The new US Airways Group Inc. made its debut yesterday on the New York Stock Exchange, plunging more than 8 percent as investors fretted over whether the carrier's merger with America West Holdings Corp. could create a low-cost airline to compete in a tough market.

"Now that the marriage has taken place, the honeymoon ends quickly. Now comes the hard part in making this airline work," said analyst Ray Neidl of Calyon Securities Inc. "If they pull this off successfully, then the value will go up. If they don't, the value will decline."

Investors drove down US Airways shares by 8.3 percent, or $1.75, to close at $19.30. The airline's shares trade under the symbol LCC, for low-cost carrier. Their decline yesterday mirrored losses in the broader airline sector which is under pressure from high fuel costs.

The merger was completed yesterday after the carriers worked for four months to convince shareholders, employees and government officials that the union would be beneficial to both airlines.

After a year in Chapter 11 bankruptcy protection, US Airways officially emerged from court protection, having restructured primarily to prepare for the merger.

Doug Parker, former president and chief executive of America West and now head of the new US Airways, called the merged airline a "new chapter in aviation history."

The combined airline becomes the nation's fifth largest carrier and the nation's biggest full service, low-cost, low-fare airline, the airline's executives said.

Merging the two airlines' operations will proceed gradually. Each will continue to operate separately for about two to three years. They will maintain separate Web sites and reservation systems for several months.

Wall Street analysts said obtaining approval from government and bankruptcy court regulators was the easiest part of the merger. The difficult part lies ahead as the new carrier seeks to be profitable in a troubled industry.

Standard & Poor's yesterday gave US Airways a corporate credit rating of B-minus with a watch-negative designation.

"The combined entity will face significant hurdles, particularly integration of its labor forces," wrote Standard & Poor's credit analyst Betsy Snyder.

The new US Airways will be based in Tempe, Ariz., the headquarters of America West. During the next several months, the airline will be shutting down the former US Airways corporate offices in Crystal City. The 600 employees who worked there will have to move to Tempe or find employment elsewhere. Friday will be the last day for many of the senior executives of the former US Airways.