737 Volcanic ash bleed demand

[nick14]

Hello all,

I am after some technical advice on the 737NG Volcanic Ash checklist.

There are several references to increasing bleed demand thereby increasing surge/stall margins (packs in high,wing and engine anti-ice on). My question is how does the increase in bleed demand increase the surge margins?

Not being an engine man I have been searching for an answer as our company sim training is focusing on this scenario.

Thanks

[hetfield]

At low power settings the compressors would be providing too high volume of compressed air than the combustor could process.


The airflow may already be disturbed by damage of the ash.

[rudderrudderrat]

Hi nick14,

By increasing the bleed demand, you reduce the pressure of the air in the compressor for the same engine RPM. Similar effect as reducing your weight whilst flying at the same IAS - your stall margin is bigger.

The compressor blades may stall (causing a surge) due to the ash contaminating the blades - similar effect as ice on your wing.

[lomapaseo]

By increasing the bleed demand, you reduce the pressure of the air in the compressor for the same engine RPM. Similar effect as reducing your weight whilst flying at the same IAS - your stall margin is bigger.

agree

but the major effect of the ash is the closing down of the Turbine inlet vanes due to plating of the remelted ash on the stationary turbine inlet vanes. Thus the concern is to reduce the turbine inlet temperature (by throttle back) before the ash builds up and to reduce the compressor output pressure in case the ash has already built up.