One of the most visible elements of the new security effort was the requirement that reinforced doors be installed. By March 2002, the FAA reported that all major U.S. airlines had complied. Critics, however, say a stronger door is only half the solution. "People have this illusion hardened cockpit doors work, and they don't," Dzakovic says. "If you want to have a secure door, you need to have a double-hulled door."

Consumer Reports searched NASA's Aviation Safety Reporting System and found 51 incidents since April 2002 in which flight crews reported problems with the hardened doors.

In many instances, the door unexpectedly opened in flight or the locking mechanisms failed. In one case, the door spontaneously opened twice on a Bombardier CRJ200 regional jet, interrupting a takeoff. And a captain said the doors on two DC9s were broken after flight attendants slammed them.

A 2006 study of aviation security by DFI International, a Washington, D.C., security consultancy, found that a drunken passenger kicked a hole in a door panel and that aircraft cleaners "broke a fortified door off its hinges by running a heavy snack cart into it on a bet."

In October 2007, Rep. Steve Israel, D-N.Y., introduced a bill that would require the FAA to order the installation of secondary barriers on commercial airliners, at a cost he estimates at $5,000 to $10,000 per aircraft. "Without secondary cockpit barriers, the door is literally wide open to terrorists," he stated in proposing the measure.

Hesselbein, of the Air Line Pilots Association, agrees, saying "We believe it's the No. 1 thing that can be done for security."