volcanich ash

[uchy]

I read that in some aircraft there is the procedure to switch on the anti-ice in case that volcanic ash is suspected...For which reason? Thank's

[BARKINGMAD]

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?:confused:

[Shell Management]

Perhaps you mean auto-ignition.

[BOAC]

No - he/she MEANS anti-ice. Read post #2.

[Shell Management]

I have and the recommendation makes no sense.

[lomapaseo]

Firstly this is an aircraft manufactuer question to answer.

However, in my consideration the number one priority if you find yourself in known volcanic ash (inflight symptoms) is to decrease the engine turbine temperatures. Failure to do so will cause internal damage (glassification) which is permanent. Adding bleeds without decreasing the throttle position will likely increase engine turbine temperatures.

Now if the damage is already done (engines have already stalled) then adding bleeds might help. But first of all get the hell out of the ash by decreasing throttle

[Pub User]

One of the earlier actions in the Volcanic Ash QRH drill is to CLOSE the thrust levers, thus reducing the turbine temperatures, to satisfy lomapaseo's concerns. This action would also tend to reduce both the temperature and pressure of the bleed air, so a Bleed Trip Off is very unlikely.

Like Barkingmad, I believe the idea is to increase stall margins within the compressor, rather like the 'bleed-band' system used by some gas turbine engines.