After 9/11, Ellen Saracini, widow of the pilot of United Airlines flight 175, which flew into the World Trade Center, was pleased when Congress ordered all cockpit doors reinforced and heartened when United, although not mandated, outfitted its airplanes with secondary doors.

“The secondary barrier is a lightweight wire-mesh door that is locked in place to act as a barrier for when the cockpit door is opened during flight when the crew comes out for a meal or bathroom breaks,” Saracini explained to me. “It is not a replacement for a reinforced cockpit door. It is just a door intended to provide enhanced security for a few extra seconds until the cockpit door can be closed.”

When United took the lead in this regard, it earned plaudits from the Air Line Pilots Association International (ALPA). Delaying a hijacker for just five seconds could make a difference, ALPA said in a 2007 report.

But United has changed course. The airline took possession of the first of 50 ordered 787 Dreamliners from Boeing in September, and the aircraft were delivered without a secondary door.

Why the change?

That’s the “question of the century,” Saracini said. On Oct. 11, she wrote to United CEO Jeff Smisek, seeking an answer. In August, two ALPA representatives raised similar concerns with Smisek. They wrote: “The events of 9/11 showed us that there are significant threats to aviation that should not, that cannot, be ignored. . . . Following the tragedy of Sept. 11, United Airlines . . . made a commitment to protecting the cockpits so that 9/11 could never happen again. United management established a goal to install ‘secondary security barriers . . . on all aircraft’ and in doing so, set the example for the industry.” A union representative told me it did not get a reply from Smisek.