Basic question on B777 Engine starting

[extricate]

Hi there,

Just want to know more.. B777 Engine starting, Engine to START and Fuel Control Switch to RUN... and the whole process is automatic.

My question is, what causes the N1 and N2 fan blades to start rotating? The starter motor?

Thanks for your time

[NSEU]

Depends on how many rotors the engine has, but the starter only rotates one spool (rotor) mechanically. The other rotors turn due to airflow through the engine (created by the first spool).

If you only have N1 and N2 spools, the starter rotates the N2 spool.

Rgds
NSEU

[tdracer]

And if it's a Trent, the starter rotates the N3 spool.
There is logic in the autostart that checks for rotation of the other spools when the high rotor reaches some limit- if rotation of the other rotor(s) is not detected by that point (it's a function of the engine type) it's assumed the rotor is locked and the start is aborted.
BTW, while autostart is basic on the 777, it can be turned off (flight deck switch) and manual start procedures used. On the later Boeing widebodies (787, 747-8) we got rid of the switch - it's autostart all the time.

[barit1]

If parked in a taiiwind, N1 may be slowly turning backwards. When the starter is engaged, core airflow will slow and stop N1, then start it turning the right way.

[FlightDetent]

I always wondered, but never checked, what are the N1 cockpit readings with significant counter-rotation? Now and then you see engines spinning backwards with what seems to be 3% ... (not t7 specific)

[BleedingAir]

I always wondered, but never checked, what are the N1 cockpit readings with significant counter-rotation? Now and then you see engines spinning backwards with what seems to be 3% ... (not t7 specific)

On the 73 we'll frequently see N1 around the 1.5-2% mark with a decent wind on start, which is in the vicinity of 70-100 RPM on the fan.

[extricate]

Thanks all

[Cough]

And if it's a Trent, the starter rotates the N3 spool.
There is logic in the autostart that checks for rotation of the other spools when the high rotor reaches some limit- if rotation of the other rotor(s) is not detected by that point (it's a function of the engine type) it's assumed the rotor is locked and the start is aborted.
BTW, while autostart is basic on the 777, it can be turned off (flight deck switch) and manual start procedures used. On the later Boeing widebodies (787, 747-8) we got rid of the switch - it's autostart all the time.
Unless its a Trent attached to a 787, where the VFSG turns the N2 spool...

[tdracer]

Cough, not to be anal, but the OP did specify the 777....
The Trent 1000 gearbox arrangement is a rather interesting arrangement, but that's food for another thread.

[single chime]

TD, I think you said you were about to retire. I hope you stick around pprune, you are one of the best contributors.

[Cough]

TD - Fair call! Love to hear your angle on the gearbox...

[tdracer]

TD, I think you said you were about to retire. I hope you stick around pprune, you are one of the best contributors.
Thanks for the kind words. Yes, I'm now retired (officially starts today - first of November), although my final work day was about 10 days ago.
As far as PPRuNe, I'm not going anywhere although I no longer have access to as much information as I did as an active engineer (although in most cases I couldn't share it anyway - see thread on the UPS 747-8F order where I knew we had something going with UPS but couldn't talk about it until it was announced...)

[tdracer]

Cough, without too much thread drift, on previous aircraft programs the Product Development people would come in with a wish list based on things they've been working on. Then those of us with real world experience evaluate their wish list to determine what's reasonable and practical, then throw out the rest. On the 787, the PD people and bean-counters were given free-reign and the rest were told to make it work - basically throwing out 50 years of lessons learned in the process. :ugh:
I've been in a few 787 post-EIS design reviews (:mad:), and a common theme was "What the :mad: were you thinking? We had a beautiful system on the 777 and you totally F-ed it up for no benefit!"
IMHO, the N2 gearbox falls into that category, although it could have been worse - at PDR they had a clutched drive between the N2 and N3 shafts for starting that would disengage after the engine was running. As I suspect you can imagine, the failure modes were horrid - fortunately it turned out to not be needed and deleted...

[Cough]

TD - Firstly I hope you enjoy your freedom from the grind! Most I have spoken to on the subject find they have less free time post retirement than they did before, but all had bigger smiles!

Anyhow!! I can't imagine how a clutched drive would prove reliable and I don't wish to dream of those failure modes... Fortunately I only operate said beast and don't have to fix it! Genuine question (If I may!) What was the motivation for switching from N3 to N2?

[single chime]

Congrats TD, I am sure it is well deserved. Enjoy YOUR time.

[tdracer]

Genuine question (If I may!) What was the motivation for switching from N3 to N2? I never worked the 787 and the Trent 1000 (although I did follow the GEnx-1B engine program fairly close since it was so similar to the GEnx-2B that I was responsible for on the 747-8). I'll always remember when we held CDR for my system on the 747/GEnx-2B, I got up and said that our plan had been that the GEnx-1B would be flying around, generating experience and data, before we got to this point on the -2B (the 787 program had slid so far that 787 first flight was still nearly a year away at the time :mad:). My counterpart from the GEnx-1B looked at me and simply said "Bad Plan" :ugh:

Anyway, I seem to recall they were concerned about pulling all that gearbox power (and those two big starter/generators) off the high pressure spool and thought it would be more efficient to pull all that power out of the intermediate spool.
Something the PD people seem to forget is in most cases the devil you know is easier to deal with than the devil you don't know...

[TURIN]

Thanks for that TD. I know next to nothing about the Trent 1000, but I'm intrigued as to how they pulled that off.

(spending the next few hours googling for a cutaway drawing)

[lomapaseo]

Basic starting theory is to get enough air moving to pressurize the burner area and atomized the fuel within the combustor.

and then the three S Spin Spark and Spray.

The air flow you want is that going through the turbines (not out the fan) as the core flow helps to overcome the drag in the compressors.

I suppose that if you put enough starting torque on any rotor you could generate enough positive torque in the turbine to drive the compressor in front of the burner high enough to pressurize the burner and lite it off