The primary concern: door transitions, or times when the cockpit door is opened and closed during a flight. Although most transitions are over before passengers even realize they're happening, Folsom and others, including Capt. Rob Powers, said they believe airplanes are not as secure as they can and should be during these brief exchanges.

"Just like in medieval times, the castle is secure until you open the drawbridge," said Powers, a commercial airline pilot and former chairman of the Air Line Pilots Association's National Security Committee. "If you don't have a moat or something else around to add a second layer of security, then you have some vulnerabilities. When the door's open, it's just a hole."

Post-9/11 FAA regulations do permit flight crews to open and close cockpit doors during flights, but only when it is necessary to do so. Typically, this means meeting pilots' physiological needs -- for example, lavatory use and food and beverage service -- though airline officials said pilots may need to exit the cockpit if mechanical problems arise during a flight.

Currently, each airline has its own FAA-approved procedures that specify how crews should go about opening and closing the cockpit door mid-flight. While the FAA doesn't specify how long or how many times the door can be opened per flight, FAA officials said it is assumed crews will do it as quickly and minimally as possible, usually for under five seconds.

But numerous passenger complaints, obtained from the Transportation Security Administration through a Freedom of Information Act request, suggest this isn't always the case.

On April 28, 2007, a passenger flying from Dallas/Fort Worth International Airport to San Diego filed a complaint saying that the cockpit door was open for five minutes mid-flight, according to TSA records.

More recently, on May 3, 2010, a passenger flying between Columbus, Ohio, and St. Louis said the cockpit door was opened a minimum of five times during the flight.

"Once that door comes open, you're vulnerable to another attack like 9/11," said Capt. Paul Onorato, a commercial airline pilot and former president of the Coalition of Airline Pilots Associations.

In recent years, according to Onorato, the coalition has set up scenarios where a handful of would-be terrorists are seated in the first few rows of an airplane. These scenarios have shown that terrorists are able to "blow past" both a flight attendant and galley cart and get into the cockpit within three seconds.

What's more, Folsom added that the drills, while grim, likely offer a best-case scenario.

"We looked at some highly trained individuals playing the role of flight attendants and crew members and even they couldn't prevent a breach of the flight deck," Folsom said. "If they can't do it, then little 57-year-old, 40 percent body fat Susie isn't going to do it."