Lawyers for US Airways and its pilots union squared off in federal court in Charlotte on Friday, with the company trying to convince a judge that its pilots are engaged in an illegal work slowdown.

Tempe-based US Airways presented what it says is statistical evidence that certain flights have been delayed more frequently since May 1 as a result of the union's action. The union's goal, US Airways says, is to force the company's hand in contract negotiations.

The US Airline Pilots Association, based in Charlotte, has denied any work slowdown and says any delays are the result of legitimate safety concerns. The union says the airline is trying to intimidate pilots into flying unsafe planes and speed up performance. The union has also sued US Airways in federal court in New York.

On Friday, the union presented testimony from a pilot who says the company tried to force her to fly a plane with a faulty electrical system and disciplined her when she refused to do so.

"We're here about safety," said union president and US Airways pilot Capt. Mike Cleary. When asked if there was a slowdown under way, he said, "Absolutely not."

Robert Siegel, attorney for US Airways, said the union's safety allegations are a "subterfuge" for a "slowdown under the moniker of safety."

The two sides are expected to wrap up their cases before U.S. Chief District Judge Robert Conrad on Monday. Conrad will then decide whether to order the union to halt the alleged slowdown.

Friday's legal battle is the latest chapter in a six-year fight that stretches back to US Airways' 2005 merger with America West. Pilots from the two companies have been unable to agree on a combined seniority list or reach a new contract with the company, leading to bitter legal battles.

US Airways flies about 630 daily flights out of Charlotte/Douglas International Airport, about 90 percent of the airport's total. The airport is US Airways' busiest hub, and the airline has said passengers traveling to and from Charlotte are being disproportionately affected by delays, flight cancellations and missing baggage resulting from the alleged slowdown.

Under federal labor law, pilots are forbidden to take any action such as a strike or slowdown until contract negotiations have reached a government-declared impasse. Negotiations between the union and US Airways are currently under federal mediation.

On Friday, US Airways presented a statistical expert who analyzed flight data for the company. He testified that delays and cancellations since May 1 could not be explained by any factors other than a pilot slowdown.

The union's lawyers tried to poke holes in the statistical analysis, arguing that other factors such as weather and airport construction that could cause flight delays had not been properly accounted for.

Several US Airways executives also said that anonymous messages have been sent to pilots encouraging the alleged slowdown, and pro-slowdown decals have been left in cockpits.

Under cross-examination, however, they said they could not say with certainty that the messages were actually sent by pilots.

US Airways Vice President Lyle Hogg testified that he had received a box sent to the chief pilots' office in Charlotte that contained tags. On one side, the plastic tags read "I'm On Board Safety First" and on the other "I've Had Enough I'm Ready to Strike."

When asked where the box had been mailed from, Hogg said the mailing label showed USAPA's offices in Charlotte. When the union's lawyers objected on the grounds that the box wasn't in the courtroom, US Airways' attorneys pulled out the U.S. Postal Service flat rate box.

Attorney Brian O'Dwyer, representing USAPA, said he didn't think any pilots would have sent the box and said the episode "stinks to high heaven."

Valerie Wells, a US Airways Airbus A-330 captain, testified that in June she was told to fly a plane with a faulty electrical system on a night flight from Philadelphia to Rome.

"Our screens went black. All our lights went out," Wells said. "Since we had no radio, we opened our sliding window ... and yelled for help."

After refusing to fly the plane, Wells said she was escorted out of the airport by US Airways security and removed from her flying duties for three weeks with no official explanation.

US Airways declined to comment on Wells' testimony.

The hearing will resume Monday morning. Observer researcher Maria David contributed