Originally Posted by
SMOC
Besides the fact that the starter would crap itself.
Is it true, that if by mistake an ignition switch is placed to ground start, while inflight, the engine will go to idle, regardless of thrust lever position?
Was told this recently but can't find it in any manual.
Cheers.
You won't find it in a manual because it's not true.
Firstly, here's what happens if you have a starter failure, a starter shut-off valve failure ( same as selecting "ground start" whilst engine is running)
Over Speed Failure
The turbine rotor wheel has a reduced cross section (fuse) below the
turbine blade rim to effect separation of the outer rim and blades in
the event of bearing malfunction or overspeed.
Two cutters are positioned so as to cut into the fused area in the
event of a rotor wobble.
The containment housing is designed to reduce the impact of turbine
blade separation on the:
• Starter;
• Adaptor Housing; and
• Engine Gearbox.
Shut off Valve Failure
If the starter shut off valve fails to close the starter turbine will
continue to run in a no-load condition until the Turbine Nozzle is
choked, this removes the driving torque of the turbine releasing the
clutch pawls causing the turbine to overspeed which if allowed to
continue to operate under these conditions eventually causing a rotor
bearing to fail. If this occurs the turbine shaft will wobble making
the turbine disc contact the cutters causing disc rim and blade
separation.
Starter Clutch Failure
If the Starter Clutch fails to disengage the starter will be driven by
the engine permanently, up to the starter rotor burst speed of
approximately 95,000 RPM, even with the Starter shut off valve
closed, this will cause the lubricating oil in the starter to overheat
causing a bearing failure. The ’wasting’ of the turbine disc is designed to
separate below this RPM to allow separation as an assembly in order to
reduce overall damage, particularly if the engine is accelerated to
Take Off RPM with the starter still being driven by the engine.
As for the engine returning to idle when you select "Ground Start" - this is impossible. A "Classic" 747 has full manual throttles from the cockpit to the Fuel Control Unit on each engine. Just like an old car. Nil, none, no electrical input at all to power setting of engine other than the movement/position of the throttle. (save for a small solenoid to give a slight fuel flow increase to enable "Flight Idle" when flaps are out) This would be just like driving your car down the Motorway with your foot flat on the accelerator and turning your ignition key......... sure the starter will try to engage but the engine power setting won't change.
Don't forget that the "Ground Start" position of the Ignition switch is a 'Momentary' position so you'd have to be fairly determined to hold it there for a long time to blow the starter up - Flight Engineers are a little smarter than that!!