RB211 Autostart (747-400)

[QAVION]

Hi, folks.
I'm seeking clarification on a line in my training notes with regards to the RB211 Autostart system:

"Fuel on conditions are met when:

N3 (is) 25% _or_ max motoring
EGT less than 100deg C
N2 greater than 8%"

(this applies to ground start).

In my manual, the normal max motoring speed of an RB211 is quoted as 30~34% N3. I can't see how the Autostart system can predict what the actual max motoring value will be, so will it always use the lower value of 25% under normal circumstances. Hopefully someone with a wealth of experience in the real world would be able to tell me at what point the fuel flow indications start ticking over (presumably this would happen very shortly after the fuel metering valves had opened(?)).

Under ABnormal circumstances, like low bleed duct pressure, would fuel be automatically applied if max motoring was less than 25%?

Thanks.
Rgds.
Q.

[VnV2178B]

Qavion,

In the case of motor assisted starts the control logic monitors the N3 acceleration and compares this with a threshold value. As soon as the acceleration (N3 dot) falls below this it is assumed that the engine is at N3 max motoring. This is done to account for low starter air pressure, so normally the 25% value will be reached first.

VnV...

[QAVION]

Excellent! Much appreciated, VnV.

Cheers.
Q.

P.S. Maybe that was too easy a question for you?... I'll try harder next time :D

[VnV2178B]

Yeah,

no problem, if you want values, the N3dot threshold is 0.275 %/sec and strictly the IP shaft (N2) has a variable threshold for fuel on between 7 and 10 %, depends on oil temperature and also a max. motoring restraint too, determined in the same way as N3.

As a aside, in an early autostart version the fuel was commanded on before the igniters which lead to the comment on the change note that 'the resultant ball of fire would be disconcerting to the passengers'! and that's why there are all these restraints on the fuel-on condition.

VnV...

[gas path]

Excellent description VnV......
You don't work for Lucas 'The Prince of Darkness' per chance?? :D :D :D

[VnV2178B]

AAAAAAAAAAAAAAAAAAGGGGGGGGGGGGGGGGHHHHHHHH !

Can you keep nothing secret ??

As I said on the FADEC thread the -400 doesn't have real FADECs but Full Authoruty Fuel Controls, the development contract team all got numbers... and my original username was FAFC1.
Now that dates me !

VnV...

[ 16 August 2001: Message edited by: VnV2178B ]