No ice protection is “for ever” or even for the duration of the fuel tanks. It is to protect you while you are in icing conditions. Obviously turboprops and jets have the performance to climb up through those quickly.
But anyone can get stuck in a holding pattern for a while and be collecting ice. The pilot of an unprotected aircraft should refuse a holding instruction in such conditions “due to icing conditions” however, and ATC will normally assist because they are trained to know what the risk is.
For most of us, the name of the game is to climb up to VMC (or, in IMC, at least to a level where icing should not be possible) and fly enroute there.
But that isn’t the whole story. I used to think that -15C was safe (and it usually is from the airframe icing point of view) but the TB20 engine stops at -15C due to fuel servo icing unless either using alt air or prop TKS. The DA42 does exactly the same (both engines stop) unless you use alt air. There is a long thread here suggesting the SR22 does the same, at some lower temperature.
Peter wrote:
No ice protection is “for ever” or even for the duration of the fuel tanks.
In the sense that you run out of de-icing capability or in the sense that you will eventually pick up enough ice on unprotected parts of the airframe to make continued flight impossible?
In terms of running out of de-icing capability, TKS certainly is not “for ever” 
But boots, bleed air and electricity will be available as long as your engine runs and you do not develop a system fault. Which is why I like the Thermawing/Evade system.
In the sense that you run out of de-icing capability or in the sense that you will eventually pick up enough ice on unprotected parts of the airframe to make continued flight impossible?
Both, although the latter should take quite a long time.
I don’t think boots work for ever because the ice will still build up on the wing surface. This is what makes TKS so effective, due to it running back over the wing, but it runs out.
Peter wrote:
I don’t think boots work for ever because the ice will still build up on the wing surface.
Typically not actually. While it is possible in certain supercooled liquid droplet (freezing rain) scenarios, I have not had that occur on any of mine.
I dont think the SR22 data has been published, but will check COPA.
I would have thought icing behind the boots is only likely with SLD, and piston aircraft are not certified to operate in these conditions ;-) TKS may be more effective but I doubt any certification work has been done for these conditions so it is down to a few Guinae Pigs to let us know if they can.
I think there are some reports suggesting with SLD a ridge can form on the wing behind the booots which causes control issues with disruption to the air flow over the ailerons. I suspect airflow disruption is far more significant than weight.
I’ve had it a couple of times in an MEP were ice has accumulated aft of the leading edge and therefore was not able to be dealt with by the boots. This happened during commercial operations in the UK in the winter, not being able to climb above FL100 due no pressurisation/oxygen and on at least one occurrence led to a descent to MSA to try to escape the icing conditions. I remember being fairly shocked when we landed at just how much ice was on all parts of the airframe.
Pirho – it would be interesting if you could recount the conditions and where exactly the ice accumulated.
I have had a situation (and someone took a photo, but it was in the days of film and printing, and I can’t find it) where I taxied a C404 in after a night mail flight in mid winter and there was so much ice that when it fell off it formed a beautiful silhouette of a C404 on the tarmac, rather like you see in black and white murder movies.
So I suspect @Pirho’s answer will be “everywhere”!
However, it is no joke. On that occasion and on several others, in both C404 and PA31, I have carried so much ice that I have been at full power and Blue Line and unable to maintain altitude. On one occasion I had to declare a full emergency to be able to descend through the airspace below me. I daresay that I was not very far from a stall/spin. It is not a situation to get yourself into lightly :-(
Thanks for sharing Timothy. This is food for thought to those who venture into icing conditions in non-deiced airplanes, and even in deiced ones.
@Fuji_Abound, Timothy is correct saying everywhere! It was in fact in a PA31. Ice accumulated on the spinner, on the leading edge despite the best efforts of the aged boots, aft of the leading edge, tail, fuselage, and especially I remember the nose because when we got out of the aircraft we noticed it was a many many inches longer than it was when we left thanks to a large build up of ice there.
Conditions I can’t exactly remember I think it was around November time in the UK, a 2 hours flight which went into IMC a few seconds after takeoff and only out of IMC late into the approach. It wasn’t convective weather it was just general clag. Initially we tried FL90 and then descended lower and lower due to ice – dropping out of controlled airspace on the way and as Timothy describes exactly, ending up flying blue line just to maintain MSA. Very unpleasant scenario which unfortunately happens now and again in that sort of year round operation. A few of us ended up bringing OS maps on tablets/print outs so that should we need to descend below MSA we stood a chance of somewhere flatter. Luckily I never had to resort quite to that.
