Friday September 14 5:57 PM ET

By Jane Light

CHICAGO (Reuters) –

U.S. airports and airlines struggled on Friday to cope with heightened security measures, passenger confusion and nerves as the industry scrambled to resume services following this week's hijack attacks.

While the two airlines -- American and United -- whose planes were involved in the coordinated strikes on New York and Washington flew again with sharply reduced schedules, several key airports remained shut.

These included Boston's Logan International Airport, from where two of the hijacked planes took off, and Washington's Reagan National Airport, because of its proximity to the Pentagon and other key government buildings such as the White House. New York-area airports Kennedy, LaGuardia and Newark, which had been closed, reopened on Friday.

In Boston, passengers who had been told they would get on flights began arriving on Friday only to be turned away by state police.

``Logan is not open,'' said Phil Orlandella, spokesman for the Massachusetts Port Authority. ``We are meeting with the airlines this morning. And when we do reopen, we will give the airlines two to three hours' notice before we open.''

Hijackers commandeered aircraft and crashed them into New York's landmark World Trade Center twin towers, which both collapsed. Another flight was rammed into the Pentagon and a fourth plane crashed in Pennsylvania, apparently missing its target. Hundreds were killed and thousands more were missing.

Passengers, who had endured days of chaos from the unprecedented grounding of all commercial planes, were jittery on Friday, a day after agents in combat gear stormed a plane at New York's Kennedy airport and detained a number of people. Officials later said it was a false alarm.

In Chicago, Joe Lopresti landed at O'Hare Airport on a United flight from Portland, Oregon, where the United Airlines crew had been escorted in a group by security guards onto the plane.

``I have to tell you, I read my book and tried not to think about it. Everyone was concerned and scared on the plane,'' he said.

Malcolm Kilminster of Britain, another passenger at O'Hare who travels between Chicago and London every three months, has been trying to get to London since Tuesday. ``I've been very struck by the spirit and the courage of people here,'' Kilminster said. ``It's hard to talk about it without getting emotional. I admire the attitude of everyone.''

CHICAGO SCARE

Chicago police said they detained two people at the city's Midway Airport on Friday. The airport was closed and no further information was immediately available.

Indianapolis-based American Trans Air, a unit of Amtran Inc. reported one of its agents identified a passenger at Midway who might pose a security risk. The carrier said that as a result of its security procedures, the agent immediately telephoned the Chicago Police Department.

Police officers wearing military-style uniforms and carrying pistols patrolled the halls of Atlanta's Hartsfield, the world's busiest airport, for a second day in a row. On a normal day, about 2,500 planes take off and land at the airport.

At San Francisco International Airport, lines of passengers snaked through the terminal buildings patiently undergoing increased security checks for the relatively few flights permitted to depart.

Operations at the airport, which handles about 1,200 flights per day, would probably be only at about 40 percent capacity. ``We're still looking at probably seven days to get the whole thing straightened out,'' Airport spokesman Ron Wilson said.

In Washington, aviation officials said airlines hoped to operate up to 50 percent of their regular flights on Friday.

The U.S. Transportation Department ordered U.S. national airspace reopened to commercial and private aviation on Thursday, but flights could only resume after airports implemented new security measures.

Air traffic controllers early on Friday were tracking 1,700 flights, the Federal Aviation Administration said. Normally about 4,000 flights would be in the air on a weekday morning.

FAA spokesman Paul Takemoto said 423 out of 451 airports offering regular scheduled air service had reopened after having certified they are meeting new security requirements.

After some foreign airlines' planes were turned back in mid-air on Thursday, Takemoto said foreign passenger and all-cargo airlines could resume flights to the United States once they and the airports they depart from meet tighter security requirements.

MAJOR CARRIERS OPERATE ON REDUCED SCHEDULES

American Airlines, the world's largest airline, planned to operate a sharply reduced flight schedule on Friday after two of its planes were hijacked on Tuesday. American, a unit of AMR Corp., said it would operate about a third of its regular schedule, while its regional affiliate, American Eagle, would operate about half of its schedule. TWA, also owned by AMR, will fly about 65 percent of its schedule.

American planned to resume other flights gradually until it reaches about 80 percent of the schedule it flew before the hijackings. The FAA's new security measures necessitate a reduced schedule in order to minimize delays and customer inconvenience, the airline said.

The second airline involved in the attacks, United Airlines, was operating some 500 flights in North America, about 20 percent of its normal daily flights. ``Through the weekend we will continue to be substantially reduced as we ramp up,'' said Steve Fushelberger, spokesman for United, the second largest air carrier, which is a unit of UAL Corp.

Houston, Texas-based Continental Airlines Inc said it was prepared to operate up to 50 percent of its regularly scheduled flights on Friday. The approximately 1,000 flights planned by Continental and Continental Express include service to most of their destination cities.

Northwest Airlines, the Minneapolis, Minnesota-based carrier expected to operate about 20 percent of its domestic and international scheduled flights on Friday and 50 percent on Sunday, said Mary Beth Schubert, a Northwest spokeswoman. The airline, the fourth largest, hopes to increase that number, but declined to speculate when that might be. ``Right now, we're concentrating on today and tomorrow,'' she said.