The 737NG QRH currently requires this action, apparently to increase the bleed on the compressors which I understand assists in maintaining compressor stall margins.
However, the same QRH also requires the WING Anti-Ice be switched ON, apparently for the same purpose.
Some pages away in the same document the caution AGAINST switching ON the WING Anti-Ice is listed, as the possible effect above FL350 is a double Bleed Trip Off leaving the hapless Volcanic Ash victims with no pressurisation!!!
I have referred this query to my company´s Fleet Management and Tech Pilot for clarification from Mr Boeing and Mr CFM, but reply came there none.....
I know what I will do in the event, and wait for the desk-flying lawyers to shoot at my orifice, but we can but try to exercise prudent judgement and earn the deep respect and the inflated salaries we are reputed to receive........
I'm sorry, I appear to be on the wrong planet, time for a lie-down in a darkened room til the haze clears?
