The suspected intentional destruction of a Germanwings jet by a pilot shows the need for new protections against suicidal crew members—a threat emerging as one of the worst in modern aviation, accident investigators say.

With French prosecutors saying the co-pilot apparently flew the Airbus A320 into a mountainside and evidence pointing toward the intentional ditching of Malaysian Airlines Flight 370, there have now been three such disasters in 17 months. The incidents killed 416 people.

“We have viewed these kind of events in the past as a one-off aberration and not something that should generate new policies or new procedures,” said Peter Goelz, former managing director of the U.S. National Transportation Safety Board. “I think this has changed all that. The industry, worldwide, has to look at this.”

The challenge is how to guard against a pilot willing to destroy a plane and kill others while taking his or her own life. Even as safety specialists debate steps such as more psychological tests and a possible redesign of cockpit doors, the industry confronts the reality that an aircraft soaring miles above the earth is always vulnerable to the humans at the controls.

A U.S. requirement to always have two people on airliners’ flight decks—with an attendant or a pilot stepping in during a restroom break—”is a potential deterrent to extreme or unusual activity by the pilot remaining in the cockpit,” said Richard Healing, a former NTSB member who now runs Washington-based R Cubed Consulting LLC. “But it’s a limited deterrent.”

Cockpit Rules

Hours after French prosecutors offered their theory on the end of Flight 9525, Norwegian Air Shuttle ASA said it was accelerating a change to require two crew members in the cockpit at all times. Air Canada is making the same change immediately, CTV television reported.

With the Germanwings crash, there have been at least seven such fatal crashes caused intentionally by pilots since 1982 and another unsuccessful attempt on a FedEx Corp. plane in 1994, according to Malcolm Brenner, a former NTSB investigator who has studied such accidents.

The most recent case was a LAM Mozambique Airlines crash on Nov. 29, 2013, when a pilot programmed an Embraer E-190 to descend into the ground after the other pilot left the cockpit. All 33 people on board were killed.

Common Threads

The list doesn’t include Malaysian Air Flight 370, a Boeing Co. 777 carrying 239 people that disappeared without a trace on March 8, 2014, and flew to one of the most remote parts of the globe, the Southern Indian Ocean. Investigators suspect that its flight path was deliberately changed.

Brenner, a psychologist who investigated the human side of accidents, said the pilots involved all exhibited mental-health issues. There were two other motivations as well, he said.

The most common is a pilot who wants to exact revenge on the airline. This was the case in EgyptAir Flight 990 on Oct. 31, 1999, which killed 217 people when the plane went down in the Atlantic Ocean off Massachusetts, according to the NTSB. Brenner was part of the investigation.

Another driver: Causing a crash in hopes of ensuring insurance payments to family members, Brenner said.

It would be easy for carriers to add psychological screenings to the physical checkups pilots undergo regularly, according to Jurek Grabowski, who was part of a suicide-by-aircraft study at Johns Hopkins University in 2005. Systems that let pilots report mental health concerns without fear of penalty also would help, he said in an interview.

Trust Issue

“Anytime you’re in a very stressful situation, there is a lot of trust being put into the captain and co-pilot, as well as the cabin crew,” Grabowski said.

The extent to which those exams would prove effective is another matter.

“A person who is absolutely determined to keep it from being discovered can probably do that,” said Healing, the Washington-based consultant. “They become comfortable with the idea and they realize any change in behavior might point people in their direction.”

Goelz, the former NTSB managing director, said pilot-caused crashes were a “straggering problem” for the industry.

They’re unfolding against the backdrop of decades of improvements in technology and training that have made air travel safer than ever.

“Now that everything else is becoming more reliable, we’re starting to see the failure of humans,” said Brenner, the psychologist and former NTSB investigator. “We belong to a species that murders other members of the species.”