While a majority of airports did open for business Friday, aviation officials said most of the airlines would operate only 50 percent of their regular flight schedule as they headed into the weekend.

On a regular weekday morning in America, more than 4,000 aircraft would be in the sky, compared to a scant 1,700 on Friday. Some major airports are still closed, notably Boston's Logan Airport where the two airliners that struck the World Trade Center originated.

The news is yet another blow for the beleaguered airline industry, which is already facing huge financial losses and sky-high fuel prices.

On Thursday already, experts had warned that rescheduling problems and new security measures coupled with an expected drop-off in air travel would leave U.S. and world airlines with unprecedented losses.

UBS Warburg estimated the U.S. airline industry will take a $2.1 billion hit, leading to the largest loss ever in the industry — $4.4 billion, adding there would be residual impact in 2002 and forecast a $400 million loss for the industry in 2002. 

"Not all airlines will make it outside of bankruptcy protection,'' the research note said. 

East Coast, New York and long-haul operations will probably be hurt disproportionately, the report also said.

The International Air Transport Association (IATA) had an even more somber outlook. The body whose member carriers include all major airlines said that, worldwide, the industry could be facing losses of up to $10 billion. 

The U.S. market is worth about $1 billion per day, according to IATA. But the slowdown on Canadian and Mexican markets as well as in transpacific and transatlantic traffic must also be taken into account. 

The IATA annual figures cover only international scheduled flights and do not include the U.S. domestic market. 

The world's airlines suffered heavy losses in the period of 1990-94. The worst year was after the 1991 Gulf War, with IATA airlines reporting combined losses of $7.5 billion in 1992, the spokesman said. 

Raleigh/Durham-based Midway Airlines on Wednesday became the first victim of the new economic landscape. The embattled carrier, which announced it would be halting operations and laying off 1,700 employees, cited the impact of the disaster as a major factor in its decision. Midway — which had filed for chapter 11 bankruptcy in mid-August — also said that because of the recent attacks, "demand for air transportation is expected to decline sharply."

The airline had gained a reputation for service and first class-like amenities, on-time performance and reliable service to a handful of East Coast and Midwestern cities including Boston, New York, Washington, Miami, New Orleans, Atlanta and Denver with a fleet of mostly Boeing 737-700s and Canadair Regional Jets.