I have been flying the 737 NGX for a few months and have picked up a lot of information from various YouTube videos, is there a “correct" sequence for the overhead panel set up when going from ground power to APU and then from APU to engine start.

 

The problem I have hit is sometimes – after I have started the APU – I disconnect ground power and find that I have no power coming from the APU, even though before I disconnect I check the amps etc in the overhead panel (usually shows 400 coming from the APU. If fiddle about with the recirc fans and the Hyd pups I can get it going so I am doing something wrong. Also occasionally I have an issue with the engine start, I wait for about 20 in the MFU panel but when I lift the cut-off switch it does not seem to start (obviously I have the overhead panel set to GRND and the ignition switch in the correct position).

 

Does anyone have a step by step document that will tell me what order to do each thing?

 

I realise that this is a bit of a noobie question but I am hopeful that someone can give me the correct sequence

 

John

Hi John,

 

Once APU is up and running you need to connect both APU buzz's before dropping grnd power. If you look at fwd overhead when APU starts you will see 2 blue lights come on when APU is stabilized. The 2 switches by them are to tx APU power to the A/C. Then you can disconnect gnd power.

 

As regards eng start. Make sure APU is on. APU bleed is open, both Packs are off and crossbleed is on. This should give you the required pneumatics to turn the eng. Put eng start switch in gnd position. Wait for 25% N2 then put eng cutoff switch to run.

 

Disclaimer. All the above is from the top of my head so may not be 100% accurate.

 

Cheers

 

Neil

The 400 you are reading is the frequency of the AC power (~400 Hz in this case, and 115 VAC). What you are looking for regarding AC load is below those two numbers.

The APU and external power can never be connected to the system at the same time, system logic ensures that connecting one trips the other. So even if the APU is running it is not supplying power while external power is connected. So if you disconnect external power you have no AC power source, which is where your troubles start..

 

Once the APU is stabilised and the APU GEN OFF BUS light illuminates you can close the APU generator switches and this will automatically trip the external power contactor. No need to manually trip it unless you are powering the aircraft down.

Believe this was covered in Tutorial 2...

Hey John,

after the APU successfully running, you can turn on the generators. once you're working with the APU, you're free to disconnect the ground power. then when you successfully did engine start, turn the engine generators on, and you're free to turn of the APU. hope it helped, cheers.

Also as a tip after arriving at the gate you can connect the ground power right away and shut down the engine thus skipping the APU and going straight to GPU, my airline does this to avoid APU usage on arrival.

Believe this was covered in Tutorial 2...

 

Came in here to say the exact same thing (surprise!).

Firstly I would like to say thank you for all the replies, yes I forgot that this was in Tutorial 2, but still there has been some really useful input

 

Kevinh – your post was really helpful in getting me to understand why I was having trouble

Thanks guys I will be writing this down as a procedure to keep me on the right track

 

John

As regards eng start. Make sure APU is on. APU bleed is open, both Packs are off and crossbleed is on. This should give you the required pneumatics to turn the eng. Put eng start switch in gnd position. Wait for 25% N2 then put eng cutoff switch to run.

 

Not to nitpick...but you do NOT need to use the crossbleed for a normal APU engine start; the APU is capable of feeding both sides perfectly. You only need crossbleed when you start with ground air since the ground air doesn't feed both sides and you would have to use bleed air from No2 engine to start No1.

 

Please someone correct me if I'm wrong.

Not to nitpick...but you do NOT need to use the crossbleed for a normal APU engine start; the APU is capable of feeding both sides perfectly. You only need crossbleed when you start with ground air since the ground air doesn't feed both sides and you would have to use bleed air from No2 engine to start No1.

 

Please someone correct me if I'm wrong.

The crossbleed valve must be open to allow the APU to supply pressure to engine 2.

The Isolation valve does need to be open to start engine number two from the APU. The bleed air valves do not need to be on to start the engines though.

Kevinh – your post was really helpful in getting me to understand why I was having trouble

Always glad to be able to help.

Hello again,

 

My apologies I meant the isolation valve. Not the crossbleed valve. Was writing from memory hence error.

 

Cheers

 

Neil

Hello again,

 

My apologies I meant the isolation valve. Not the crossbleed valve. Was writing from memory hence error.

 

Cheers

 

Neil

The isolation valve is the crossbleed valve I referred to. Correct terminology is isolation valve, I tend to think of it as opening the crossbleed duct which is why I call it a crossbleed valve. Whatever you call it, it must be open.

 

FCOM vol 2 pages 2.20.3 and 2.20.4 illustrates the layout.