Should the Engine bleed valves be off for engine start and takeoffs! then turned off on Shutdown ?

Would appreciate a reply from one of the pilots who actually flew the D model for the airlines.

thanks

ress

Well I teach 'em and don't currently fly 'em. But, the Bleed Air would normally be in the ENVIR and INST OPEN position and is generally only turned to either an ENVIR or ENVIR and INST OFF position during abnormal procedures.

 

Environmental air is needed by the Air Cycle Machine for cooling. Cooling/Heating and Pressurization are all the same system. The Air Cycle Machine bypass valve has to be fully closed before a signal is transmitted to the Vapor Cycle System to further cool the air using the evaporator coils. So if you are going to cool the air you need Bleed Air. Of course if you want the cabin warmer the only source of warm air comes from the engines.
 

Now it is my understanding that a certain operator normally starts the engines with the bleed air switches in the ENVIR OFF position when operating at high density altitude airports (such as KTEX.) They reason that they want maximum air going through the engines during start and none of it being tapped off for environmental. Since the AFM does not specifically prohibit this it is an acceptable technique.