TOGA
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Pressurization & Bleed Air Question.

Wed Aug 28, 2002 10:18 pm

Under what circumstances would a "bleeds off" take off be made. As I understand it, if there is no bleed air then the cabin cannot be pressurized and there will be no airconditioning available. At what stage is bleed air selected on, in order to pressurize and supply air conditioning ?. Also, can anyone explain in lay-mans terms, what purpose the "Engine Electronic Control" serves. I believe it prevents over thrusting of engines on take off.
 
Guest

RE: Pressurization & Bleed Air Question.

Wed Aug 28, 2002 10:56 pm

The "no bleed" takeoffs are sometimes done to gain a little extra power, in turn, this permitting a higher gross weight for takeoff...
Some airplanes doing that will simply "put the bleed air ON" thereby starting pressurization and air conditioning use, a few seconds after takeoff, say 500-1000 feet above the airport of departure...
In the 747, we use another technique, we do not use bleed air from the engines for takeoff, we use our APU during the takeoff to provide bleed air for air conditioning and pressurization purpose, once in the air, landing gear and flaps retracted, the engine bleeds are selected "ON" and the APU is then stopped...
(s) Skipper
 
LMP737
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RE: Pressurization & Bleed Air Question.

Thu Aug 29, 2002 1:01 am

The Electronic Engine Control is basically the brain of the engine. The EEC monitors engine parameters during start such as hot starts and wet starts. It also monitors engine operation, and controls engine operation. It does this by getting data from the various components on the engine such as the HMU, speed sensors, temp sensors etc. And yes it will prevent excedences during takeoff. If you want more info don't be afraid to ask. Smile
Never take financial advice from co-workers.
 
avioniker
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Joined: Tue Jan 01, 2002 5:38 am

RE: Pressurization & Bleed Air Question.

Thu Aug 29, 2002 9:46 am

http://www.b737.org.uk/powerplant.htm

Depending on the aircraft and engine the EEC may be as little as a governer or as much as the sole controling unit. The next generation 737's have no cables to control the engine. It is known as a FADEC (Full Authority Digital Engine Control) engine with the EEC providing total control of the engine through the HMU (Hydro Mechanical Unit).
The pilots move the throttle levers causing an electrical signal to be transmitted to the EEC which then commands the engine to do what the pilots want. There's lots of backups.
Check out the link above. It's about the 737 engines from Next Generation to Jurassic and should be very informative for you.
One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
 
avioniker
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RE: Pressurization & Bleed Air Question.

Thu Aug 29, 2002 9:56 am

Sorry, I forgot to mention your original question.
"Bleeds off" take offs are made primarily for noise reduction. Many planes are actually considerably quieter and have more available thrust with them off. Airports like Orange County California have very strict noise rules and with the older aircraft it's sometimes the only way to get legal. You're right that you can't pressurize without bleed air and there have been many embarrassed crew members at their destinations after writing up the pressurization only to have some wise a... mech reach over the F/O's head and throw a switch.
One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
 
BR715-A1-30
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RE: Pressurization & Bleed Air Question.

Thu Aug 29, 2002 11:35 am

The BR715 is also another engine with FADEC with EEC as the brains of the engine. It does work alot better than a -9A (JT8D)
Puhdiddle
 
EssentialPowr
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Joined: Wed Sep 06, 2000 10:30 pm

RE: Pressurization & Bleed Air Question.

Thu Aug 29, 2002 1:38 pm

Avioniker,

""Bleeds off" take offs are made primarily for noise reduction. Many planes are actually considerably quieter and have more available thrust with them off."

What?
I've never seen a "bleeds off/noise reduction" profile. Pls enlighten me.

 
exPratt
Posts: 306
Joined: Wed Mar 08, 2000 6:15 am

RE: Pressurization & Bleed Air Question.

