When's it too cold for Carb Heat?

[Gaseous]

Headsethair,
PC6s advice comes from Lycoming and as he says, is fine for fixed wing. It is not however, appropriate for helicopters fitted with a gauge. No one said it was. I should have emphasised that.

Incidently, one is not necessarily a 'whacko' to lean a helicopter. It is standard operating procedure detailed in the pilots operating handbook for some piston helicopters.

Edit. I have found Lycomings article on this. 3 passages are relevant. (Flyer Reprints 1991-Related factors when using carb heat - page 47) I think these confirm mongoose237s post below but see my comments on SN25 below.

1) In known or suspected icing conditions follow the POH
2) At a temperature of 20deg F or below any moisture in the air is frozen and heat should not be used.
3) partial heat melts these crystals and they form carburettor ice when they come into contact with cold metal....

In short. I interpret this as: Keep the needle out of the yellow arc. If its above, keep it above. If its below, keep it below. This agrees with Section 4 of my old Astro POH says simply keep the needle out of the yellow.

However:

SN 25 says the induction system should be warmed to above the yellow before flight and kept there using carb heat.

Does the SN override the POH? I think it might.
Clear as mud.