monitor N1 for 30 sec during engine start, why???

[elpilote]

is there anybody who can tell me why on B737 during engine start after having put engine start switches to GRN do we have to monitor it for 30 sec and what do we have to check, i cannot remember and i could not find the info in my OM?
thanks

[TyneQNH]

Firstly, you're looking to make sure that you have some N1 rotation before you raise the start lever to idle. This is to ensure that there is not some form of seizure.

After you raise the start lever you're looking to ensure a reasonable rate of increase in N1, and also N2, once you have an EGT indication to ensure that you don't have a hung start.

TyneQNH

[elpilote]

ok but what is the aim of the 30 sec before setting start lever to idle???

[BOAC]

You don't! The start lever goes on at a minimum of 20%N1, a max of 25N1% or max starter motoring rpm achieved. It so happens this is around 30 secs from starter engagement normally.

[captjns]

Right out of the Boeing 737-800 Flight Crew Operations Manual:

Position engine start lever to IDLE detent when:

• N1 rotation is observed and N2 RPM reaches 25% or (if 25% N2 is not achievable) at max motoring and a minimum of 20% N2.

Max motoring occurs when N2 acceleration is less than 1% in approximately 5 seconds.

At 56% N2 RPM, check ENGINE START switch moves to OFF.

Once the engine is stable we look for about:
20% N1
400 degrees on the EGT
About 60% N2