The nation's leading airline pilot union Tuesday began asking passengers to get more involved in security on board planes, including possibly intervening in hijackings.

        Officials from the Air Line Pilots Association, which represents more than 67,000 pilots in the United States and Canada, said that given what happened last week, “the model has changed.”

        “What we are saying is to keep your antenna up and to be alert,” said Henry Gasque, spokesman for the Washington, D.C.-based union. “We're not asking people to charge the bayonets, but to use their best judgment.”

        But federal officials are not advocating such behavior.

        The Federal Aviation Administration isn't recommending any course of action for passengers, spokeswoman Kathleen Bergen said.

        Mr. Gasque said each situation is different, and that the union was not offering specific advice to passengers.

        There is evidence that passenger involvement might have caused the crash in Pennsylvania of United Airlines Flight 93, which authorities say might have been headed toward a target in Washington, D.C.

        Previously, pilots were trained to tell passengers to be passive, since most hijackers wanted money or to fly to a different destination.

        “This is a totally foreign concept to us, too, but the contemporary world has changed in the past week,” Mr. Gasque said.

        The union is expected to take out nationwide ads over the next few weeks asking for passengers' help in identifying potential security risks.

        In addition, the union wants:

        • The reinforcement of cockpit doors.

        • Training to be revised, given the circumstances of last week's attacks.

        • The FAA to investigate letting pilots be armed.

        Rather than follow old policies, which call for negotiation and cooperation, the union also is recommending that pilots take aggressive steps to combat suicidal hijackers, including depressurizing cabins at high altitude, which can quickly cause unconsciousness and eventually kill anyone not using an oxygen mask.

        Other aggressive steps could include barrel rolls or steep dives.

        The FAA's Ms. Bergen said training procedures for pilots and flight attendants in hijackings, which previously stressed compliance with demands, are under review, but she would not offer specifics.

        USA Today contributed to this report.

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