Whatever happened to Germanwings Flight 9525 happened very fast.

The Airbus A320 had followed a normal flight path from Barcelona north up to the Alps, climbing steadily to 30,000 feet on its way to a cruise height of 36,000 feet. Then, according to the airline’s CEO, as it approached the most mountainous region it began descending for about eight minutes until it disappeared from radar without any communication from the pilots. The flight should have continued at cruise height into southern France before beginning a descent to Duesseldorf.

An eight-minute loss of altitude like that does not suggest a dive but some kind of controlled descent until something more catastrophic occurred. All of this was happening in some of the most closely monitored airspace in the world. The flight would have been monitored on the radars of air traffic controllers in Spain, France, and Switzerland.

So why were the pilots unable to send a distress call—or contact controllers during their descent? The one certainty in this sequence of events is that the end came suddenly—not from flying into a mountain but because the airplane was unable to maintain a safe altitude. This means that there must have been either a structural failure or an engine failure.

One of those failures could involve a sudden loss of cabin pressure caused by a rupture in the skin of either the cabin or the cargo hold. The first thing a pilot needs to do in this event is lose altitude: get below 10,000 feet to equalize the atmospheric pressure outside and inside the cabin (difficult to do if you are flying over mountains). As the pressurized air in the cabin escapes, the oxygen goes with it, creating the condition called hypoxia. Hypoxia quickly renders passengers and pilots unconscious.

If it was violent decompression, not a slow leak, it would have been very difficult for the pilots to realize what was happening and make a distress call.

In 2005 a Helios Airlines flight from Istanbul to Athens suffered a slow pressure loss that incapacitated the passengers and crew, leaving the airplane to fly on autopilot until it ran out of fuel and crashed into a mountain. And one of the most viable theories about the greatest aviation mystery of all time, the loss of Malaysian Airlines Flight 370, posits that it, too, suffered an oxygen loss that incapacitated everyone on board.

The most dramatic example of this recently in the U.S. was in 2011 when Southwest Airlines Flight 812, a Boeing 737, lost part of its cabin roof and the pilots made a rapid descent from 36,000 feet after radioing: “…declaring an emergency we lost the cabin.” In that case the airplane landed safely.

We haven’t seen a catastrophe like this over the Alps since the 1950s, before the age of jets, when relatively primitive piston-engine airplanes labored to gain enough altitude to get safely over the Alps—some airliners of that era were so under-powered that they had to go west and over the Atlantic to avoid the peaks. Bad weather could make this very hazardous without modern navigation aids.

So this is a crash that ought never to happen, involving an airplane with an extremely safe record, same for the airline, plus state-of-the-art navigation aids and air traffic control and an absolutely routine route.

Although the wreck is in a very inaccessible site the black boxes have been recovered, according to authorities. This time they didn’t have to be fished out of an ocean.