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Icing in VMC (and detecting pitot heater failure)

A fellow club member said he experienced icing in VMC last weekend on his cross country training flight. The weather was fine with some low clouds, temperature at ground around 0°C.

He took a picture of the wing, which showed, what I think is hoar frost: moist air which condenses on the frozen wing. The leading edge looks clean.

I told him that icing is nothing to worry about if you stay clear of clouds (with the exception of freezing rain). However, he is not convinced. What do you guys think? Could this potentially grow to something serious?

Are there any accident records related to icing in VMC?

I have never seen that myself, but have had what I suspect was icing on the pitot tube in VMC (a loss of a few kt of airspeed, fixed by switching on the pitit heat). Now I fly with the heater on always.

That photo doesn't look like clear VMC though. It looks very hazy.

Also it would take a great deal more ice than that barely visible layer to affect most "conventional" planes. The leading edge looks spotless.

Administrator
Shoreham EGKA, United Kingdom

I've seen this before on the TB10 - it seems to be an area of condensation which freezes up if you climb above the freezing level. As far as I can tell, it is just a film that has no effect on the flight characteristics. If you move in to sunshine it sublimates instantly.

I've only seen this form in 5-8k vis on cold days, below cloud.

Interestingly, it always seems to be around (approx. equally in front and behind) the highest point on the wing surface - which I'm pretty sure is ahead of the minimum pressure area. It is also cleared instantly by going to a very low angle of attack (pushing over on the yoke).

EGEO

Looks like a tiny layer caused by cold fuel in the wings. Nothing to worry about.

EGTK Oxford

@JasonC, that is an interesting idea. Never thought of the fuel tanks. I was the one flying the plane. Lenthamen and I had a discussion wether it was real ice or just hoar frost. Even on the ground the ice was still there and I could scratch it off (like on a frozen window of a car) with my finger. I examined the aircraft before the next take off and the ice was on every horizontal surface of the plane, like the ailerons. I did not noticed any reduced performance during flight, but from a theoretical point of view, the wingshape is altered. Ice on a wing puts in weight and changes the shape of the wing, hence reducing lift, isn't it?

EHLE (Lelystad - NL), Netherlands

Nothing to worry about, ice will be mostly first build up on the front side of the wing, not on top.

The only thing could happen watching that pic is that you have natural vortex generators now, so you could land slower :-)

YSS
EGBJ

Speaking from theory rather than experience here, I understand it depends very much on the aerofoil. Laminar flow aerofoils - as used in many of the new composite wonders - are considerably more efficient than some of their older counterparts, but also much more sensitive to surface contamination. There were problems with the aerofoil Burt Rutan used for the LongEZ for example, and many people ended up rebuilding the canard to avoid problems such as trim changes in the rain, or changes to the stall speed after insect contamination. I suspect that a TB10 would use a less adventurous aerofoil section, but I don't know.

EGCW

The TB wing is certainly a non-adventurous aerofoil and can carry a certain amount of ice OK. On my TB20, with the TKS de-iced prop, I see little or no performance degradation with around 5mm of rime/clear, which usually forms only on the leading edges (wing and elevator).

Beyond that things do get worse.

As I wrote previously, I have been up to about 30mm and that would have been very dangerous if the descent-to-warm-air escape route was not available, as the stall speed was about 100kt and the max level flight speed was about 120kt at max power. And there are well documented cases of TBs landing with several inches, with one of them written up by the pilot in great detail (well to the extent he could recall it, suffering from hypoxia at the time...) but the aircraft would have only been descending; not capable of level flight.

Rumour has it that the Cirrus SR20/22 gets severely affected but I cannot vouch for that.

Administrator
Shoreham EGKA, United Kingdom

Well I had a noticeable speed drop at a reduced power setting on Friday evening in an icing layer at FL080 on descent back into Oxford. And this was with deiced prop and leading edges. Was all manageable although might be time to stick some Icex on the boots! Prop spinner still had 15mm on it when parked up at hangar.

EGTK Oxford

What are the pros and cons of a "hot prop" vs TKS prop? Any views?

EGPD / OMDW / YPJT, United Kingdom
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