During engine starting, a pack switch must either be in the auto position or in the high position to get 40psi for the start process. If both packs are off,the duct will only reach about 20psi . My question is, why must a pack be ' on-line' to reach 40psi ? Thanks ahead----

They should be off. Turn them off, then try starting one of the engines. Watch what happens to the pressure. The APU only provides bleed air as needed, so if there's no demand, it keeps the pressure to a minimum.

It is my understanding, that if pack is off, automation will lower the pressure in the ducts (so you see a decrease in indicated pressure) but as soon as you move the start switch, airflow will get higher so that demand from starter is met.What this translates to, is that you do not need to observe specific pressure with packs off for engine start. (40psi)

Correct. This is different than on the CL. Compare the NPs - you won't find any check for start pressure on the NG, unlike the CL, because there is none. As soon as the starter demands pressure, it will be provided. But as long as the packs are off, it's simply not there.

Thanks for confirmation Etienne.

off

all bleeds on, isolation valve auto, packs off

why are they switched off during engine start? APU cannot provide enough air pressure for both engine and PACKs? What is the difference between PACKs and bleed air?

why are they switched off during engine start? APU cannot provide enough air pressure for both engine and PACKs? What is the difference between PACKs and bleed air?

The APU cannot provide enough bleed air pressure to feed both the engine starter and the packs. Even if it could, this would probably also mean a too high duct pressure, but not sure about this. Hence, packs are turned off during engine start.Bleed air is the compressed air that is taken from the section after the compressor stages of a gas turbine (i. e. the APU in that case).Packs are the air conditioning units which use bleed air as input and mix it with ram air via heat exchangers to provide the necessary air temperature of the compressed air which is fed to the pressurized compartments in the aircraft.

thanks Dave!but some aircraft with 3 packs can have 1 running while starting up, Ive seen it in a 747 tutorial

747 is another machine...Te APU are not the same.Bleed air taken from te APU is insufficient to supply air to both the systems, the risks are:Low engine N2 spool up, maybe you will not reach the requirements for fuel ignition, and then, starter will not help the N2 in the initial phases of the starting (until 55% of N2), so with the risk of hung or hot starts that may damage the engine and create risks of fire.For the APU there are also problems as you are removing a great amount of air from its compressor, air that normally must go to the combustor chamber and then to the turbine. This means that APU must increase fuel flow and temp to mantain speed, and this stresses te APU itself.There are some protections against those problems, but is good practice to not reach limits to increase aircraft life.All "small" airplanes I seen needs the packs to be off for engine start.

thanks Dave!but some aircraft with 3 packs can have 1 running while starting up, Ive seen it in a 747 tutorial

As the previous poster said: a 747 is not a 737. Comparison between these two are mostly moot point. Each aircraft has its own, distinct characteristics and respective procedures. That's why you need a separate type rating for each of the two.

As was said, the APU bleed air system on the NG is demand based, it will never be at a constantly high value.

What is the difference between PACKs and bleed air?

PACKS are the systems that provides warm/cold air por passengers and airflow for pressurize the aircraft, they uses hot air coming from pneumatic ducts.Bleeds are holes in the engine/apu compressor stages that are used to provide pressurized air (normally a bit hot) and are connected thrugh valves and ducts to the pneumatic system.The valves open, close, modulate to mantain a target pressure when the system is able to mantain it. On the engine the system is a bit more complex, but the description above is sufficient.Bleed air is used for some reason like Anti ice, air conditioning and pressurization (packs), hyd and water tank pressurization, engine starting.

I'd guess people get the 40psi from the APU MEL and transfer it.You can use a PACK during the second start if you CLOSE the ISO VALVE and bleed the #2 ENG. Just a technique if you're hot or cold.