each site. The demonstrations are not expected to affect scheduled flights as flight attendants are protesting in their free time.

The final straw that prompted the demonstrations, said union spokeswoman Dawn Deeks, was when the Chicago-based airline requested that some of the union flight attendants remain on call in the last week of August under a special "critical coverage" provision. The airline said the provision was enacted because of crew shortages caused by the unusually high number of delays this summer. The union complains that some flight attendants who were unable to come in to cover a shift were either fired or "severely disciplined."

The carrier's flight attendants also claim that United violated their contract by failing to assign hotel rooms for overnight stays, failing to notify attendants about flight cancellations or delays, and lying to flight attendants when abuses and violations occurred. They say they were forced to change their work schedules frequently and without much advance notice because of scheduling problems created by mismanagement.

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"Flight attendants stood by United in its summer of turmoil," Linda Farrow, president of the master executive council of the United branch of the union, said in a statement. "In return, management has wreaked havoc on our family and personal lives by systematically breaking the rules it has agreed to in our contract and by heavy-handed discipline."

About 600 of United's 25,000 flight attendants, or less than 1%, were called in to work on their time off under the critical coverage provision in August, according to the airline.

United spokesman Matt Triaca said that although the "critical coverage" provision was enacted twice this year, affecting about 1,000 flight attendants, it had last been used in 1996. The provision is intended to be used when the airline is unable to cover its scheduled flights due to extraordinary circumstances -- like bad weather and labor disruptions.

"It's not something we use liberally," Triaca said.

Triaca added that the airline is in talks with executive members of the flight attendants' union, and has made a commitment to investigate any alleged contract violations or incidences of unnecessarily harsh disciplinary actions against the flight attendants.

Deeks said the union has not planned any work stoppage actions. She emphasized that all of the flight attendants participating in the union-organized protests were doing so on their own time, so as not to disrupt United's flight schedule.

Still, Deeks added that resolving the violations in the contract, which does not expire for another half-dozen years, is of "the utmost importance."

The airline is also in the midst of contract negotiations with its groundworkers' union. United's contract expired in mid-July with its 44,000 union ground workers -- including the airline's mechanics, ramp workers, reservation and ticket agents and baggage personnel.

The airline has blamed labor disruptions for the forced

cancellation of thousands of flights. In August, United acknowledged that it would cancel 1,980 flights in September and an additional 1,940 flights in October, in addition to the 4,800 flights removed from its summer schedule after union pilots began refusing to fly overtime when their contract expired. Early this month, they agreed to a tentative contract.

Shares of United's parent company,


(UAL) - Get United Airlines Holdings, Inc. Report

, closed down 38 cents, or 1%, at $46.63.