It's a terrifying scenario. A pilot is flying an airliner carrying hundreds of unsuspecting passengers. He decides to intentionally crash the plane, killing everyone on board.

Pilot suicides are thankfully very rare — and are getting rarer — but they do happen occasionally, sometimes with horrific results. The Aviation Safety Network identifies at least eight instances worldwide since 1976 where pilots appeared to have deliberately crashed airliners, sometimes taking dozens or hundreds of people with them.

The co-pilot seemed to have "an intention to destroy this aircraft"

Now French prosecutors think something similar may have happened with Germanwings Flight 4U99525, which crashed into the French Alps on Tuesday, killing 150 people.

Investigators now believe the plane's co-pilot, Andreas Lubitz, locked the pilot out of the cabin after the latter had left for some unknown reason. The pilot knocked and tried to get back in, but the doors were fortified — a security precaution taken after the 9/11 attacks. The voice recorder indicated that Lubitz had been breathing up until the moment of the crash, suggesting he meant to destroy the plane. We still don't know why.

"I haven’t used the word suicide," said prosecutor Brice Robin at a press conference. But, he added, it was "a legitimate question to ask."

Pilot suicides are rare — but unnerving when they happen

Flying, as many people know, is one of the safest forms of travel. Yet the possibility of a pilot suicide is incredibly unnerving, since there's seemingly nothing any of the passengers can do.

In the United States, most pilot suicides are committed by people flying solo. The National Transportation Safety Board (NTSB) has identified eight cases of pilot suicide between 2003 and 2012. In only one instance was there a passenger on board.

Four of the eight pilots had been drinking at the time, while two had been taking antidepressants. The report noted that "factors involved in aircraft-assisted suicides may be depression, social relationships, and financial difficulties, just to name a few problems." The good news? The frequency of these suicides has declined sharply in recent decades, the NTSB said.

Even rarer — but more gruesome — are times when pilots appear to commit suicide while carrying dozens or hundreds of passengers on board. Here's a partial list:

As the Egyptair flight went down, the

co-pilot muttered, "I Rely on God" over and over

In November 2013, Mozambique Airlines Flight TM470 crashed in Namibia, killing 33 people on board. Investigators initially couldn't figure out why the plane had crashed, since the weather was so nice.

But as the International Business Times reported, the plane's black box recorder offered some disturbing clues. The co-pilot had left the cockpit for the bathroom only to find that the door was locked when he returned. The pilot then altered the autopilot to bring it to below ground level and manually switched it to maximum speed. Someone was pounding on the cockpit door as the plane went down. The pilot never once called for help.

In 1999, EgyptAir Flight 990 crashed near Nantucket, Massachusetts, killing 217 people. Before the crash, the plane's pilot had apparently excused himself to go to the bathroom. The black box recorder then picked up unintelligible commotion and banging on the door. The co-pilot, Gamil El Batouty, could be heard muttering over and over, "I rely on God. I rely on God. I rely on God. I rely on God." The captain eventually forced his back way in and could be heard saying, "What is this? Did you shut the engine[s]?" As the plane crashed, the captain was heard trying to right the plane, saying, "Pull with me. Pull with me."

In the EgyptAir case, the NTSB concluded that the crash occurred because of the co-pilot's "manipulation of the airplane controls." But they did not explicitly call it suicide, and Egyptian officials have disputed that it was deliberate.

In December 1997, Silk Air Flight 185 crashed in Indonesia, killing 104 people on board. Indonesian authorities weren't sure exactly what had happened, though US investigators suggested the captain may have switched off the flight recorders and caused the plane to dive — possibly after his co-pilot had left the cockpit. At the time of the crash, investigators noted, the pilot had been experiencing significant financial difficulties and had work-related problems.

Why the cockpit doors are fortified

In a number of these pilot suicides on airliners, there's a recurring pattern — the pilot or co-pilot leaves the cockpit and gets locked out by the other person, who intentionally crashes the plane.

Paradoxically, recent security measures may have made this even easier to do. After the attacks of September 11, 2001, international regulations required all airliners to reinforce their cockpit doors with steel and cypher locks. The idea was to prevent terrorists out in the cabin from gaining control of the plane.

The New York Times points to an an Airbus video showing how this would have worked on the Germanwings flight: The cockpit door is normally locked and accessible only by code. But anyone inside the cockpit can also disable this access pad for five minutes, preventing people outside from gaining entry. At that point, the only way to contact the person in the cockpit is via intercom. So if one of the pilots is intentionally locked out, he can't gain entry.

Pilots rarely leave the cockpit unless there's a pressing need, like going to the bathroom (and even then, most pilots don't take bathroom breaks on short flights). When that happens, a spokesman for the European Aviation Safety Agency told the Times, there's currently no requirement for European domestic flights that one of the cabin crew come into the cockpit and stay with the remaining pilot.

In the US and on international flights, by contrast, regulations do require two people to be in the cockpit at all times. If one pilot leaves, a flight attendant steps into the flight deck. In the wake of the Germanwings crash, some European airlines like EasyJet and Norwegian Airlines have announced that they'll now adopt similar policies.

This post has been updated to note that a number of European airlines are now adopting policies requiring two people to be in the flight deck at all times.

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