Tue Sep 03, 2002 7:38 am

B747skipper is correct. On the 4-engined airplanes like the 707s, DC-8s, early model 747s, and even the 727s, it was a relatively common practice to take off with the bleeds off to get a little extra thrust for takeoff, especially on hot days. The bleeds in question are not the engine bleed valves, but the airframe supplied air valve in the engine pylon or strut that is controlled from the cockpit and not by the electronic control. The bleed air controlled by the air valve is commonly referred to as customer bleed air and is typically used for air conditioning and cabin pressurization. It can also be used for cross bleed engine starts. With the valve open, it does take away air from the compressor that in turn reduces the engine's thrust. With the valve in the closed position, then all of the air going through the compressor will be available to go through the remainder of the engine in the generation of thrust. In any event, the engine bleed valves, which on older engines would be controlled by the pressure ratio bleed control (PRBC) and on new engines by the EECs or FADECs, would be scheduled closed when the engine was at max power. The bleed valves are scheduled open for various conditions such as starting, idle power, off idle accel, top of descent decel, etc. No engine in the world would have an engine-mounted bleed valve scheduled open at max power.

And the bleed valves, on the engine or in the pylon, have nothing to do with noise reduction.
 
ismangun
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RE: Pressurization & Bleed Air Question.

Tue Sep 03, 2002 8:05 pm

Well, with bleeds-off T/O the EGT will be lower than bleeds-on with the same thrust produced, but this is more to saving engine life. Bleeds-off could also give more margin to ATOW (you can take more weight) with respect to those engine-life saving.

Noise reduction can be achieved by red-t/o-thrust or with hushkit offcourse.
If it's an Airbus, I'll take the bus
 
AAR90
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RE: Pressurization & Bleed Air Question.

Fri Sep 06, 2002 1:24 am

>"Bleeds off" take offs are made primarily for noise reduction. Many planes are actually considerably
>quieter and have more available thrust with them off.

Turning engine bleed air off does nothing to actual noise produced by the engine. It is done to increase maximum thrust an engine is able to create.

>Airports like Orange County California have very strict noise rules and with
>the older aircraft it's sometimes the only way to get legal.

By increasing available thrust, the aircraft will climb more quickly so that it passes by the noise monitoring stations at a relatively higher altitude. That relatively higher the altitude causes the noise monitor to record a relatively lower noise level. This is a by-product of the higher available thrust.

*NO CARRIER* -- A Naval Aviator's worst nightmare!
 
LMP737
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Joined: Wed May 08, 2002 4:06 pm

RE: Pressurization & Bleed Air Question.

Fri Sep 06, 2002 1:48 am

All commercial aircraft flying into SNA have to be Stage 3. No hushkitted aircraft are allowed. Therefore there are no issues with older/nosier commercial aircraft. That does not stop the folks in Newport Beach from complaining about the "noise" however.
Never take financial advice from co-workers.
 
EssentialPowr
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Joined: Wed Sep 06, 2000 10:30 pm

RE: Pressurization & Bleed Air Question.

Fri Sep 06, 2002 2:54 am

So I guess Avioiker isn't going to respond...Bleed position had nothing to do with noise reduction.
 
AAR90
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Joined: Fri Jan 21, 2000 11:51 am

RE: Pressurization & Bleed Air Question.

Fri Sep 06, 2002 6:06 am

>All commercial aircraft flying into SNA have to be Stage 3. No hushkitted aircraft are allowed.

Correct.

>Therefore there are no issues with older/nosier commercial aircraft.

Older.... correct, there are no "older" airliners permitted into SNA.
Noiser.... incorrect. SNA had more strict noise standards than FAA.

Without getting into the complexities of SNA noise standards (slot allocations/violations are way too complex for this pilot to fully understand), one must understand that the use of a Bleeds-Off take-off is utilized by some airlines to keep some aircraft quieter than "normal" to improve noise slot utilization. For example:

--AA MD80s [slot "A"] operate using maximum thrust and are weight limited to prevent slower than planned for climbs. Crossing the noise monitors lower than normal=higher than planned for recorded noise level. Bleed air remains ON at all times.

--AA 737s [slot "C" or "D", I don't recall] operate using special maximum thrust limit --"27K Bump"-- and are _sometimes_ weight limited. When approaching a weight limit, 737 pilots are authorized to use Bleeds-Off procedures in order to increase that weight limit slightly without a performance decrement (lower than expected climb). The result being a slight increase in gross weight permitted with the same recorded noise level at the monitors. I believe similar procedures apply for AA 757s [slot "C"].

OTOH, AA MD90s [slot "E"] operated at maximum thrust, _never_ weight limited and Bleed Air on all the time! Ahhhh, the advantages of being "Exempt" at SNA.  Big thumbs up

Confused? Welcome to government regulation.... California politics style.  Sad
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