More than 120 commercial flights headed to Washington's Reagan National Airport have been diverted for security violations since 2001, mostly because pilots failed to send a secret code to air traffic controllers before landing, the Transportation Security Administration said yesterday.

Twenty flights headed to National were diverted because a passenger stood up during the last 30 minutes, a violation of a security rule put in place at the airport after the Sept. 11, 2001, terrorist attacks, figures released by the Federal Aviation Administration show. An additional 107 flights were turned away because pilots did not follow proper procedures, the TSA said.

None of the incidents involved a serious security threat. Government procedures require that if either rule is broken the planes must land immediately at nearby Dulles International Airport.

Two carriers have been diverted to Dulles so far this month for violating the 30-minute rule, and two others were diverted in April, the FAA said. Five flights were diverted in 2003, 10 in 2002 and one in 2001. Nearly every airline at National had violated the rules, including Air Wisconsin Airlines, American Airlines, Continental Airlines, Continental Express, Delta Air Lines, Midwest Express, Northwest Airlines, United Airlines and US Airways Express, the FAA said.

"We didn't see any pattern" indicating that one airline was frequently falling short of following security procedures, said FAA spokesman William Shumann.

The FAA and the TSA said they did not have annual figures for diverted flights caused by pilots.

Pilots said National is one of the most challenging airports during the approach because the flight path borders restricted airspace over the White House, Capitol and Pentagon. Pilots must navigate manually above the Potomac, unlike the skies over other metropolitan airports where aircraft can be guided by autopilot.

"Because of the more demanding workload we have flying into National, perhaps in some or many of those cases, the errors doing the correct security procedures might have been the result of that," said John Mazor, spokesman for the Air Line Pilots Association.

Gary Boettcher, an American Airlines pilot who regularly flies out of National, said the airport is the only one that requires a unique code that must be communicated to air traffic controllers before landing. The procedure may throw off some pilots unaccustomed to it, he said. "Pilots do things out of standardization. We do certain things at certain times," Boettcher said. "Sometimes guys forget -- particularly if they don't fly out of National" regularly, he said.

Some lawmakers, pilots and airlines criticize the 30-minute rule as costly and inconvenient for airlines and passengers who wind up at Dulles instead of National. Airlines say it costs $29,000 to $182,000, depending on the size of the aircraft, each time a flight is diverted. Costs include refueling the plane, busing passengers to National and sometimes changing flight crews.

"The 30-minute rule is out of date," said Rep. John L. Mica (R-Fla.). He said it was imposed as a condition for reopening National after the terrorist attacks. "Most of the flights coming out of National have air marshals on them. We tend to do things in a reactive mode and nobody bothers to remove it."

The TSA defended its rules for National, saying they add layers of security to the aviation system and for the Washington area. "While we certainly would like to see fewer occurrences of false alarms, and we're working with the airlines to achieve that, it doesn't negate the value of having those rules in place," said TSA spokesman Mark O. Hatfield Jr.

Some passengers said they have seen other passengers violate the 30-minute rule without the plane being diverted, because flight attendants or passengers told the passenger to sit down. Dan Dussault, a Boston resident who flies to Washington frequently, said he saw a flight attendant tell a passenger who had violated the rule to sit down. "I was glad," Dussault said. "If she didn't do it, I was going to. I was late for a meeting."