CHARLOTTE, N.C. TheStreet) -- The battle between US Airways(LLC) and its pilots over the airline's safety culture is continuing, this time focused on an incident in which a captain declined to fly a transatlantic flight.



On June 16, captain Valerie Wells, a 30-year-pilot, was scheduled to fly an Airbus A330, which can carry nearly 300 passengers, on a flight from Philadelphia to Rome. But she declined to fly because of failures of both the auxiliary power unit, a backup source of electrical power, and the "hot battery bus," a primary source of electrical power.



After the crew and passengers had returned to the gate Wells, in a particularly unusual event, was escorted out of the airport by security officials. Subsequently, a second crew of three pilots also declined to fly; the aircraft was repaired and underwent a rigorous inspection, and a third crew took off about six to seven hours late.



In seeking to publicize the incident, the U.S. Airline Pilots Association took out a full-page advertisement in Friday's edition of USA Today. The ad proclaimed that US Airways put "revenues first, safety second.



"The intimidation of flight crews is becoming commonplace at US Airways, [which] works to maximize their revenues by pushing their employees to move their airplanes regardless of the potential human cost," said the text. The ad referred readers to a website, www.USAirlinePilots.org/SafetyFirst.



In a letter to employees on Friday, Robert Isom, chief operating officer, wrote that "USAPA has embarked upon a smear campaign that in reality is all about contract negotiations, not safety.



"I can tell you unequivocally the union's claims are outlandish, false and a disservice to the 32,000 hard-working employees of US Airways," Isom wrote. "Safety has been and always will be the top priority at US Airways, as it is at any airline."



Union spokesman James Ray said that initially, Wells could not possibly fly the airplane because it lacked cockpit electrical power, but a chief pilot nonetheless encouraged her to fly. He said the incident symbolized US Airways' desire to enhance on-time performance and revenues. "This is not just an isolated incident," he said. "It has been going on on a daily basis, and is the kind of practice we've been fighting for a number of years now."



Airline spokesman John McDonald said the incident is under investigation. He said "the fact that [Wells] was escorted off the property had nothing to do with safety," but declined to elaborate. Ray speculated the airline did not want Wells to tell the replacement crew of the problems she had with the aircraft.



McDonald said that to be cautious, the airline "took the batteries off site and had them tested, and they were found to be in proper working order." He said the real issue in the incident was about the union "negotiating and trying to get leverage against the company, using safety as an issue, which it is not."



In a statement issued late Friday, the Federal Aviation Administration said the APU shutdown in the aircraft "is a failure that pilots are well aware can happen and that they are trained to recognize. The battery apparently was depleted by attempts to restart the APU." The agency said aircraft often fly with inoperative APUs, without a safety risk, but "the captain simply chose to exercise her pilot-in-command authority of not accepting an aircraft." It said US Airways maintained the aircraft in accordance with regulations. Ray noted that in addition to the APU failure, the hot battery bus also failed, causing an instrument failure that justified the Wells' decision.



US Airways recently passed the International Air Transport Association's safety audit "with flying colors," Isom noted in his letter, while the FAA has cited US Airways as an industry model for its safety management system. Ray said the IATA inspection is "nothing more than a self-evaluation by a trade association." Ray also denied that the safety claims are a negotiating tactic. "How do you think pilots are going to gain at the negotiating table by pointing out we have a safety issue at the company?" he said.



Friday marked the second time USAPA criticized US Airways safety practices in USA Today. A 2008 ad in the newspaper asserted that the airline was pressuring pilots to fly with less fuel than they might prefer.