-8 packs off before start

[chapstick 2,757]

Am I correct in thinking that you need to manually turn off the 3 packs before engine start, just like in the -400? As opposed to the 777 where they come off/on automatically.

Alex Pugh

[downscc 1,327]

The packs do not turn off automatically for engine starts.  All three off may not be required, as long as your duct pressure is above 20 psi I normally start two with two packs off.  The Boeing checklist is a good place to look for things like this.

[PinkPony 171]

1 hour ago, Chapstick said:

Am I correct in thinking that you need to manually turn off the 3 packs before engine start, just like in the -400? As opposed to the 777 where they come off/on automatically.

Alex Pugh

Yes, you manually have to turn them off for start and back on. This is to maintain commonality with the -400.

Two major 8F operators have the following SOP:

1. All PACKS OFF for dual engine start.

2. One PACK ON for single engine start (crew discretion).

3. Above 5000' pressure altitude and/or OAT >35C, All PACKS OFF and single engine start only.

 

Edited September 23, 2018 by calzonister

[downscc 1,327]

2 hours ago, calzonister said:

1. All PACKS OFF for dual engine start.

Yeah, this got kicked around between us during beta.  This is definitely in the category of operator procedures.  The Boeing checklist for engine start has a foot note indicating that all packs off MAY be required to start two engines, and honestly that APU puts out enough air to do that and maintain 30 psi under most conditions.  What really threw me at first is that under normal conditions it is not unusual to see the engines spool up to about 29% N2 and slooolwy increase before the fuel starts flowing.  Robert told me that they can take up to 45 sec before ignition.  At first I thought that this was due to lack of sufficient bleed but it turns out to be intentional.

It's interesting to get outside in spot position and listen to them start. 

[Morvious 43]

It amazes me that Boeing didn't opt for the automatic pac system for the B747-8.

 

The first time I booted the B747-8 I started the engines one by one without turning the pacs off, because I thought they were like the B777. Then on a later test I couldn't figure why the dual start didn't work :)! After clicking the packs switches I figured it out.

[thibodba57 462]

5 minutes ago, Morvious said:

It amazes me that Boeing didn't opt for the automatic pac system for the B747-8.

 

The first time I booted the B747-8 I started the engines one by one without turning the pacs off, because I thought they were like the B777. Then on a later test I couldn't figure why the dual start didn't work :)! After clicking the packs switches I figured it out.

I'm sure if they thought it would be beneficial and not alter it's Type classification (B744) they would have added it.

[Morvious 43]

1 minute ago, thibodba57 said:

I'm sure if they thought it would be beneficial and not alter it's Type classification (B744) they would have added it.

It reduces the workload of the pilots in some way but you can argue if it is really that beneficial.

I am also sure there are legit reasons why they didn't include it in the B747-8 but from an outside point of view it is weird 🙂.

[PinkPony 171]

8 minutes ago, Morvious said:

It reduces the workload of the pilots in some way but you can argue if it is really that beneficial.

I am also sure there are legit reasons why they didn't include it in the B747-8 but from an outside point of view it is weird 🙂.

The reason given below is it. 

13 minutes ago, thibodba57 said:

I'm sure if they thought it would be beneficial and not alter it's Type classification (B744) they would have added it.

 

[PinkPony 171]

1 hour ago, downscc said:

Yeah, this got kicked around between us during beta.  This is definitely in the category of operator procedures.  The Boeing checklist for engine start has a foot note indicating that all packs off MAY be required to start two engines, and honestly that APU puts out enough air to do that and maintain 30 psi under most conditions.  What really threw me at first is that under normal conditions it is not unusual to see the engines spool up to about 29% N2 and slooolwy increase before the fuel starts flowing.  Robert told me that they can take up to 45 sec before ignition.  At first I thought that this was due to lack of sufficient bleed but it turns out to be intentional.

It's interesting to get outside in spot position and listen to them start. 

Absolutely!

Both the GE90 and GEnx“suffer” from bowed rotor. As do the latest generation GTF’s on the A32X B73M family. 

This is a condition caused by bowing/bending of the HP shaft after shutdown due to differential cooling of the rotors. 

On the GE90 the autostart system will keep motoring the engine in order to straighten the the shaft out, before fuel is introduced automatically. To prevent blade rubbing due to a bent shaft. 

However, on the GEnx -2B extended high speed motoring caused additional issues, so to keep the motoring speed down, the stator vanes run on a special schedule (opening/closing), to keep N2 rotation below a certain threshold.  The length of this motoring process depends on how long the engine was shut down. 

With constant increases in required efficency, the blade clearances are getting smaller and smaller. Hence the need to ensure the shafts are nice and straight before introducing the fuel!

Edited September 23, 2018 by calzonister

[hontzt 14]

2 hours ago, calzonister said:

Absolutely!

Both the GE90 and GEnx“suffer” from bowed rotor. As do the latest generation GTF’s on the A32X B73M family. 

This is a condition caused by bowing/bending of the HP shaft after shutdown due to differential cooling of the rotors. 

On the GE90 the autostart system will keep motoring the engine in order to straighten the the shaft out, before fuel is introduced automatically. To prevent blade rubbing due to a bent shaft. 

However, on the GEnx -2B extended high speed motoring caused additional issues, so to keep the motoring speed down, the stator vanes run on a special schedule (opening/closing), to keep N2 rotation below a certain threshold.  The length of this motoring process depends on how long the engine was shut down. 

With constant increases in required efficency, the blade clearances are getting smaller and smaller. Hence the need to ensure the shafts are nice and straight before introducing the fuel!

Glad to hear this is simulated!

Todd Hontz