Safe to fly through TCU ?

You will probably be going through the top slice if airways where, as it has been said, expect ice, moderate ++ ...even if FIKI equipped the icing can be quite dramatic and test FIKI piston aircraft to their limit

I would be extremely careful with any possibility of flying through a TCU if in IMC before it i.e. it might be unexpected.

The ride is likely to be very rough - to the level of banging your head against the roof and have the headset flying off.

Obviously slow down well below Va if anything like that is expected.

Icing is likely to be rapid if the OAT is below 0C.

Any passengers are unlikely to fly with you again, which may turn out to be quite significant in the long term (depending on who it is).

Don't let ATC stop you from deviating if weather is really a concern. If necessary tell them rather than ask them. It is your right and responsibility as Captain. To be fair I have always found ATC to be very co-operative.

ATC in N Europe are usually really good, but lower down (Spain and Italy notably; Greece is usually better) they often don't understand English well enough to understand words like "icing".

One has to keep it really basic e.g. "Nxxxx request two zero degrees left to avoid".

I no longer use "Nxxxx request two zero degrees left due weather" because "weather" is not always understood.

And if you get no response after say two calls, just say "Nxxxx turning two zero left to avoid" and just get on with it. They can see you on radar.

Also it is common in Spain for ATC to not respond if they don't understand because that leaves no evidence (of sub ICAO English language proficiency) on the tape, so you have to just get on with it. I had a case a while ago where I was cleared FL180 but was unable to climb above FL170 (most likely due to a slight downdraught, just south of the Pyrenees) and ATC could not understand anything I said so stopped replying to me. I just carried on at FL170 - there would be no traffic anywhere near that low anyway and they had me on radar - all the way to Biarritz. They spoke OK in simple terms.

So, work with ATC but keep ultimate decisionmaking in the cockpit, always.

What does suprise me is that most civilian ATC have no idea of the weather. They don't seem to have weather radar access. Some do e.g. Bournemouth EGHH gave me some suggestions when I was in IMC and in very rough wx. I think this is where the USA is way ahead.

Cobalt's post touches on one BIG reason why we cannot have IFR with most non-certified planes. IFR means IMC and you need full bonding of the airframe parts, otherwise the whole thing will get charged up, with flashovers to any electrical parts, crashing or destroying avionics, messing up comms, etc. I lost all comms on COM2 in IMC because there was too much grease (no kidding) in the elevator bearings (they are now bonded with two lots of braid). That was on the TB20 which is metal except the roof and that is bonded. With lightning it gets even better, with fused or interrupted control linkages. Basically all "plastic" non-CofA planes are unbonded and should never be IFR certified because they will be death traps. The metal ones have a chance, if properly built and meeting control force requirements and many other things.

ATC could not understand anything I said so stopped replying to me.

Yeah British English is usually hard to understand to non-tuned ears. I find that English spoken by Italian ATC is very easy to understand and both sides being non-native speakers we usualy find common grounds and have no problems in understanding. So far I have experince with following ATC's English: Croatian, Bosnian, Slovenian, Macedonian, Serbian, Montenegrian, Italian, Albanian, Czech, Slovak, Hungarian (all easy to understand), Austrian (little bit of struggling), German (more struggle, they tend to speak fast), Swiss (similar to Germans), Belgium, Dutch, French (very hard for me).

Coming back to the subject, Italian ATC can be persistant in trying to settle you back on airway once you've changed heading for avoiding rather than allowing you direct to next point. I've expirienced that in few occasions - that's something you have to insist on - if it's avoiding then it's avoiding and lasts until you as PIC decide that it's over.

I try everything to not fly through TCUs. They are terribly unpleasant, dangerous etc.

I ask for deviations when I see them. If there are a lot, ATC will grant the first few heading changes and then after some time give you a general clearance to circumnavigate.

If ATC did not approve my deviation, I would do it anyway. No way I fly through a TCU because ATC wants me to.

My short awnser NO Nothing for enroute flying (and even more with pax) when you have only a littel bit fun at this trip! Take a look:

That's a horrible video.

What is not clear to me is what action the pilot was taking to get out of it. At one point it sounds like he descended by 4000ft to get into warmer air.

Most non-deiced planes would be going down fast in freezing rain lasting that long. Maybe he got away with it because he maintained a high speed in the descent, so avoided a loss of control.

What plane was it?

My only encounter with freezing rain was inside a TCU, near Bournemouth EGHH. I did an immediate left 90 and descended from about 4000ft to 2000ft. None of it appeared to stick so it must have been a very localised patch.

The pilot was Cheshunt and it's a C210. I don't think it was that dramatic as there was warm air and good weather below and you never lose your ability to descent due to ice

I wonder what your insurance would think about a hail damage claim that happened inside a TCU? On the ground it's covered. However, usually nothing happens inside a TCU as the hail doesn't have size and velocity it has on impact on the ground.