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Packs, Bleeds And Engine Starts, Oh My!  
User currently offlineWestJetForLife From Canada, joined Jun 2005, 814 posts, RR: 1
Posted (5 years 2 months 1 week 6 days 20 hours ago) and read 5553 times:

Hi, everyone. If there are any current/former 737 drivers that can help me with these questions.

1. During start up, does the 737 have to have its AC packs OFF for start, or can you have them running in the "AUTO" position? As well, do you usually turn your APU bleed off immediately after start or do you leave it on for a while?

2. What is a "bleeds-off" takeoff, and how does it benefit the aircraft during takeoff/initial departure? For those who aren't savvy, the bleeds are used for air conditioning and pressurization.

3. Between the two start switches on the forward overhead panel, there is a switch that is marked "left, both, right" for the engine start. What exactly is that for?

I hope WILCO737 and AAR90 (I think that's him) can help me on this one. Any other 737 pilots that are currently or formerly type-rated in the 737 can help as well.

Thank you,
Nik


I need a drink.
19 replies: All unread, jump to last
 
User currently offlinePart147 From Ireland, joined Dec 2008, 481 posts, RR: 0
Reply 1, posted (5 years 2 months 1 week 6 days 19 hours ago) and read 5543 times:

Hmmm?

Are you doing your homework?

1, Normally off for many aircraft

2.

Quoting WestJetForLife (Thread starter):
for those who aren't savvy, t

That sounds a little bit condecending there a 'lil whippersnapper!  Wink
Bleeds off means that all compressed air is used during takeoff, giving max thrust for a given rpm, hence no losses

3. I'm sorry but I gotta to say it - the 'left, both right' switch refers to the 'magnetos', you know, when doing a mag-check - making sure the rpm rpm doesn't drop more than a few hundred rpm  Wink  Wink

PS - Don't use these answers - you'll get an 'F' for sure

Why not look at this site instead? http://www.b737.org.uk/



It's better to ask a stupid question during training, rather than make a REALLY stupid mistake later on!
User currently onlineDashTrash From United States of America, joined Aug 2006, 1519 posts, RR: 2
Reply 2, posted (5 years 2 months 1 week 6 days 19 hours ago) and read 5525 times:

Are you looking at a MS Flight Sim 737 panel by chance?

User currently offlineHAWK21M From India, joined Jan 2001, 31667 posts, RR: 56
Reply 3, posted (5 years 2 months 1 week 6 days 18 hours ago) and read 5509 times:

1.Since you want max bleed pressure to crank the Engine,hence Packs are switched off to achieve that.

2.Bleeds off T/O,implies no bleeds used from the Engines for T/O,thus no loss in thrust.APU Bleed could be available.Both APU & Engine bleed on will cause a "dual bleed" alert,you don't want engine bleed striking a running APU in case the Apu check valve malfunctions {winks]

3.Left,right or Left&Right ignitors [Both].Normally on one sector LH is used & on the return leg RH.
In case of Ignitor malfunction leading to no light up,then the next start is carried out with the Ignitors to both.

If both was selected first one would never know if LH or RH ignitor was not working until the other working one went u/s too leading to an AOG Situation.

regds
MEL



Think of the brighter side!
User currently offlineWestJetForLife From Canada, joined Jun 2005, 814 posts, RR: 1
Reply 4, posted (5 years 2 months 1 week 6 days 13 hours ago) and read 5423 times:



Quoting Part147 (Reply 1):
That sounds a little bit condecending there a 'lil whippersnapper!

Not really, I just know a little bit about what the bleeds are, thanks to A.Net and b737.org.uk.

Quoting DashTrash (Reply 2):
Are you looking at a MS Flight Sim 737 panel by chance?

No. The default panel has no AC/Pressurization panel. I'm just curious about how the 737 pneumatics and engine start procedures are, just because I'm bored and it was bugging me.



I need a drink.
User currently offlineWILCO737 From Greenland, joined Jun 2004, 8968 posts, RR: 76
Reply 5, posted (5 years 2 months 1 week 6 days 12 hours ago) and read 5391 times:
AIRLINERS.NET CREW
HEAD MODERATOR

I am not on the 737 anymore, but I try to remember  Smile

Quoting WestJetForLife (Thread starter):
1. During start up, does the 737 have to have its AC packs OFF for start, or can you have them running in the "AUTO" position? As well, do you usually turn your APU bleed off immediately after start or do you leave it on for a while?

