WASHINGTON - The National Transportation Safety Board has opened an investigation into the aborted landing this week of a plane carrying first lady Michelle Obama.

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While aborted landings are not unusual, and Mrs. Obama apparently was not in serious danger, the episode has become another embarrassment for the Federal Aviation Administration. FAA officials have been struggling to calm public jitters about flying raised by nine suspensions of air traffic controllers and supervisors around the country in recent weeks, including five for sleeping on the job.

The first disclosed case of a controller falling asleep on duty occurred March 23 at Washington's Reagan National airport. The most recent suspension was this week when a controller at a regional radar facility near Cleveland, Ohio, was suspended for watching a movie on a DVD player when he was supposed to have been monitoring air traffic. The FAA official in charge of the US air traffic system resigned last week.

The incident involving the first lady and Jill Biden, wife of Vice President Joe Biden, occurred at about 5 pm on Monday when a Boeing 737 belonging to the Air National Guard, one of several guard planes used by the White House, came within about three miles (5 kilometers) of a massive C-17 cargo plane as the planes were approaching Andrews Air Force Base in Maryland to land, according to the FAA and Maj. Michelle Lai, a spokeswoman for Andrews.

The FAA requires a minimum separation of five miles (8 kilometers) between two planes when the plane in the lead is as large as the 200-ton military cargo jet, in order to avoid dangerous wake turbulence that can severely affect the trailing aircraft.

Andrews' civilian air traffic controllers initially ordered Mrs. Obama's plane to conduct a series of turns to bring it farther from the military jet. When that did not provide enough distance, controllers realized that there might not be enough time for the cargo plane to clear the Andrews runway before Mrs. Obama's plane landed.

Controllers then directed the pilot of Mrs. Obama's plane to execute a "go-around" - to stop descending and start climbing - and circle the airport, located in a Maryland suburb of Washington. A go-around is considered a type of aborted landing.

The required separation between the two planes "was compromised," the NTSB said in a statement on Wednesday.