Let’s look at the most important valves in the fuel
system, the Spar Fuel Valve and the Engine Fuel Valve a bit further than needed
but still at an acceptable level. It will clarify what actually happens
specifically with the Engine Valve. By all means just remember the easy way as
the FCOM explains.
The #1 most important fuel valve is the Spar Fuel Valve.
This 28 VDC valve is mounted in the front wall “spar” of the main fuel tank
supplying fuel to the fuel feed line of the engine.
The DC power comes from the Hot Battery Bus and the valve even has an own
recharging Battery Power Pack to be able to positively close the valve in case
of an emergency such as a separated engine. The valve opens when the Start
Lever is placed in the IDLE position and closes by CUTOFF of that Start Lever,
or by pulling its Fire Switch. When the valve is closed it shows a dim blue
light even with the Start Lever in CUTOFF as I always explain that any blue
light is a “not standard flight condition light”, knowing that the book says
it’s a status light.
The Engine Fuel Valve is actually the High Pressure Shut
Off Valve (HPSOV) and is integral with the Hydro Mechanical Unit (HMU) on the
accessory gearbox. The valve opens and closes by the same controls as the Spar
Fuel Valve but its actual opening is a bit more complicated. It relies on the
so called Fuel Metering Valve (FMV) which is under control of the EEC. So . .
when conditions meet the requirements to open the HPSOV, the EEC signals the
FMV to open up the HPSOV by servo fuel pressure.
On
the other hand the closing of the HPSOV is achieved by the Start Lever or Fire
Switch, the EEC energizes the CLOSED SOLONOID of the HPSOV which uses 28VDC
from the Battery Bus. During engine start this FMV is controlled by the EEC and
when conditions dictate the HPSOV (Engine Fuel Valve) to close, the EEC
commands the FMV and thereby the HPSOV to close in the following conditions:
• A Hot Start occurs (>725°C) on the ground (exceedance protection)
• If the engine decays after idle speed during start below 50% N2 speed and EGT exceeds the start limit
• The EEC senses a “wet start” meaning no EGT rise within 15 seconds after the Start Lever is at Idle (YOU are the start limit for the EGT rise which is 10 seconds!!!)
All
of these conditions will be indicated by a bright ENG VALVE CLOSED light.
Note
that with an updated EEC software (7.B.Q and later) the EEC also provides a
protection when approaching a Hot Start meaning a rapid increase in EGT.
