proxypilot the term inner wing de-ice is a bit misleading. As you know when wing de-ice is used, all of the leading edge is heated; the inner wing de-ice supplements this, primarily with flaps down – the amber caution (wing not de-iced?) comes on with flap selection.
There are several subtle issues:
The design of the wing de-icing system was for a fully evaporative system, thus it should more correctly be an ‘anti-icing’ system, which was intended to be on at all times in icing conditions. Note the significant difference in operating philosophy with turbo prop boots. However, with experience and industry trend, wing de-icing is more often used as a de-icing system and only switched on when ice is detected (alerted or visually). This does not necessarily follow for the tail, thus always consider the conditions carefully, and if in doubt put the system on.
The severity of icing conditions vary, in some instances runback ice could form; therefore as a precaution the inner wing system boosts performance to remove any residual ice when leaving icing conditions (use for 1 min when leaving icing). In moderate to severe icing then the basic system (as with any aircraft) could be overwhelmed with ice accumulation, thus the inner wing should always be on. The definition of severe is when any system cannot maintain a clean wing; aircraft should leave these conditions immediately.
The ice detect light is not always a good guide as to the conditions; some crystalline ice may not stick to the detector, but the particles could melt on the wing or affect other systems / engines. These are rare conditions, but often if encountered the circumstances could result in an incident. Always look outside for ice, use the wing lights at night, follow temperature related SOPs, and don’t forget that ice accumulates more readily on thinner tail surfaces, so if the wing is clear the tail may not.