The president of a US pilots' union has accused American Airlines of cutting its safety margin by "manipulating" flight plans with tactics that include faster speeds.
Dan Carey of the Allied Pilots Association suggested that American was trying to avoid cancelling flights when crews pushed the limits of their legally-allowed work shifts.
American did not respond directly to the union leader's claims but a spokesman said the airline took safety and regulations seriously.
Mr Carey, who has started a three-year term as president of the association, said in a note to members that pilots were reporting pressure to approve faster flight plans.
Federal rules generally limit airline pilots to shifts of nine to 14 hours, depending on when the pilot's work day starts and the number of flights.
They can exceed their limit for unexpected delays such as bad weather, but cannot take off if they know they will not complete the flight under the limit.
The rules, designed to prevent accidents caused by fatigue, were adopted after a Colgan Air flight for Continental Airlines crashed in 2009, killing 50 people.
When crews do not have enough time left in their shift to complete a flight, the airline must find replacement pilots or cancel the flight. Either option angers customers and can cause later flights to be delayed or cancelled.
In the latest US government figures, for May, American trailed Delta Air Lines, United Airlines and Southwest Airlines in on-time arrivals and had a higher cancellation rate.
Earlier this month chief operating officer Robert Isom outlined steps American was taking to operate on schedule during the busy summer season.
He told employees in a memo that planners and pilots would "utilise 'speed up' flight plans to reduce delays involving crew duty times" on "critical flights".
Mr Carey said managers had been directing pilots to redraw flight plans to keep their shifts legal by insisting on faster speeds that were "nearing aircraft limitations", even if routes went through turbulence, which usually forces pilots to slow down.
"These last-minute manipulations are used to make a flight appear legal when in reality it's not or is, at best, on the ragged edge," he said. He said the actions caused an "erosion of the safety margin".
American Airlines Group spokesman Joshua Freed said "We take safety very seriously and we are absolutely committed to working together with our pilots and all other employees."
Robert Mann, a long-time aviation consultant and former American Airlines executive, said airlines had long used quicker flight plans to catch up when they fell behind schedule "and it's done safely".
But he added that he understood the union's concern because pilots could face sanctions if they signed an unrealistic flight plan and an accident then occurred.