The book say:
"FUEL CONTROL Switch
RUN (AUTOSTART ON) –
• Opens the spar fuel valve
• arms the engine fuel valve (the EEC opens the valve when required)
• arms the selected ignitors(s) (the EEC turns the ignitors on when
required).
RUN (AUTOSTART OFF) –
• opens the spar fuel valve
• opens the engine fuel valve
• turns ignitors on.
CUTOFF –
• closes the fuel valves
• removes ignitor power
• unlocks the engine fire switch."
B777 fuel control switch actually control the spar valve as well as the engine fuel valve. Thus, not required to pull the fire handle.Of course, fire handle will also do the job of the fuel control switch.
If the centre tank has fuel and either wing tank loses fuel profusely, it is likely to be a tank leak as engine fuel leak would deplete centre tank fuel instead of wing tank fuel.
Have a look at the fuel schematic, notice how near are the spar valves to the engines? Personally, spray observed downstream of the engine pylon can be treated as engine fuel leak as closing the spar valve (by fuel control switch or fire handle) would stop the leak. But tank or pipe line leak upstream of the spar valve have no way to plug it. Also, very hard to pin point the actual leak location.Thus, a land ASAP may be a best option if unable to make destination. Don't forget fuel gauge error can appear to be fuel leak too. Leak downstream of fuel metering unit can cause a reduction of thrust even with thrust level at full forward.Fuel flow would be normal. Shutting it down would stop the leak.
Had a actual wing tank crack some time back in a B737 on a 2hrs flight over water. Captured the leak rate for 30mins then projected the total lost.Proceeded to destination with 700kgs lesser than planned. Saw the ground crew jumping like frog-on-a-hot-wok while docking in as the right wing was dripping.