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Why do we have to PACKS OFF during Engine Start?

Posted: Wed Nov 13, 2019 3:02 pm
by icejuice25
Hello.

Simple Question.

Why do we have to PACKS OFF during the

Engine Start?

Re: Why do we have to PACKS OFF during Engine Start?

Posted: Wed Nov 13, 2019 4:16 pm
by n901wa
The Eng uses Bleed Air to turn the starter. If you run the packs ( They run on Bleed Air too) You don't have enough air pressure to get the starter to turn fast enough to get the eng turning fast enough to add fuel to the engine, so it can start. Not sure on the 787 since that uses Electric. Hope that helps

Re: Why do we have to PACKS OFF during Engine Start?

Posted: Wed Nov 13, 2019 4:22 pm
by unimproved
n901wa wrote:
The Eng uses Bleed Air to turn the starter. If you run the packs ( They run on Bleed Air too) You don't have enough air pressure to get the starter to turn fast enough to get the eng turning fast enough to add fuel to the engine, so it can start. Not sure on the 787 since that uses Electric. Hope that helps

The 787 runs both on electric so you won't have enough current from just the APU.

Re: Why do we have to PACKS OFF during Engine Start?

Posted: Thu Nov 14, 2019 4:23 am
by Max Q
I believe some aircraft, perhaps the 747 can leave one pack on during engine start and we never tried it but
theoretically we could do the same on the 727 by re configuring the #2 engine bleeds

Re: Why do we have to PACKS OFF during Engine Start?

Posted: Thu Nov 14, 2019 4:57 am
by RetiredWeasel
Max Q wrote:
I believe some aircraft, perhaps the 747 can leave one pack on during engine start and we never tried it but
theoretically we could do the same on the 727 by re configuring the #2 engine bleeds


Yes you could, but not for the first engine started at least according to our ops. I posted before, after getting one started, you could isolate the pneumatic duct on that side so the APU supplied air only to the opposite side to start it's engines . The one engine running would then provide bleed air to run the one pack on its side of the ducting. The one engine and one pack provided a little relief while starting in a very hot environment. It got some air moving at least. That technique was used infrequently.

Re: Why do we have to PACKS OFF during Engine Start?

Posted: Thu Nov 14, 2019 5:04 am
by zeke
You don’t touch the packs when starting an Airbus normally.

Re: Why do we have to PACKS OFF during Engine Start?

Posted: Thu Nov 14, 2019 7:03 am
by fr8mech
zeke wrote:
You don’t touch the packs when starting an Airbus normally.


Yes, but on the Airbus I'm familiar with, A300, the Packs automatically shut down when you're starting engines. I'm pretty sure our new Jumbos do the same.

Re: Why do we have to PACKS OFF during Engine Start?

Posted: Thu Nov 14, 2019 7:59 am
by Max Q
RetiredWeasel wrote:
Max Q wrote:
I believe some aircraft, perhaps the 747 can leave one pack on during engine start and we never tried it but
theoretically we could do the same on the 727 by re configuring the #2 engine bleeds


Yes you could, but not for the first engine started at least according to our ops. I posted before, after getting one started, you could isolate the pneumatic duct on that side so the APU supplied air only to the opposite side to start it's engines . The one engine running would then provide bleed air to run the one pack on its side of the ducting. The one engine and one pack provided a little relief while starting in a very hot environment. It got some air moving at least. That technique was used infrequently.



Thanks for that RW,

Interesting, I though I’d heard that before. Theoretically you could do the same on the 727 after starting one


It wasn’t approved so I never tried it

Re: Why do we have to PACKS OFF during Engine Start?

Posted: Thu Nov 14, 2019 10:21 am
by unimproved
Max Q wrote:
RetiredWeasel wrote:
Max Q wrote:
I believe some aircraft, perhaps the 747 can leave one pack on during engine start and we never tried it but
theoretically we could do the same on the 727 by re configuring the #2 engine bleeds


Yes you could, but not for the first engine started at least according to our ops. I posted before, after getting one started, you could isolate the pneumatic duct on that side so the APU supplied air only to the opposite side to start it's engines . The one engine running would then provide bleed air to run the one pack on its side of the ducting. The one engine and one pack provided a little relief while starting in a very hot environment. It got some air moving at least. That technique was used infrequently.



Thanks for that RW,

Interesting, I though I’d heard that before. Theoretically you could do the same on the 727 after starting one


It wasn’t approved so I never tried it

You can do the same on any aircraft with more than 1 engine if the bleed valves can be controlled manually. Crossbleed start with packs on APU.

Re: Why do we have to PACKS OFF during Engine Start?

Posted: Thu Nov 14, 2019 11:23 am
by GalaxyFlyer
I hope the OP isn’t a pilot of airliners. Appalling lack of mechanical sense, if so.

Re: Why do we have to PACKS OFF during Engine Start?

Posted: Thu Nov 14, 2019 4:27 pm
by Tristarsteve
The Tristar had a PT6 engine for its APU. It produced bleed air all the time, but if there was no demand, or the APU bleed valve was closed, there was a surge valve on the APU that opened and blew all the excess air very noisily overboard. So we tried to not open the surge valve by selecting the relevant pack on as the the start valve closed on the engine. With two cross bleed valves and everything manually controlled on the FE panel it was possible.

