I know everybody's thinking this already, but I might as well be the one to write it down here.

First: what's a "hijack alarm?" One would presume that it's a switch or something similar that a pilot or crew member can throw to let air-traffic control know that the flight is being hijacked. Is that what they're talking about?

Second: if the planes were hijacked or in the process of being hijacked, why did they crash? Was it a Flight 93 scenario of a failed hijacking leading to a crash, or was it something else?

Finally: assuming this was an act of terrorism, thank God it wasn't any worse than this. The loss of 100 lives is a tragedy, but we all know now that it could have been so much worse.

Actually, my question is: How will this affect the upcoming Russian elections? Aren't they due by now? I don't count days very well... it was five days in the future X days ago...

Don't rule out missiles in this scenario. The first thing I thought when I heard the 3 booms description was "Mach 2 -- 2 sonic booms and the impact." Of course, that suggests bigger hardware than a shoulder fired missile like an SA2 or a SA6, but I wouldn't rule it out.

I don't think you get two sonic booms. I think there's a boom when the aircraft or whatever exceeds the speed of sound and then that's it.

First: what’s a “hijack alarm?”

As a GA pilot, I might be able to help here. I don't know of a specific 'hijack alarm', but here in the UK (and I'm sure it's the same elsewhere), you can use specific transponder codes to indicate a hijack (and other situations like lost comms, etc).

Modern commercial airliners also have data links back to home base that could also be used to pass messages. Maybe they have a 'panic' code or other configuration mode.

I haven't been able to find out if domestic Russian airliners use the same type of equipment, but all Western civil aircraft use a device called a "transponder" that sends a 4-digit code number back to the air traffic control system every time the ATC radar "illuminates" the aircraft. (In most areas of the U.S., an aircraft en route will get "illuminated" by a radar at least 3-4 times every second. I suspect the radars are slower and farther apart in Russia, so it may be less there.) The pilot sets the transponder's code number based on standard flight procedures or on specific instructions from ATC. When the radar system receives the transponder response, it correlates that with the aircraft's detected position and it puts the number up next to the plane's blip on the ATC radar screen.

In the West, there are some standard code numbers that pilots can set their transponder at to indicate an emergency. For general emergencies, 7700 is used. For a hijack, I think 7500 is the proper signal value. (This allows the pilot to send back a hijack signal while under duress from the hijackers, while pretending that everything is normal on the radio. Unless the hijackers know about aircraft avionics, they won't realize it. ATC is trained to ignore the pilot's voice communications and assume a hijack if 7500 pops up on the radar display. This system has been in place since the early 1970s, and very few hijacked pilots have ever gotten caught doing this by a hijacker.)

The catch is, since the transponder is getting pinged at a fairly high rate most of the time, the pilot has to be careful not to accidentally go through one of the emergency code values while switching from one value to another. For example, if the airplane had been "squawking" 0500 ("squawk" is the word that is used to refer to setting the transponder code), and then the pilot was told by ATC to "squawk" 7020, the pilot might execute this by first turning the knob for the most significant digit to 7, and then turning the second knob to 0 and the third knob to 2. The trouble is, in between turning the first knob and turning the second knob, the transponder will in passing be squawking 7500 for a fraction of a second. Even if the controller doesn't actually see it during this interval, the emergency code will cause an alarm to go off in the ATC system and the radar display will do something on the screen to draw attention to that aircraft.

Pilots get cautioned about this during their training. But in the heat of the moment, one could forget -- it's not always an obvious thing. It's possible that the one aircraft had a general emergency, say an explosion that disabled the aircraft but didn't cause it to break up immediately. The pilot goes about setting the transponder to squawk the mayday code, 7700, but in the process of flipping the knobs he goes through the 7500 code for a moment, and that sets off the ATC hijack alarm. (The 7700 code would also set off an alarm, but that's a different alarm and controllers are trained to react to it differently than for a hijack alarm.) That might be what they saw. Until more evidence is released, it's hard to say.

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