United Airlines is threatening to fire some flight attendants it suspects are engaging in a sophisticated effort to grab the best trips — such as lucrative and comfortable long-haul flights to London, Sydney, or Tokyo — and selling them to colleagues, according to a new memo.

“We have zero tolerance for this prohibited behavior,” wrote P. Douglas McKeen, senior vice president for labor relations. “When we discover that it’s occurring, we will fully investigate and take appropriate action, up to and including discharge.”

This marks the second time in the past month United has warned employees not to commit fraud against their employer. Early in March, United admitted it fired 35 employees it said had sold their employee travel passes. Employees can give the passes, which permit inexpensive travel, to friends and family but cannot sell them. According to United, some passengers paid as much as $4,000 for a year’s worth of discounted flights.

The recently uncovered fraud was more of an inside job, United said, with flight attendants scheming to take advantage of colleagues.

Both the union and the company said in separate memos that upset flight attendants brought the matter to their attention. Neither memo said how may flight attendants were accused of unethical behavior, but it is believed to be a small number. United said an “overwhelming number” of flight attendants followed rules.

“Over the past few months, we have been aware that many of you have voiced concerns about illicit trip brokering where certain individuals have been improperly ‘parking’ and holding trips for their personal gain,” the Association of Flight Attendants (AFA) said in its note, dated March 23. “We’ve heard your frustration and recognize how many of you have exercised restraint in reporting this egregious activity to management because we are unionists.”

The Scheme

At United, as with most U.S. carriers, crews bid for trips based on seniority, so flight attendants with the longest tenure usually take the best trips. Flight attendants often prefer the longest flights, both because they pay more in a shorter time period — they’re paid by the hour but only when in the air — and they have longer layovers, often in better hotels than for domestic trips.

United’s contract allows flight attendants to trade trips to colleagues if they cannot fly them. The system is designed to ensure coverage for “unforeseen events,” McKeen said.

“Those trades are not the issue,” McKeen said. “What we’re addressing is the growing problem of selecting, trading, or parking a pairing to broker, buy, or sell it to another flight attendant.”

Neither United nor the the union representing United’s cabin crew, would say much about the circumstances. But McKeen’s memo, along with a similar one from the union, suggests this practice is more involved than some flight attendants simply asking for payment from friends in exchange for a cushy trip.

United said it searched social media and found flight attendants using code words to describe the kickback approach, promising “hugs,” “kisses,” “candy canes,” and “expressions of appreciation” to flight attendants who participated.

“This is about fairness, plain and simple,” McKeen said. “No flight attendant should have an unfair advantage beyond their seniority rights when it comes to managing their schedule or accessing flying opportunities.”

Usually, organized labor protects members on the verge of disciplinary action but the union said it would not stand by behind members who committed fraud against other flight attendants.

United, meanwhile, said it would continue to investigate.

“We know schedules are very important to our flight attendants, and we work closely with AFA to make sure our flight attendant scheduling is fair for all of them,” a United spokesman said in an email.

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