Cockpit Door Panel from EFB Desktop
AVIATION DISASTERS, like the recent unfortunate loss of Germanwings 4U9525, sadden me and cause me to rewind back through human factors I investigated while a Flight Safety Officer with the US Air Force. Today, I'm thinking a lot about the way the Airbus cockpit is protected from entry because locked cockpit doors might be cited as the single cause of failure in some aircraft disasters. 
During my career as an instructor pilot in the US Air Force, I walked through wreckage of fatal aircraft accidents while investigating their causes and I walked through reconstructed crash scenes during recurrency training as an accident investigator. My role in these investigations was to dig into the human factors behind aircraft accidents. That meant trying to burrow into the flight scenario and to see the incident from the perspective of the pilots behind the controls. I would look into the personal lives of aircrew members, including their financial transactions, home lives, crew rest, duty periods and medication. I was an instructor pilot in two different types of military aircraft during my time in uniform, including the Lockheed C-141 Starlifter. I've seen highly qualified pilots make critical errors while airborne. In fact, I would induce some of those errors during local training flights or in the flight simulator. It was part of my job.
Switch positions became part of my job too because I discovered one of the easiest ways I could inject turmoil into a flight profile was through incorrectly throwing a switch or by removing circuit protection from a switch. The pilot flying the aircraft would either fail to see the anomaly or would be distracted by a minor malfunction and permit the aircraft to enter a dangerous condition.
On the heels of the German Wings crash, I thumbed through an A320 manual to read more about the way its cockpit door can be secured.
A320 Cockpit Door Switch
Unlocks door when raised above the detent and held in this position. Door must be pushed to open. UNLOCK is an override and reset selection of any previous action.
When NORM is selected, it allows the door to be locked when closed. It also allows the door to be opened after an emergency access code entry and 30 second delay in case of pilot incapacitation.
Momentarily placing the cockpit door switch to LOCK illuminates the red cockpit access panel light, rejects keypad entry request, inhibits aural alerts, and prevents further access code entry for 20 minutes. The cockpit door switch returns to NORM when released, but remains in locked mode for 20 minutes or until UNLOCK is selected.
There is no cockpit indication when LOCK is selected; therefore, if the door is closed and the cockpit is unattended during a locked period, the door cannot be opened until the timer expires or power is removed from the airplane.
The cockpit door became an issue after Nicola Clark from the New York Times filed her report from the crash site. 
PARIS — As officials struggled Wednesday to explain why a jet with 150 people on board crashed in relatively clear skies, an investigator said evidence from a cockpit voice recorder indicated one pilot left the cockpit before the plane’s descent and was unable to get back in.
A senior military official involved in the investigation described “very smooth, very cool” conversation between the pilots during the early part of the flight from Barcelona to Düsseldorf. Then the audio indicated that one of the pilots left the cockpit and could not re-enter.
“The guy outside is knocking lightly on the door and there is no answer,” the investigator said. “And then he hits the door stronger and no answer. There is never an answer.”
He said, “You can hear he is trying to smash the door down.”
While the audio seemed to give some insight into the circumstances leading up to the Germanwings crash on Tuesday morning, it also left many questions unanswered.
“We don’t know yet the reason why one of the guys went out,” said the official, who requested anonymity because the investigation is continuing. “But what is sure is that at the very end of the flight, the other pilot  is alone and does not open the door.”
Do pilots deliberately crash airplanes? Contemporary history and professional pilot discussions  suggests people fly aircraft into the ground (or water).
SilkAir Flight 185: Lead accident investigator Greg Feith concluded from an interpretation of the absent black box data that the crash was the result of deliberate flight control inputs, most likely by the captain.
EgyptAir Flight 990: The National Transportation Safety Board found the crash was caused by deliberate action of the relief first officer Gameel Al-Batouti.
LAM Mozambique Airlines Flight 470: Investigators concluded Captain Herminio dos Santos Fernandes had a "clear intention" to crash the jet and that he manually changed its autopilot settings. The plane's intended altitude was reportedly changed three times from 38,000 feet (11,582 m) to 592 feet (180 m), the latter being below ground level, and the speed was manually adjusted as well. 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 until shortly before the crash.
Japan Airlines Flight 350: The 35-year-old Captain Seiji Katagiri (片桐 清二 Katagiri Seiji), 33-year-old First Officer Yoshifumi Ishikawa, and 48-year-old flight engineer Yoshimi Ozaki were on the flight deck. The cause of the crash was traced to Katagiri's deliberate engaging of the number 2 and 3 engines' thrust-reversers in flight. The first officer and flight engineer worked to restrain him and regain control. Despite their best efforts, the DC-8's descent could not be completely checked, and it touched down in shallow water 300 meters (980 ft) short of the runway.
Germanwings 9525: unofficial recovered audio indicates that one of the pilots left the cockpit before the start of the descent and found the door locked when he came back. He then tried to break down the door, with no response from the other pilot.
1. Cockpit Door Panel information comes from The A320 Displays and Panels ebook, written and developed by Mike Fisher, a United Airlines A320 Captain. Fisher's ebook incorporates the latest changes to the A320 cockpit control panels and displays.
2. Nicola Clark and Dan Bilefsky -- "Germanwings Pilot was Locked Out of Cockpit" in the New York Times, March 25, 2015.
3. Andreas Lubitz was the copilot at the controls on the fateful flight. His Facebook page was in Google's cache.
4. Professional Pilots Rumor Network -- "Airbus A320 Crashed in Southern France" on PPRuNe, March 25, 2015.
5. Related Twitter chatter used #4u9525 with condolences for all those who experienced a tragic loss when the flight crashed into the French Alps.