WASHINGTON (AP) _ The Air Force resumed training flights over the East and Gulf coasts Tuesday after a four-day suspension that included a quick course in the dangers of shadowing airliners with sensitive collision avoidance systems.

One of the things investigators discovered was that many commercial planes' alerts can be triggered by fighter jets at distances the military pilots may not be counting on, Pentagon spokesman Kenneth Bacon said.

In two cases last week, collision alarms sounded in airliners when jet fighters came too close, causing one airline pilot to maneuver so sharply that three people were thrown to the floor. In the other case the pilot went into a descent to avoid the military planes. In two other cases alarms did not sound.

``The military needs to remind their pilots of the effect of close approaches to aircraft like that,'' said Tony Broderick, an industry consultant who formerly headed the Federal Aviation Administration's regulation and certification office.

Bacon, the Pentagon spokesman, said the Air Force is redesigning its training to take into account the sensitivity of collision avoidance systems on civilian planes.

Training in areas along the East and Gulf Coasts was suspended Friday for a review of safety procedures, but Air Force spokesman Capt. Leo Devine said many units have now resumed flying.

Authorities insist that none of the planes was in danger. But the incidents raised concern about close calls in the sky _ particularly since all four cases involved F-16 fighter jets.

The case causing most concern occurred last Wednesday when two Air National Guard fighters doing interception training off the coast of New Jersey discovered a Nations Air flight nearby and one fighter approached it.

Like all airliners with 30 or more seats, the Nations Air Boeing 727 had a collision alert system. It went off and the pilot followed instructions to dive.

The military pilots may see an airliner and think they can practice interceptions by flying in behind without being seen, said Jim Burnett, a former National Transportation Safety Board member and now an aviation consultant. ``They may not know there is a computerized radar that will pick them up and advise the commercial pilot to do a recovery maneuver.''

David Stempler, an aviation attorney, said the incidents indicate military aircraft have been in the habit of practicing their intercepting techniques with commercial aircraft.

``They have been doing this all along, but they've just never been caught,'' Stempler said.

Bacon said, ``The flying public, I believe, has nothing to worry about. The military is trained to stay well away from civilian airliners.''

Eliot Brenner, a spokesman for the Federal Aviation Administration, said the recording of four incidents in three days was ``just a statistical anomaly and probably drew attention only because the media is focused on it at the moment.''

Pilot reports would have led to disclosure of the Nations Air incident and one Friday involving military jets passing above and below an American Eagle flight. But information on the other two cases _ one over Texas and the other over New Mexico _ was made public only when reporters asked if there had been other cases.

It is relatively common for planes to get closer than specified in regulations, and air traffic controllers routinely direct them to change course or altitude to correct the problem. Called pilot deviations, there were 1,277 incidents in 1996 when planes strayed in that way from their assigned course or level, the FAA reported.

While that indicates an average of 3.5 incidents a day, aviation experts point out that things don't happen that smoothly and incidents tend to occur in bunches for no apparent reason.

The FAA also records what it calls ``near midair collisions,'' which are reported by pilots. There were 202 last year, down from 241 a year earlier.

Even here, though, the report is subjective and FAA officials say it may not indicate a real hazard. Factors that influence whether a pilot submits a report include how close the airplanes were, whether the pilot was surprised and ``heightened alertness of the flight crew ... because of publicity surrounding a near or actual midair collision,'' the FAA reports.

Planes are supposed to get no closer than 5 miles horizontally and 1,000 to 2,000 feet vertically, and coming closer than that triggers the collision warning.