B747 Classic Ignition switches?

[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.

[Spanner Turner]

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!!

[vapilot2004]

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?

I am not aware of this - I do know that the GND start switch is indeed hardwired to the bleed and start valve - no logic intervention here.

[Flight Detent]

Guys,

Isn't it great to hear about the 'hard wired' and 'no electrical input at all' and 'no logic intervention here', aaah, just the way I always liked to fly!

Pity about these 'modern' airplanes, eh!

Cheers, FD

[Spitty42]

Hey SMOC. Whoever told you this little known fact is spot on! Whilst ST has explained the ramifications from a starters point of view, there is one other function that happens when the switches go to Ground Start. On the RR engines the FMV in the FFR is cycled to the low speed stop everytime start is selected. It is only a momentary cycle, however is enough to run an operating engine down to idle for a brief moment. Scary if all 4 are done at once, such as entering icing conditions.

[SMOC]

Hey Spitty42,

Cheers !! It was via a mate via QF which makes sence due to the Rollers.

Seeing I'm using Rollers, it would be nice to know, for the exact reasons you gave!

Thanks again.

SMOC.

[TRAVENGINEER]

Spitty 42 I think you mean the VMO not the FMV and how can this be when there is only one elec plug for the idle reset solenoid valve (GND idle / FLT idle ) air/gnd controlled and TE flap position on the FFR ? Explain your reason for this to happen.

[Spitty42]

Ah the beauty of forums such as these! Spot on Travengineer. I'm too long on the 400's. I was referring to the Engine Limit Control trim valve on the EDFP, not the FFR. My mistake. Now I've actually had to look it up, I better give you the details. Full trim applied at altitude could in fact flame the engine out (assuming a failure in the STA circuit). To prevent this, there is a bypass around the trim valve that always allows enough fuel to maintain at least idle power. Each selection of "Gnd Start" on the ignition switches exercises the actuator within the fuel pump to the full bypass position. This function was introduced via a SB well after EIS of the engine. Experience showed that lack of use of the actuator meant when the functionality was required for engine limiting, often the actuator would fail to operate. Hence the mod.
Always good to have these discussions, helps blow out the cobwebs!

[TRAVENGINEER]

SMOC Here is a description + SPITTY42 Well done. During any Engine Start, when the Start Switch is selected to the
Ground Start position, the Control Amplifier is enabled, and runs the
Variable Restrictor to the full trim position, and then back to Normal,
to exercise the variable restrictor located in the HP fuel pump.
If the variable restrictor remains in the full trim position (closed),
Engine start will be normal, however the engine thrust output as the
thrust lever is moved up to the Take Off power range, will be limited
to 1.30 EPR. This is caused by the fuel flow increase being restricted
by the trim valve being modulated into its extended position as the
fuel flow increases. This results in Throttle Stagger which is an
apparent shortfall in the power output of the engine for a particular
throttle lever position.

When operating the eng limit control sw to test this also functions the variable restrictor actuator to reduce fuel flow to the ffr to simulate fuel trimming if you have an N2 approaching overspeed.

[Willit Run]

Having an engine go to idle would be a good thing at that time,(except for a critical phase of flight) so the starter had something to drive, hence provide some drag on the starter and maybe prevent a starter from blowing up.
If you have seen the damage a starter can do after coming apart to an engine and accessories and cowlings, its quite remarkable.

The only reason you would go to ground start in the air, is the need for an inflight start and you were below 250 knots.(or so)
And, you have to physically hold the switch in the grd. start position as it is not a lever lock position.
Some folks just like to play with too many things!