Packs OFF. The 737 is not automated that the packs would automatically swtich to off and the APU is not strong enough to supply air to the starter and to the packs. After starting one engine you can do a "single pack operation during engine start". Then you suplly air from the running engine to the aircondition pack and the APU air to start the other engine.

Quoting WestJetForLife (Thread starter):
2. What is a "bleeds-off" takeoff, and how does it benefit the aircraft during takeoff/initial departure? For those who aren't savvy, the bleeds are used for air conditioning and pressurization.

Packs off during take off. That means you do not need to take air from the engine which increases its performance and you maybe can carry more passengers, cargo etc out of an airport if you are performance limited.

Quoting WestJetForLife (Thread starter):
3. Between the two start switches on the forward overhead panel, there is a switch that is marked "left, both, right" for the engine start. What exactly is that for?

It is for the engine ignition, there are 2 systems. They are used alteratly daily (at my airline).

I hope that explains it  Smile

wilco737



It it's not Boeing, I am not going.
User currently offlineLuke From United Kingdom, joined Apr 2005, 181 posts, RR: 0
Reply 6, posted (5 years 2 months 1 week 6 days 3 hours ago) and read 5287 times:



Quoting WILCO737 (Reply 5):
Packs OFF. The 737 is not automated that the packs would automatically swtich to off and the APU is not strong enough to supply air to the starter and to the packs. After starting one engine you can do a "single pack operation during engine start". Then you suplly air from the running engine to the aircondition pack and the APU air to start the other engine.

So as air from the APU is required to start the engines, how does the APU get started in the first place? As it is itself a (very small) jet engine, does it not also require air for start? And what role exactly does the air play in starting the engines?

Thanks,
Luke


User currently offlineWILCO737 From Greenland, joined Jun 2004, 8968 posts, RR: 76
Reply 7, posted (5 years 2 months 1 week 6 days 3 hours ago) and read 5285 times:
AIRLINERS.NET CREW
HEAD MODERATOR



Quoting Luke (Reply 6):
So as air from the APU is required to start the engines, how does the APU get started in the first place? As it is itself a (very small) jet engine, does it not also require air for start?

The APU is pretty much a small jet engine, but does not require external air to be started, electric and fuel is enough.

Quoting Luke (Reply 6):
And what role exactly does the air play in starting the engines?

Air will be blown into the engine until a certain RPM and then fuel will be ignited and then the engine can stabilize on its own and doesn't need external air anymore.

wilco737



It it's not Boeing, I am not going.
User currently offlineMir From United States of America, joined Jan 2004, 21529 posts, RR: 55
Reply 8, posted (5 years 2 months 1 week 6 days 3 hours ago) and read 5272 times:



Quoting Luke (Reply 6):
So as air from the APU is required to start the engines, how does the APU get started in the first place?

A battery and an electric starter.

Quoting Luke (Reply 6):
And what role exactly does the air play in starting the engines?

The air spins the N2 section of the turbine up to a certain speed to get some air moving through the engine core, then the igniters go on and fuel is added, and the engine lights off and accelerates to idle from there.

You don't really need air to start an engine - you just need something to turn the N2 section. And in smaller engines, like the ones found on some business jets, electrical power is enough. But the size of the motor that would be required to start an engine the size of those found on an airliner is impractical, and since you have the APU there anyway, you might as well just use its bleed air to do the job.

-Mir



7 billion, one nation, imagination...it's a beautiful day
User currently offlineEireRock From Ireland, joined Nov 2007, 301 posts, RR: 4
Reply 9, posted (5 years 2 months 1 week 6 days 1 hour ago) and read 5232 times:



Quoting Mir (Reply 8):
and since you have the APU there anyway, you might as well just use its bleed air to do the job.

On the 737NG you can start an engine off the battery if you have external air, may be the same on most boeings actually, not 100% sure on that.


