First, the man can be heard knocking on the cockpit door, to no response. Then, as the jet plunges lower toward the French Alps, he can be heard trying to break down the door, also without success.
The cockpit door of the Germanwings Airbus A320 jet was doing exactly the job it was designed to do: stop intruders.
But the man banging on the door wasn't a hijacker or a terrorist. He was the pilot, who had stepped out of the cockpit and whose copilot, authorities say, was deliberately steering the jet into the mountains.
Officials who revealed the scenario say the tragedy that took 150 lives was caused by one man, copilot Andreas Lubitz, 27. But in an industry dominated by safeguards, backup systems and redundancies, the crash also appeared to be the result of strategic failures.
One factor was Germanwings' cockpit policy, which, unlike that employed by U.S. carriers, allows a pilot or copilot to be alone in the cockpit if the other leaves to go to the restroom.
A second factor is that cockpit doors are so heavily secured that they can prevent entry by bad guys and good guys alike.
How secure are cockpit doors?
Incredibly secure — so secure that they can stand up to gunfire or even small grenades.
After the Sept. 11, 2001, attacks, when hijackers took control of four U.S. airliners to crash them, American aviation officials issued new regulations requiring cockpit doors to be reinforced. European air carriers — including Lufthansa, the parent company of Germanwings — followed suit.
"I know in the U.S., post 9/11, the cockpit door is the most secure door on the plane," said Anthony Brickhouse, associate professor of safety at Embry-Riddle Aeronautical University.
How do flight crews normally open a cockpit door?
Since the Sept. 11 attacks, pilots and flight attendants no longer have keys to open the cockpit door, which remains locked during flight, U.S. experts say.
Access for Airbus A320 jets is controlled from the cockpit, with only a limited option for outside override.
On Airbus jets, access is requested via a keypad outside the cockpit door. An Airbus manual and instructional video available online shows that a buzzer then sounds in the cockpit, and the pilot or pilots at the controls can decide whether to toggle a switch to unlock the door.
Is there any way to open a cockpit door from the outside?
If there is no response from the cockpit for a request to open the door, the flight crew can enter an override code on the keypad, which also sets off a 30-second alert in the cockpit, according to the training video.
If there's no response from the cockpit by the end of the alert, the door automatically unlocks for five seconds and can be opened from outside, according to the training video.
But without the emergency access code, "you're not going to get in," says Glen Winn, an instructor at the University of Southern California's school of aviation safety and security. "It's not going to happen."
What happens if the pilots in the cockpit do not want someone to open the door?
The flight crew in the cockpit also has the option of denying emergency access by hitting a "lock" toggle, according to the Airbus training materials.
If the "lock" toggle is selected, the outside keypad is then disabled for five to 20 minutes or until the pilot decides to unlock the door, according to the training materials.
"If the LOCK position has not been used by the pilot for at least 5 to 20 min, the cabin crew is able to request emergency access to open the cockpit door," the Airbus manual states. "The UNLOCK position overrides and resets any previous selection. In case of an electrical supply failure, the cockpit door is automatically unlocked, but remains closed."
Investigators have not said whether the pilot locked out of the Germanwings cockpit tried to use a keypad to access the cockpit, or whether the copilot toggled the lock switch to override emergency access.
What are the policies for leaving someone alone in a cockpit?
It's forbidden in the United States, according to the Federal Aviation Administration. If a pilot steps out of the cockpit, "another qualified crew member must lock the door and remain on the flight deck until the pilot returns to his or her station," the FAA said in a statement Thursday. "A qualified crew member could be a flight attendant or a relief pilot serving as part of the crew."
Until this week's crash, that didn't happen on Lufthansa planes in Europe, according to the airline's top official, who spoke Thursday at a news conference. Within hours, however, German airlines agreed to change the policy and will now require a flight attendant in the cockpit if a pilot steps out to go to the restroom or to get a drink.
Times staff writer Christine Mai-Duc contributed to this report.