My thoughts
1- The ADS-B data shows an 11,000 ft/min descent before the crash. For comparison, the terminal velocity of a falling skydiver is about the same, and a bowling ball falls at 17,000 ft/min. Granted, the jet isn't in freefall, but the point is that a 11,000 ft/min descent is ridiculously high.
2- The ADS-B recorded airspeed was above 200 knots up until impact. That is well above the stall speed of a learjet, which is somewhere between 100 and 130 knots (hard to pin this down exactly). This doesn't absolutely rule out a stall, since you can technically stall an airplane at any speed. But it is a lot easier to stall if you are slow.
3- The Learjet has a T-tail design that is prone to "deep stalls", which is a stall where the horizontal stabilizer is in the wake of turbulent air aft of the stalled wings. This causes the elevator to have no impact because there is no smooth air for it to grab onto. Deep stalls are difficult to recover from. They typically feature rapid descent above 10,000 ft/min, but not much horizontal speed over the ground.
4- To counter these disastrous stall characteristics, the original Learjet 55 came with a stick pusher and stick shaker. If the plane approaches a stall, the stick shakes to alert the pilot. If the pilot ignores the shake, the stick pusher will engage to push the yoke forward, and push the nose of the plane down before the plane stalls. I don't know much about these devices. Never heard of them before today.
5- The horizontal stabilizer functions as an upside down wing that pulls the tail down and the nose up. When the center of gravity (CG) of the plane is where it is supposed to be, this tail down force almost exactly balances the nose of the plane, and the aircraft stays level and stable. If the CG moves too far forward, the tail can't provide enough down force to keep the nose up. If the CG moves too far aft, then things just get wonky and astable. Both are very bad, but the CG moving aft of its limits is probably worse.
6- The balancing act between the horizontal stabilizer and the nose is critical. If the tail suffers structural failure, or the elevator sticks, then ... well... that just isn't good.
7- Multi engine planes become unwieldy when one engine loses power.
8- The pilot of the learjet signs off with the tower, but never makes contact with departure. The jet was ascending through the signoff, and starts to lose altitude right after the signoff. Ground speed stays about the same throughout.
Departure and tower are on different frequencies, so the pilot has to type in a new 4 digit frequency to make the switch. Usually these would be preprogrammed in before takeoff, so the pilot just has to tap one button two switch frequencies, and then make a radio call. This is normally done right after the signoff with tower. Within 10 seconds or so. With a two person crew, managing the radio comm shouldn't be a big deal.
It is suspicious that everything starts going wrong at this exact moment.
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Some theories on what happened
- Some aspect of the payload broke loose and shifted the CG out of whack
- Bird strike or failure of the tail structure
- Fire or loss of one of the engines
- Pilot disorientation due to instrument flying conditions (clouds and low visibility)
- Pilot incapacitation (was this a 2 person crew?)
- Pilot became distracted while making the radio switch
- Major wind shear
- Cabin fire
The problem with the first 4 is that absence of an emergency radio call. I am leaning towards one of the last 5.
I don't know if I buy the theory that the pilot simply stalled a perfectly functional airplane. You don't go from stable flight to a stall without making some kind of change to the flight controls, engines, or flaps. On top of that, the plane would have worked against him, and there is still the issue of the missing radio call. But pilot induced stalls are one of the most common causes of aviation accidents.
H