When airlines make money, everybody is happy: shareholders, employees and, most important -- at least for this column -- travelers.

But when airlines lose money, passengers often experience a sharp decline in customer service.

The drop in customer service has been pronounced at some of the carriers with deep financial problems.

For the first six months of 2005, US Airways, United Airlines, Delta Air Lines and Independence Air were among the carriers that had the highest rates of consumer complaints filed with the government. The complaint rate for each of those airlines rose during the period.

Those four carriers are also among the most chronically financially troubled: US Airways and United are already in bankruptcy court protection, while Delta and Independence Air have in recent weeks mentioned the possibility of filing for Chapter 11.

"As an airline's fortunes decline, it sometimes creates a bad environment for your fliers," says Darryl Jenkins, visiting professor at Embry-Riddle Aeronautical University.

Complaints filed with the Department of Transportation against Arlington-based US Airways more than doubled in the first six months of this year compared with the same period last year. As calculated on a per-100,000-traveler basis, US Airways' complaint rate ranked second-worst among the 19 major airlines in the report.

Delta's complaints rose 39 percent from a year ago, while United recorded a nearly 11 percent increase. Independence Air's complaints increased nearly 7 percent from when it operated as Atlantic Coast Airlines and served as a feeder carrier for Delta and United. The Dulles-based carrier's complaint rate ranked in the bottom three of the airlines in the report, just ahead of US Airways.

Overall, complaints against the nation's top airlines over lost bags, delayed flights and employees' attitudes increased 30 percent between January and June, compared with the same period last year.

Customer complaints were not limited to carriers reporting a loss. American Airlines, which reported earnings of $58 million in the most recent quarter, had a 24 percent increase in grievances filed with the government. And America West, which has agreed to merge with US Airways and reported a profit in the most recent quarter, had a 14 percent increase in complaints from last year.

It has been a difficult year for the airline industry, with fuel prices soaring to record levels and summer thunderstorms wreaking havoc on operations along the East Coast and in the South. As fuel costs continue to climb, airlines are searching for cuts elsewhere.

When an airline looks to cut costs, it often targets labor first, because workforce expenses are usually the largest and most manageable. Front-line employees typically take the biggest hit -- losing pay, benefits and sometimes even their jobs. These cuts can affect how workers relate with passengers.

"The employees give up and become surly and more resentful about the company," Jenkins said.

It may not be a coincidence that the airlines with the fewest complaints this year are among the strongest financially: Southwest Airlines and JetBlue Airways. Both airlines reported a profit in the second quarter.

Ray Neidl, analyst with Calyon Securities, said travelers become irritated when they take longer flights and find fewer amenities such as hot meals and blankets. And, he said, travelers are likely to see even more service cuts until carriers are able to reverse their losses.

"Airlines are going to start looking at everything they can to cut until they start making money," Neidl said.

If You're Flying Northwest Soon: The countdown has begun. Northwest's mechanics could strike at 12:01 a.m. Saturday if they do not reach a new labor agreement with the financially ailing airline.

If its mechanics do strike, Northwest says it will be business as usual and it will fly its full operations. The airline says it has 1,500 replacement mechanics ready to work. "We have been working on this plan for 18 months," Northwest spokesman Scott Tennant said.

Both sides returned to the negotiating tables yesterday. Meanwhile, the White House said that President Bush has no plans to appoint an emergency board that could prevent a strike or a lockout.

Terry Trippler, airline expert with Minneapolis-based Cheapseats.com, said travelers should prepare for "some bumps" this weekend. Trippler said those passengers who have to fly Northwest during the next week, should travel earlier in the day, even if that means flying standby. Travelers on late-afternoon or evening flights risk being stranded if a flight is delayed or canceled.

America West-US Airways Merger: Yesterday, America West Airways began showing a seven-minute in-flight video telling passengers about its planned merger with US Airways. The video explains the airline's name change to US Airways and reviews frequent-flier information. On Sept. 1, US Airways will start showing a three-minute video to its passengers explaining what the new airline will look like. The merger, which still must receive approval from America West shareholders and US Airways creditors, is scheduled to be completed by early October.

Northwest Airlines employees support Northwest mechanics at a rally. The carrier says a strike won't affect flights.