Re: Why do we have to PACKS OFF during Engine Start?

Posted: Thu Nov 14, 2019 6:23 pm
by unimproved
GalaxyFlyer wrote:
I hope the OP isn’t a pilot of airliners. Appalling lack of mechanical sense, if so.


The ones that have just a basic understanding of how things work are more dangerous IMO.

Re: Why do we have to PACKS OFF during Engine Start?

Posted: Thu Nov 14, 2019 6:57 pm
by ual777
On the 737, the PACK #2 is usually on when you start engine #1. You crank engine #2 first, then "isolate and ventilate". Close the isolation valve and run PACK #2 off of the engine bleed and use the APU to start engine #1.

Re: Why do we have to PACKS OFF during Engine Start?

Posted: Fri Nov 15, 2019 4:28 am
by speedbird52
GalaxyFlyer wrote:
I hope the OP isn’t a pilot of airliners. Appalling lack of mechanical sense, if so.

I clicked on this thread just to post this. The reason why is literally in the FCOM. If you can't be assed to read that (and somehow pass all your tests...) this is something that can be answered with 5 seconds of Googling. The only airliners I have ever flown are virtual airliners, and even I managed to learn why just by reading the manual.

Re: Why do we have to PACKS OFF during Engine Start?

Posted: Fri Nov 15, 2019 9:09 am
by Loran
Tristarsteve wrote:
The Tristar had a PT6 engine for its APU. It produced bleed air all the time, but if there was no demand, or the APU bleed valve was closed, there was a surge valve on the APU that opened and blew all the excess air very noisily overboard. So we tried to not open the surge valve by selecting the relevant pack on as the the start valve closed on the engine. With two cross bleed valves and everything manually controlled on the FE panel it was possible.

I've experienced this live in 2013 when flying on a Tristar 500 ex-NBO. Significant issues in starting the engines off the APU plus the GPU combined with the altitude. Eventually #3 started and provided sufficient cross-feed pneumatics to start #1 and #2. I've documented it on video:

https://youtu.be/IP05DjM0vFk

Regards,
Loran

Re: Why do we have to PACKS OFF during Engine Start?

Posted: Fri Nov 15, 2019 1:05 pm
by Nicoeddf
speedbird52 wrote:
GalaxyFlyer wrote:
I hope the OP isn’t a pilot of airliners. Appalling lack of mechanical sense, if so.

I clicked on this thread just to post this. The reason why is literally in the FCOM. If you can't be assed to read that (and somehow pass all your tests...) this is something that can be answered with 5 seconds of Googling. The only airliners I have ever flown are virtual airliners, and even I managed to learn why just by reading the manual.


It shouldn't be necessary to read any manual to understand the connection, if you are a pilot.
If you are not, perfectly reasonable question.

Re: Why do we have to PACKS OFF during Engine Start?

Posted: Sat Nov 16, 2019 12:44 am
by strfyr51
n901wa wrote:
The Eng uses Bleed Air to turn the starter. If you run the packs ( They run on Bleed Air too) You don't have enough air pressure to get the starter to turn fast enough to get the eng turning fast enough to add fuel to the engine, so it can start. Not sure on the 787 since that uses Electric. Hope that helps

the B787 started is also electric and it's just a precaution just like no airline ever started all 4 engines on the 747-400 at once even though it could do it,, But? At What Cost?

Re: Why do we have to PACKS OFF during Engine Start?

Posted: Sat Nov 16, 2019 1:54 am
by BravoOne
Some stations had such strong air that you could in fact start all 4 at the same time. (707)

Re: Why do we have to PACKS OFF during Engine Start?

Posted: Sun Nov 17, 2019 2:21 pm
by GlobalMoose
It all comes down to how the aircraft was designed!

In my jet, we have two packs (left and right); during engine start, the on side pack automatically shuts down when an engine is started on that side of the aircraft (i.e. when we are starting number 1, the left pack automatically shuts down) and the other pack is left running to provide environmental air to the avionics and cabin. The silent shutdown occurs regardless of the type of start we are doing (APU air, engine cross bleed, or external huffer cart) and is designed to give the maximum amount of air to the engine while still providing acceptable airflow to the cabin. If we know we are working with a weak bleed source or a low-margin engine, we will normally manually shut down the opposing side pack to give the best possible chance for he engine to start on the first go.

Re: Why do we have to PACKS OFF during Engine Start?

Posted: Sun Nov 17, 2019 6:12 pm
by thepinkmachine
I once forgot to switch off the bleeds/packs before engine start on EMB145. The result was a hung start (or 2, I don’t remember).

We only could start it when we figured out the reason... :banghead:

Re: Why do we have to PACKS OFF during Engine Start?

Posted: Sun Nov 17, 2019 11:45 pm
by Starlionblue
thepinkmachine wrote:
I once forgot to switch off the bleeds/packs before engine start on EMB145. The result was a hung start (or 2, I don’t remember).

We only could start it when we figured out the reason... :banghead:


I think these things happen to all of us. In the sim, I once forgot to turn the APU bleed on before trying to start the engines. Realisation dawned when I heard badly muffled chuckling from the instructor after I'd stared blankly at the engine instruments for a bit.