I am tempted to ignore 411A but as a flight safety matter is under discussion it is important that his misleading information is not taken as gospel by anybody. Although I bow to his greater experience of the subject (of course they have much more icing in his Arizona than here in Northern Europe), his assertion that thermal anti-ice does "NOTHING" at idle is plain wrong. I note that his second posting modifies this assertion more than somewhat.
Captain SandL is, I believe, on the right lines that thermal anti-ice complements externally-applied measures, but it remains odd that some operators/states have radically different procedures and equipment options for the same aircraft (B737).
If 411A doubts the effectiveness of thermal anti-ice at idle I suggest he starts up a 737 CFM engine without a squat switch inhibit, selects engine anti-ice on, idles the engine for a minute or so (albeit it might be necessary to open the thrust lever momentarily by a small amount to open the valve at the beginning of the period, but then idle can be resumed), shuts down the engine and then puts his bare hands on the engine intake lip. If his hospital Emergency Room has email facilities, perhaps he'll let us know the result. I don't have the manuals to hand to check the details of which stages are involved, but on the CFM 737s, if bleed air from the 'normal' stage is insufficient at idle, bleed air from another stage is added. 5th and 9th stages ring a bell. Perhaps pilots of other a/c might like to add their experience/knowledge.
I can't be bothered to check the original accident investigation report just for the benefit of 411A, but I recall that a crucial factor in the Potomac accident to which he refers was that thermal anti-ice was not selected on at all, hence the erroneous EPRs etc (200 series a/c?).
[This message has been edited by Hew Jampton (edited 01 January 2001).]