Question about APU start up

[Vespa]

A quick question about APU start: in the Mike Ray's book I read ( page 119 ) 

- Step 1 Battery switch ON

- Step 2 APU bleed Switch OFF

- Step 3 AFT No.1 Fuel pump ON ( check LOW PRESSURE lights go OFF)

- Step 4 APU Control Switch START

 

Jan, in his video tutorial about cold and dark, after battery switched ON, jumps to the step 4, APU control switch START.

I'm wondering what's the real procedure followed in real flight; I know that fuel pump it's not really needed to start APU/engines due the engine onboard fuel pump ( I guess ) but I'm curious to know what's the correct procedure and why.

Thanks a lot for any feedback

[Alpha Floor]

Well, the "real procedure" is the "Electrical Power Up" procedure, which is to be found in the Supplementary Procedures chapter of the FCOM V1. Pilots don't usually do this procedure from memory, they rather follow it because there's a series of things they have to ensure and going through the list is safer.

I imagine that APU bleed is left off to let the APU start-up without a "load", so to speak. Depending on how "smart" the aircraft is, even if you leave the cockpit switch in the "on" position, the system will wait until the APU has started and is online.

The fuel pump is turned on the "help" feeding the fuel to the APU. However, as you well say, even if the pump is left off the APU pump will suction the fuel from the tank just as the engines would. The fuel pump ON is therefore something "optional".

Interestingly enough, the Electrical Power Up doesn't mention either APU bleed nor fuel pump, so...

 

[Vespa]

Thanks for feedback Alpha Floor.

I'm not able to find FCOM V1 Supplementary Procedures  ...is this into the Mike Ray's book?

[Alpha Floor]

21 minutes ago, Vespa said:

I'm not able to find FCOM V1 Supplementary Procedures

A quick search on Google and you'll find it!

21 minutes ago, Vespa said:

is this into the Mike Ray's book?

I don't think so. Mike Ray's book is a supplement upon the Boeing official manuals. I don't own his 737 one, but at least on his 757/767 guide it's like that.

[stefans85]

The APU has it's own DC fuel pump which get's in a sleep mode after you enable the AC pump. Normally no crew would jump into a C&D cockpit. The maintenance crew will power up the aircraft, make the first coffee and sometimes align the IRU for you. Then the aircraft is handled from one crew to the other in the powered state. The DC pump is pretty nasty to change on failure so you won't use it and use AC pump's instead to length it's livetime. You can also (currently not possible in IXEG) switch on right AC pump and open crossfeed valvue to balance fuel. Normally it is not neccecary as the scavange pump should pump fuel from center to left fuel bay after switching off center fuel pumps.

[stefans85]

On 23.5.2016 at 3:09 PM, stefans85 said:

The APU has it's own DC fuel pump which get's in a sleep mode after you enable the AC pump.

I have to correct myself. Only the later 500 (and all NG were I worked on) has had this DC pump. The 300 and 400 models does not have an DC pump and the AC pump is required to have enough fuel in the lane. But you won't connect a GPU when you're on an small airport. You'll start the APU right way and see.