User currently offlineWILCO737 From Greenland, joined Jun 2004, 8968 posts, RR: 76
Reply 10, posted (5 years 2 months 1 week 5 days 19 hours ago) and read 5139 times:
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HEAD MODERATOR

Quoting EireRock (Reply 9):

On the 737NG you can start an engine off the battery if you have external air, may be the same on most boeings actually, not 100% sure on that.

Yes you can, but the battery needs to be fully charged After that start the battery will be pretty empty. Some kind of an air source is mandatory for the 737.

wilco737

[Edited 2009-05-20 15:44:35]


It it's not Boeing, I am not going.
User currently offlineOsiris30 From Barbados, joined Sep 2006, 3192 posts, RR: 25
Reply 11, posted (5 years 2 months 1 week 5 days 17 hours ago) and read 5116 times:



Quoting Mir (Reply 8):
And in smaller engines, like the ones found on some business jets, electrical power is enough. But the size of the motor that would be required to start an engine the size of those found on an airliner is impractical, and since you have the APU there anyway, you might as well just use its bleed air to do the job.

* excluding the power plants on the 787, which AFAIK are electric start engines. I believe on the 787 the generators attached to the engine are also motors for start though.



I don't care what you think of my opinion. It's my opinion, so have a nice day :)
User currently offlineArmitageShanks From UK - England, joined Dec 2003, 3611 posts, RR: 15
Reply 12, posted (5 years 2 months 1 week 5 days 14 hours ago) and read 5075 times:



Quoting WILCO737 (Reply 5):
Packs off during take off. That means you do not need to take air from the engine which increases its performance and you maybe can carry more passengers, cargo etc out of an airport if you are performance limited.

How much more thrust would you have with packs off compared to packs on? Is it noticeable?


User currently offlineDescendVia From , joined Dec 1969, posts, RR:
Reply 13, posted (5 years 2 months 1 week 5 days 14 hours ago) and read 5071 times:



Quoting ArmitageShanks (Reply 12):
Is it noticeable?

Its noticeable to the people that didn't get bumped  Smile


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 14, posted (5 years 2 months 1 week 5 days 12 hours ago) and read 5043 times:



Quoting WILCO737 (Reply 7):

Air will be blown into the engine until a certain RPM and then fuel will be ignited and then the engine can stabilize on its own and doesn't need external air anymore.



Quoting Mir (Reply 8):
The air spins the N2 section of the turbine up to a certain speed to get some air moving through the engine core, then the igniters go on and fuel is added, and the engine lights off and accelerates to idle from there.

Just to clarify, on modern engines you don't actually blow the air directly into the core...the air drives a starter motor (a small pneumatic turbine) on the gearbox. This backdrives the gearbox shaft to spin the N2 spool (not sure if RR spins the N2 or N3). Unless it's a 787, in which case you just run the generators backwards.

Quoting EireRock (Reply 9):

On the 737NG you can start an engine off the battery if you have external air, may be the same on most boeings actually, not 100% sure on that.

Except the 787, yes, anything that will provide adequate pneumatic duct pressure and volume can be used to start the engine. The airplane doesn't really care where the air is coming from, it just needs air to run the starter.

Quoting WILCO737 (Reply 10):
Yes you can, but the battery needs to be fully charged After that start the battery will be pretty empty.

Not on an engine start...an engine start with pneumatic ground power isn't particularly hard on the battery. You may be thinking of APU start on battery...that's a tough one to do more than once or twice.

Tom.


User currently offlineWILCO737 From Greenland, joined Jun 2004, 8968 posts, RR: 76
Reply 15, posted (5 years 2 months 1 week 5 days 11 hours ago) and read 5031 times:
AIRLINERS.NET CREW
HEAD MODERATOR



Quoting ArmitageShanks (Reply 12):
How much more thrust would you have with packs off compared to packs on? Is it noticeable?

Not really no. It is just the little bit more you get, but not really noticeable.

Quoting Tdscanuck (Reply 14):
Just to clarify, on modern engines you don't actually blow the air directly into the core...the air drives a starter motor (a small pneumatic turbine) on the gearbox. This backdrives the gearbox shaft to spin the N2 spool (not sure if RR spins the N2 or N3). Unless it's a 787, in which case you just run the generators backwards.

