Topic Author
Posts: 31
Joined: Wed Jul 18, 2001 9:12 pm

Bleed Air

Wed Jul 18, 2001 9:21 pm

Can any one explain when the bleed air system is selected on and off. I realise bleed air is switched off for engine start up and is activated when the engines are operating. Is bleed air switched off just before take-off to maximise thrust ?. By having bleed air off, does this affect the de-icing system ?. I would like to be enlightened.
Posts: 1952
Joined: Sat Mar 18, 2000 10:09 pm

RE: Bleed Air

Wed Jul 18, 2001 10:48 pm

no bleed air is usually left on the entire flight ( at least on the plane i fly ) its used to run the A/C and other pnuematic systems. on rare occasions bleed air is truned off for performance but turned on shortly there after. as far as anti-icing it doesnt matter.
"damn, I didnt know prince could Ball like that" - Charlie Murphy
Posts: 980
Joined: Sat Jan 27, 2001 6:53 am

RE: Bleed Air

Thu Jul 19, 2001 5:03 am

On DC-9's,MD-80's and -90's,you need bleed air for airfoil anti-ice,no doubt about that.We've got weight penalties if we need to use airfoil anti-ice during take-off and early climb due to the loss of power when bleed air extraction is taking place.Engine anti-ice is a different matter.
Switching bleed air off will give some extra T/O performance,but it's seldom used.In 16 years as a jet pilot,I haven't done that yet!
"Luck and superstition wins all the time"!
Posts: 1443
Joined: Mon Jun 18, 2001 1:29 pm

RE: Bleed Air

Thu Jul 19, 2001 11:43 am

Bleed air is an absolute necessity for anti-ice. The anti-ice valve is usually located on the engine side of the bleed air shut-off valve, also called the pressure regulating shut-off valve (PRSOV). The bleed air switch can be off and the engine nacelle still be anti-iced, but airfoil de/anti-ice must come from bleed air. In fact if we have to de-activate an engine bleed system we usualyy take an icing penalty, i.e. you cannot fly into icing.
Posts: 694
Joined: Sun Nov 21, 1999 11:44 pm

RE: Bleed Air, Part 4

Thu Jul 19, 2001 10:06 pm

Hi TOGA, Buzz here. On the engines i work on, the bleed valves close pneumatically if there's pressure in the duct (so it doesn't leak through a non-working engine). So ever since the DC-8 days we haven't had to manually close the valve before start. Once the engine's running the valve opens up on it's own. (OK, on 727's if you close the bleed valve you can't start the engine) But those swithces are normally left open.
At DEN we had a procedure with the oldest 737's (which had JT8D-7 engines) and DC-10's to do a no-bleed takeoff if the weather was hot (and DEN is at 5200 ft above sea level). It helped a little, the APU would provide cabin pressurization for the first minute or so. But everything since the DC-10's came out has stronger engines and engine bleed is normally left on for takeoff.
And as the others have said, you need a source of bleed air from somwhere to provide wing anti-ice, on the 757 and A320 series ther's a helpful amber indication on the engines screen to remind you to push the thrust knobs forward to provide enough bleed air. Remember, the bleed air is also the air you breathe. Once in a while pilots have forgotten to turn on the bleeds and return to the airfield because the airplane didn't pressurize. It's hard to get them to admit to it, unless we look in the computer that monitors engine performance.
Engine anti ice is "important", got to be careful when you deactivate it (mechanically) and go flying. Many years ago at Christmas time a PeopleExpress 747 was waiting for a new set of fan blades for #4 engine. Seems that the engine anti ice valve wasn't working and they flew through some "fairly moist" clouds. A large chunk of ice built up on the inlet, then broke off and was eaten by the engine. What was left of the fan blades wasn't pretty, about 30% were severly damaged.
Buzz Fuselsausage, Line Mechanic by night, DC-3 Crew Chief by choice.

RE: Bleed Air

Thu Jul 19, 2001 10:20 pm

The bleed air for AirCon and Anti/Deicing come from different sources. IE., you can have one on and not the other. Flying 727's out of short fields on hot summer days, we normally turned off AirCon bleed air to improve takeoff performance.

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