737 NG question (or other aircraft)What EXACTLY are the engine bleeds and what specifically do they do?Do they direct pneumatic air to go somewhere in the engine? Do they have anything to do with starting the engines at all? etc.Thanks.Chris Catalano

Engine bleeds are valves that allow compressed air to exit the engine to be used for various purposes. Engine bleeds are not used to start the engine (think about it no engine start, cant get air from it :) ) but the APU also has a bleed which supplies air to the starter motor. THink of the starter as a air wrench, it gets pressurized then spins the motor to ignition. Engine bleeds are used for air conditioning and pressurization. They do take away a bit of power, so on takeoff if the extra power is needed they often turn them off.Hope i helped

Some engine bleeds from the hot air stages of the turbines can also be used for wing/structural deicing. When that kind of deicing is engaged you'll see somewhat of a loss of power.On the 737NG when using bleed air from the APU you may see only one air conditioning pack on line so there is enough air pressure remaining to spin up the first jet engine. After one turbine engine is started, the bleed air for that is turned on and the APU bleed off, the second jet is started and after spinning up its bleed air is turned on and the second air conditioning pack is turned on. Electrical jet engine generators are placed on line and the APU electrical bus opens. Generally the APU remains on so its bleed output and electrical output is ready for emergency use until after take-off and established in the climb after which the APU is shut down until the final arrival stages.For a discussion on these issues go to www.b737.org.uk and click on systems and other sections.Also bleed air is hot being taken from the later compressor stages of the turbines but cooled for cabin air use as necessary. It is left hot for wing/structural deicing where used.

you need your engine bleeds on for engine start to draw air to the engines

Hi,================AIR BLEED SYSTEM================Compressor Section------------------The N1 compressor, or booster section, produces low temperature, low pressure air and delivers it to the N2 compressor which produces high temperature, high pressure air. The single stage fan, which is an extension of the first stage of compression, produces very large volumes of bypass air. Each compressor section is driven by its own separate turbine at its own best speed. The high pressure compressor (N2) is governed by the MEC / EEC while the fan and low pressure compressor (N1) is driven by its turbine and is free to select the best speed to ensure optimum airflow. This airflow matching feature allows the compressor sections to adjust themselves automatically throughout the operating range of the engine. It also minimizes interstage bleeding preventing stalls and surges, and with the front and rear rotor sections working in harmony, the compression ratio can be increased without decreasing efficiency. Fan Bypass / Bleed Air----------------------Fan bypass air is used for thrust reversal, generator drive and generator cooling as well as bleed air cooling. Fifth stage bleed air is used for the Environment Control and Anti-Ice systems. However, at low thrust settings, fifth stage air pressure is inadequate, so ninth stage bleed air is used. When fifth stage air pressure becomes adequate, a crossover from ninth to fifth stage air is made.-------FAN AIR-------GENERATOR DRIVE AND GENERATORCOOLING AIR CONDITIONING PRECOOLER--------------------------------5TH STAGE OR 9TH STAGE BLEED AIR--------------------------------AIR CONDITIONIING & WING ANTI-ICEENGINE ANTI-ICEHYDRAULIC SYS TANK PRESSUREWATER SYSTEM PRESSUREHope this helps.

Great expansion on the knowledge.Now I have one question. I understand some turbine inputs have an available trap that can be opened to catch and detour ice chunks sucked into the intake. Is this also controlled by the engine de-ice switch?Thanks.

Hi,Fan blades and front-end compressors are designed to cope with ingested ice. Lightweight ice impact panels protect the fan-case lining from ice shed from the fan blades.There are two critical points where icing is most likely to occur and presents the greatest hazard. The air inlet in the narrow region between the shroud and blade, and the struts connecting the fan casing to the core engine.For those two spots, hot compressor air is being ducted into the space between the shroud and fan, that will quickly melt whatever ice there may be.The only engine traps I'm aware of, is for sand, along with thermal barrier coatings.

no you don't.The bleed air valves are used to draw air out of the engines and into the pneumatic systems of the aircraft itself (among them the airconditioning and pressurisation systems).If you're using a crossbleed procedure to start your engines you need to start one using an external source of bleed air (like an APU or GPU, or a startercart), and after that can use the bleed air from the started engine(s) to start the other engine(s).

I found the reference to the ice traps. They are "inertial separators" used on turboprops. They have their own pilot activation or are spring loaded and opened when a foreign object strikes them. They are located at the bends of the induction flow and the object inertial vector makes them tend to go straight following the path to the bypass run. They still sap engine power since part of the induction air gets separated out with the ice chunks. It is not used in jets AFAIK.This same reference book (The Turbine Pilot's Reference Manual 2nd Edition - great book for the whole regional aircraft theory and application beyond engines and into all systems including electronics) states that some jet engine inlets are electricly heated.Turboprops go their own way again with a few types using pnuematic boots on their inlets.Thanks for expanding on the more familiar jet engine inlet ice protection scheme.