Freezing rain
Some of the posts above show some confusion about freezing rain.
To be sure, it is a killer. Why?
True freezing rain, as opposed to normal rain freezing on cold ground/surfaces forms when a winter warm front drops rain through the cold - below zero wedge of air below. Then super cooled droplets form - still water although below zero degrees. The environment is then filled with these highly unstable droplets - waiting only for the slightest vibration (your aircraft is more than is needed) to turn them into solid ice.
When a wing hits freezing rain, the drops start to run back across the surface, freezing as they go. The next drops follow and pretty soon you have a tenacious thick layer of ice.
This is why most airlines forbid take-off in freezing rain and why any HOT (hold over table) quoting a time for freezing rain is highly unreliable.
Anti icing systems vary - thermal ones often cannot cope in the take-off phase. Boots have the following effect:
If the boots are running when entering freezing rain, the droplets still freeze and run back along the surface. As the ice becomes thicker, a tube of ice develops around the maximum radius of the boots. You can run them all day after that but they are powerless to remove the tube.
If, despite the warnings to AVOID freezing rain, you find yourself flying in it, then you have to allow ice to begin to build BEFORE operating the boots. Then the boots can crack the layer away. the boots have to be deactivated and the process repeated. Obviously not a recommended scenario.
The ice doesn't only form on the wings - it will attack your prop, any intakes and sensors and cover your windshield pronto. All the aircraft's defences are at or beyond their limits in the severest cases.
So to those of you who are already familiar with this phenomenon, excuse the post and move on. Those who are not - be very respectful of freezing rain, freezing drizzle or heavy wet snow falls. All these scenarios have super cooled water in the game.
Lastly, this is a general note and in no way intended to be speculation on the cause of this sad crash.