Yeah, I know. Didn't want to go too deep into detail  Smile

Quoting Tdscanuck (Reply 14):
Not on an engine start...an engine start with pneumatic ground power isn't particularly hard on the battery. You may be thinking of APU start on battery...that's a tough one to do more than once or twice.

Two APU starts on the battery can be indeed tough. Our book says though that even an battery start for the engine the battery should have a certain amount of voltage.

wilco737



It it's not Boeing, I am not going.
User currently offlineJetstar From United States of America, joined May 2003, 1642 posts, RR: 10
Reply 16, posted (5 years 2 months 1 week 3 days 21 hours ago) and read 4884 times:
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Quoting WILCO737 (Reply 7):
The APU is pretty much a small jet engine, but does not require external air to be started, electric and fuel is enough.

I’m not nitpicking, but just to clarify this, an APU is not a jet engine, but a gas turbine engine and produces a negligible amount of thrust.

To make a gas turbine engine into a jet engine, an exhaust nozzle or tailpipe has to be added aft of the engine, the aft end of the tailpipe is smaller than the inlet end which forces the exhaust gas to accelerate out of the tail pipe creating the thrust needed for propulsion.

I do not know of any jet engine that used air directly into the engine to start, air pressure or in some early jet engines a shotgun type cartridge which used the expanding gases is directed into a small air turbine motor which turned the engine for starting.

As an A&P mechanic, we always used a pneumatic drill using air from an air compressor, instead of an electric drill motor for sheet metal work. These drills were very light and powerful and almost torque free and use the same principal as a pneumatic starter.

Almost all small biz jets up to, but not including jets like the large Gulfstreams and Challengers use electric starter/generators. These starter/generators are DC motors and product 24 volts DC for aircraft electrical power. The have one armature with 2 sets of brushes, one for starting and one for generating. I am sure out there are some different systems used, but starter/generators are the most common because they can do 2 different applications using 1 component.

JetStar


User currently offlineDL From France, joined Feb 2008, 18 posts, RR: 0
Reply 17, posted (5 years 2 months 1 week 3 days 9 hours ago) and read 4798 times:



Quoting ArmitageShanks (Reply 12):

The ECS packs impact on fuel burn consumption is about 2.5% so I imagine this is also the impact on the thrust.

Regards,

DL


User currently offlineMender From United Kingdom, joined Feb 2004, 240 posts, RR: 0
Reply 18, posted (5 years 2 months 1 week 3 days 5 hours ago) and read 4765 times:



Quoting Part147 (Reply 1):
3. I'm sorry but I gotta to say it - the 'left, both right' switch refers to the 'magnetos', you know, when doing a mag-check - making sure the rpm rpm doesn't drop more than a few hundred rpm

I've only scan read this thread and no-one seems to have clarified this.

Only piston engines use magnitos. This might be a figure of speech but it could be misleading. The engines have two separate ignition systems. If you always started using both systems you wouldn't detect a failure so it is normal to start with just one. If the engine doesn't light off using the selected system then you select the other one. The igniter plug will wear out just like a spark plug on a piston engine so it is normal to alternately use each ignition system to even out the wear.

Quoting WILCO737 (Reply 5):
They are used alteratly daily (at my airline).

in our airline they use system one on the outbound sector and system two on the inbound sector.

The reason you can select BOTH ignitors is for in-flight restart. You really don't want to find a faulty ignition system during in-flight re-start!


User currently offlineCptspeaking From United States of America, joined Apr 2006, 639 posts, RR: 1
Reply 19, posted (5 years 2 months 1 week 1 day 12 hours ago) and read 4601 times:



Quoting Mender (Reply 18):
Only piston engines use magnitos.

He's joking...

I'm nerdy enough I actually found it amusing too  biggrin 


I spent some time in the US training center in the B734 full-motion sims. The IP ran us through the engine start procedures and we did single-pack ops after one start.

Basically, procedure was to turn all pack switches off, including the isolation valve. Once we had one engine up and running, we'd establish electrical power, then put the bleed isolation valve switch to "on", then whichever side we had started, the pack to "auto". After starting the other engine, all switches to auto and you're good to go.

I believe US uses L ignition on odd days and R on even days, both for really cold starts or in-flight restart attempts. Continuous below 10' (I think...could be remembering wrong though...).

Jonathan



...and don't call me Shirley!!
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