Bolstered cockpit doors had unintended effect in Germanwings crash

Cockpit security on passenger airliners is under increased scrutiny after a French prosecutor concluded Thursday that the co-pilot of Germanwings Flight 9525 had "intentionally" crashed the aircraft.

Tuesday's crash in the French Alps killed 150 people.

CBS News aviation and safety expert Capt. Chesley "Sully" Sullenberger said that after the Sept. 11, 2001 terrorist attacks, locking mechanisms were implemented in cockpit doors to make it impossible for anyone, even the pilot, to enter the cockpit if someone inside was actively denying them entry.

"If there's someone in the cockpit who actively denies them that and keeps the door locked, they cannot enter, and of course this is a result of the 2001 terror attacks," Sullenberger told "CBS This Morning" on Thursday. "We had to beef up the cockpit doors, come up with elaborate locking mechanism but also we came up with human systems."

Sullenberger said that in the United States and a few other countries, protocols require that there always be two people be in the cockpit and if one pilot needs to leave for some reason then another crew member comes up so that they can safely get back in the cockpit.

"Apparently that was not done in this case," Sullenberger said.

French prosecutor Brice Robin said in Marseille on Thursday that the co-pilot, Andreas Lubitz, requested control of the aircraft about 20 minutes into the flight. The pilot then left the cockpit, leaving the co-pilot in full control of the plane.

Lubitz manually and "intentionally" set the plane on the descent that drove it into the mountainside in the southern French Alps. It was the co-pilot's "intention to destroy this plane," Robin said.

Sullenberger said that alleged pilot suicides are "extraordinarily rare" but not unprecedented. In 1999, EgyptAir Flight 990 crashed off the coast of Nantucket and the NTSB found the crash was caused by the first officer.

"This may unfortunately be another one of those," Sullenberger said. "I should say that pilots are among the most scrutinized of all professionals, certainly more than medical professionals, and yet in very rare occasions something happens that's really out of the ordinary, out of character, and it's really difficult to predict in advance which person is going to act in an very bizarre and harmful way."

According to The New York Times, the pilot tried aggressively to get back into the cockpit, but was denied access. Eventually, according to The Times account, "you can hear he (pilot) is trying to smash the door down."

Sullenberger said that if that account is true, some passengers would have been aware that something was wrong and "it would have been a terrifying number of minutes" for them. However, he said passengers likely had little chance to make a call.

"I don't know if someone had cell signal," he said. "Most of the air phones had been removed years ago from those kinds of airplanes so there would have been limited opportunities for them to communicate depending upon what cell signals, what mobile signals they were able to get."