One of our planes is missing.
That's basically all we know for certain right now, but more than a week after losing contact with flight MH370, there are a lot of other suggestive bits of information. Unfortunately, a lot of it is, as usual, being reported by news readers who barely understand that you don't want to keep the pointy end aimed at the ground for an extended time. Here's some things we do know now.
The first indication of trouble was when the transponder stopped responding to radar. This is the point where newspeople are saying it "dropped off the radar," so let's get a little clarity here to start. A transponder is a device that transmits a response. In a plane, the transponder is receiving an interrogation and responding by transmitting a burst of data. The problem with "dropped off the radar" from the start is that all it indicated was that the transponder stopped transponding. Imagine for a second that you're trying to find someone in the dark. If you have a flashlight, you can use the flashlight, and hopefully see them in the reflected light. This, in radar, is called a primary radar response. It's a lot easier, though, if the person you're looking for has a flashlight too, and can turn it on and wave back at you with it. This is what a transponder does, and it's the major part of what's called secondary surveillance radar.
When MH370 "dropped off the radar," the transponder stopped responding. But transponders have an off switch. There are two independent transponders, so it isn't probable just that it was just a transponder failure.
It didn't disappear from primary radar, but primary radar is a lot harder to read. They now think that they might have tracked the plane as it turned back, did some vertical excursions, and headed off into the Indian Ocean.
Turning off the transponders didn't stop all radio transmissions, however. There is an onboard flight telemetry system that kept transmitting for a long time, as much as seven hours. It's very difficult to crash and have the telemetry transmissions keep going, so the combination is a pretty strong indication that the transponders turned off but the plane kept flying.
Unfortunately, Malaysia Airlines doesn't pay for the service, so the only responses were pings saying "nothing to say." But those pings are timestamped, and that means you can estimate the distance from the satellite to the aircraft by the time the signal arrives at the satellite. Now, PJ is ill-equipped to show a three-dimensional picture, so instead imagine a map. There's one circle centered on the last known position, which is how far the plane could have flown in seven hours. There's another circle centered on the satellite, which is all the places that are the estimated distance from the satellite. (Strictly, that's the surface of a sphere, but we can discount the parts of the sphere that are underground.)
This, by the way, is how GPS works: your GPS receives very accurate time signals from several satellites and computes where all the circles intersect; that's where you are.
The result is something that looks like this:
That line is actually fuzzy, because that distance to the satellite isn't known as accurately as say a GPS signal would be, but the last transmission does mean the plane was somewhere near that circle when it stopped sending telemetry.
So, now is the part of Malay Mystery in which we speculate.
As far as I can see, there are about four possible explanations.
First, something happened that incapacitated the pilots, and the plane flew on autopilot until it ran out of fuel. This has happened before, in the crash that killed Payne Stewart, although not on a commercial jetliner. The way it apparently happened in the Payne Stewart accident was decompression.
This is fairly unlikely just as an accident on a commercial jet because the pilots have oxygen masks immediately available -- but more speculation is coming.
There is a story today that there was a "supergrass" -- which is Brit for a highly-placed informant apparently -- who described a plot for four or five people to take over a plane by blowing open the cabin door with a shoe bomb. So, let's imagine that this is what happened. The door blows open, and cabin pressure is lost. The pilots, being a little bit distracted and busy, don't get their masks on. Everyone passes out -- and shortly dies at that altitude. The bodies rattling around in the cockpit cause the plane to make some uncontrolled maneuvers until the autopilot finally stabilizes. I explain the second possibility on the next page.