How premeditated was Andreas Lubitz’s plan to fly his Germanwings Airbus A320 into a mountain, killing himself and 149 other people in a horrific act of suicide and mass murder in the manner of a kamikaze pilot?

The shocking revelation from the French prosecutor that the cockpit voice recorder confirms that this is what happened did not deal with this question, however. There is one salient feature of Lubitz’s behavior that suggests that he had carefully planned his actions.

The prosecutor said that while the copilot was alone with the captain locked out of the cockpit “he manipulated the flight management system to manage the descent.” The highly sophisticated computerized system of the A320 would not have detected any anomaly from these actions.

And here is the key clue to Lubitz’s preparations. Although the system uses a “flight protection envelope” to prevent pilots from what is called “over-corrrecting”—forcing the airplane into a maneuver that could destabilize it—the Germanwings A320 did not breach this envelope in its descent.

In the two most recent cases of pilot-directed crashes (see below) the chosen method was to suddenly make a vertical dive—the quickest route to destruction. Lubitz knew that the Airbus’s flight protection envelope would have prevented that maneuver. He might still have taken that route by disconnecting the computerized flight management system and taking over the flight manually, but that would have taken time. Instead, he simply input a descent using the same steps as would a pilot who needed to lose altitude for navigational purposes—except that he kept on descending until the proximity warning alarm went off shortly before impact.

It is normal for a copilot to be flying the airplane once it reaches cruise altitude, but with a copilot as relatively inexperienced as Lubitz the captain would be unlikely to leave the cockpit for longer than necessary to go to the toilet, which on the A320 is immediately behind the cockpit on the lefthand side.

Nonetheless, Lubitz could not have pre-selected the Alps as the place to execute his plan—it just happened that his captain chose that moment to go to the toilet, a pretty routine thing for a captain to do. Had the captain left the cockpit 10 minutes later, the plan could still have been executed by flying the A320 for another two minutes or so into normal terrain beyond the Alps.

Nothing in what is so far known about 28-year-old Lubitz’s past gives any hint of how he reached the point of planning and executing such a ghastly crime. The French prosecutor said Lubitz was heard breathing normally throughout the entire descent. Even if he had had “a burnout, depression,” as has been reported this would hardly explain his drastic calculation. Moreover, this was more than a job to him, he was a flying enthusiast who got his pilot’s license as a teenager. He qualified as a Lufthansa pilot in 2013 and had 630 hours of flight time.

Experts had been confounded by the airplane’s behavior—the sudden, unexplained departure from cruise altitude and the nearly 10-minute long steep descent, undeviating, directly into a mountainside, hitting with such violent force that the whole airplane disintegrated into thousands of pieces. A deliberate act by a pilot would, on the face of it, be consistent with this scenario including the total absence of a message from the cockpit—and also consistent with my earlier analysis that for some reason the flight controls remained locked in one configuration during the descent.

The inexplicable becomes only a little more explicable with the terrible evidence from the cockpit voice recorder.

The two most recent examples of “kamikaze” crashes both caused disputes about the pilot’s actions:

In December 1997, a Silk Air Boeing 737 flying from Jakarta, Indonesia, to Singapore suddenly dived vertically for more than 30,000 feet into a river and 97 people were killed. Because it was a U.S.-built airplane, the National Transportation Safety Board was called in to investigate; it determined that the crash resulted from deliberate action by one of the pilots. However, the Indonesian Transportation Safety Committee disagreed, saying the evidence was inconclusive, and a private legal action in California unsuccessfully tried to reverse the NTSB’s ruling claiming that a mechanical flaw, inherent in the 737’s design, had caused the crash.

Even more disputed was the crash of EgyptAir Flight 990 in October 1999. In this case the Boeing 767, on a flight from New York to Cairo, as it neared cruise height suddenly dived vertically into the Atlantic near Nantucket Island, killing 217 people.

The NTSB determined that there had been no mechanical failure and that the Egyptian captain had gone rogue and suddenly and deliberately pointed the nose down and doomed the flight. Once again, though, the NTSB’s verdict was disputed, this time by the Egyptians, who claimed that a design flaw had been responsible. The NTSB remained adamant that it was a case of pilot action. Other analysts agreed, pointing out that the concept of suicide by a captain was repugnant in Egyptian culture.

One point to note about these events is that the airplanes both fell in steep and rapid dives, whereas in the case of the Germanwings A320 the descent, though steeper than a normal approach descent, was not precipitate, and took nearly 10 minutes.

All of which shows that any act as bizarre as a deliberate one by a pilot to destroy himself and everyone on board an airplane will never be easy to explain, and can lead to years of dispute, as well as years of pain and anger for the families of the victims for whom this is the least expected explanation for a catastrophe.

Editor's Note: This story has been updated throughout.