One of the pilots on Germanwings flight 4U 9525 could be heard trying in vain to get into the cockpit as the Airbus A320 lost altitude, before crashing into the French Alps on Tuesday morning, killing all 150 people on board.
"The guy outside is knocking lightly on the door and there is no answer," an investigator said on Thursday after listening to the cockpit voice recorder recovered from the crash site.
"Then he hits the door stronger and no answer. There is never an answer.
"You can hear he is trying to smash the door down."
It is not the first time a flight has been compromised because of errors or actions of pilots related to the lock on the cockpit door. In 2013, a pilot was found to have purposely crashed a passenger jet in Namibia, after locking his co-pilot out of the cabin.
On a Greek airline in 2005, a maintenance error deprived the cabin of oxygen, rendering the pilots incapable of flying the plane. A steward tried to save the plane after forcing his way into the cockpit, but failed.
Other incidents have seen pilots locked out after arguing with their co-pilots, or stuck outside when the door jammed.
Crashes and near misses related to cockpit door locks
August 14, 2005: Helios Airways
Helios Airways Flight 522 crashed near Grammatiko, Greece. A maintenance error meant the cabin pressure switch was not set to automatic, and as the plane climbed to cruising altitude, the pilot and co-pilot were overtaken by oxygen deficiency, or hypoxia. The effects were insidious and they were too impaired to realise a warning siren indicated a lack of oxygen.
In the cabin, the oxygen masks were deployed, and stewards tried to warn the pilots, but a locked cockpit door meant they could not enter. After the plane's oxygen ran out, one steward, with a portable oxygen bottle, managed finally to break into the cockpit. He fought to handle the controls but he was too late: the fuel ran out and the Boeing 737-300 dived into a Greek hillside, killing all 121 people.
September 6, 2011: All Nippon Airways
A Boeing 737-700 flown by All Nippon Airways, with 117 passengers and crew on board, rolled and dived 1900 metres in 30 seconds over the Pacific Ocean near the southern Shizuoka district after the co-pilot mistook the cockpit door catch for a command button.
He was trying to readmit the captain who was returning from the toilet, but instead turned the rudder trim knob. The jet rolled almost belly up and dropped 1900 metres.
Two crew members were slightly injured, while four passengers reported health problems following the incident, but the plane touched down safely at Tokyo's Haneda airport.
November 29, 2013: Mozambique Airlines
Flight 470 was an international passenger Embraer 190 flying from Maputo International Airport.
During the flight, Captain Herminio dos Santos Fernandes manually changed its autopilot settings three times from 38,000 feet to 592 feet -- which was below ground level -- as well as adjusting the speed.
The cockpit voice recorder captured several alarms going off during the descent, as well as repeated loud bangs on the door from the co-pilot, who was locked out of the cockpit. An investigation concluded that Captain Fernandes had a "clear intention" to crash the jet, and had kept his co-pilot out of the cabin so he could do so.
The plane crashed into the Bwabwata National Park in Namibia, about half way between its departure and scheduled arrival airport. All 27 passengers and six crew members were killed.
February 17, 2014: Ethiopian Airlines
Flight 702, an Ethiopian Airlines Boeing 767-300ER, was flying from Addis Ababa to Milan via Rome on 17 February 2014 when its own co-pilot hijacked it.
He waited until the pilot went to the toilet, then locked the cockpit door to keep him out, while continuing to fly the aircraft. He bypassed Rome and Milan and flew instead to Geneva, where he circled several times while communicating with air traffic control, trying to broker political asylum for himself and an assurance that he would not be extradited to Ethiopia.
With only 10 minutes of fuel left, and with one engine stalling from lack of fuel, he landed the plane safely, then climbed out the window on a rope and handed himself over to police. All 202 passengers and crew aboard were unharmed.
May 21, 2014: Air New Zealand
On flight NZ176 between Perth and Auckland, the captain locked the first officer out of the cockpit for two minutes mid-flight after an argument over a take-off delay.
The captain did not respond to requests to open the locked door, alarming crew.
The incident, on a Boeing 777-200 carrying 303 people, sparked calls for a third crew member to be added to flight decks so no one is ever alone in the cockpit. Both pilot and co-pilot were stood down over the incident.
January 21, 2015: Delta Air Lines
The captain of the Delta Airlines flight 1651 from Minneapolis to Las Vegas left the cockpit before the plane's final landing approach but "was not able to re-enter the flight deck because of a door jam," the airline said in a statement.
The first officer took control and landed the aircraft safely at McCarran International Airport.
The 160 passengers were unharmed.
