Spiny Norman said:

This video popped up in my YT feed no doubt because of the recent Learjet crash.
There’s a pretty reasonable chance that they suffered from tail icing and hence loss of control.

YT

FWIW I’d pre-programmed myself if that ever happened to anything I was flying would be to stop trying to fight with the elevator and to use the elevator trim system.

So instead of the relatively small elevator trying to pitch the plane up & own I’d use the entire horizontal stabiliser instead. It’s difficult to get fine & rapid control like that but it’s the only control you’ll have.

And indeed I did have to do that one night when descending into Archerfield Airport near Brisbane. The Metro 2 turboprop I was flying had no autopilot so it had to be hand-flown 100% of the time. When I tried to pitch down a little to start the descent the elevators refused to move. So yeah that was a bit of a problem.
But I just used the elevator trim to start the drop and I had some time to see if I could figure out what was going on. I then remembered read a report on a Beechcraft Kingair where it’d been sitting out in the rain on the ground for a few weeks and when it was flown after a while when climbing the controls froze in position, but came back when they descended again. After some investigation they found that the water drains in the bottom of the fuselage where blocked with debris and the bottom of the floor had enough water in it so the control cables were covered and when the water froze they jammed. And this Metro had also been sitting in the rain for a few weeks in Bankstown.

So that’s what I reckoned was happening. I had plenty of time before needed to land and if the elevators didn’t free up I could have diverted to Brisbane with its very long runway and just trim the thing onto the ground. A couple of minutes after flying around in lower altitudes the elevators freed up again so I was able to make a normal landing. After I got out I went under the fuselage, poked all the drain holes, and water ran out of them all for several minutes.

I figured the icing over the control cables was the problem and that was that. I flew back at a lower altitude where it was warmer and the freezing problem didn’t occur again. I left a note for the next pilot that was going to fly it in a few hours later that day and also wrote it up in the maintenance logbook.

Sorry meant to hit preview instead of submit.
There is, unfortunately, a bit more to the story.

I got it wrong, the other pilot got back and despite flying lower than usual he too had the elevators locking up randomly so it definitely wasn’t icing from water in the belly.
Straight to the maintenance shop and they pulled up the floorboards to see what was going on. It turned out to be a gadget named the Stall Avoidance System stick pusher unit. Metro’s are shits of things when they stall so the manufacturer made the elevator cables run around an electrically-actuated pulley. When activated it’d physically wind the cables so the nose would pitch down to help prevent a stall. The pulley was normally free to rotate but when needed it was connected to the electric motor by means of some kind of electrically activated powder so when power was passed from one side of the powder to the other the powder would go solid so the electric motor could turn the pulley.

However it was one of the shittiest systems ever installed in aircraft. We always flew with the circuit breakers pulled on it so it would work. The reason being that a couple of years earlier when operating a passenger flight one of our planes, when flaring for landing, had the SAS kick in for no discernible reason and thump the plane onto the ground nosewheel first and so a really rough touchdown.
The extra fun problem with the system was sometimes the powder in the pulley system would randomly lock-up all by itself and jam the elevator controls. I was the first to have that happen.
The fix was to disable that somehow.

So I was wrong about the cause, but managed to get away with it.