Originally Posted by
tdracer
The last aircraft I'm familiar with that used a cable to control fuel ON/OFF was the 737-100/200. Everything since then uses a hardwired electronic signal that actuates a solenoid (or on a few engines, motor driven actuator) right on the fuel control to control ON/OFF. Once enabled ON, then the FADEC can use the fuel metering valve to control the fuel, including taking it to zero fuel flow if needed.
No first hand knowledge of the A350, but I suspect Electronics Pro pretty much nailed it - Airbus isn't using the appropriate impedance for the flight deck ON/OFF signal - and as a result it can be compromised by liquid contamination. Sloppy design.
Thanks tdracer. You are right, a solenoid can't be activated with a liquid because the impedance is so low (typically few tens of ohms). But if a pull-up design is used the the pull-up resistor should supply at least 10mA. In a 5 volt logic the resistor value should be 500 ohms or even as low as 100 ohms.