This is my understanding of how I think the BF110 fuel system in game should work:
BF110 FUEL SYSTEM
The BF110 has a total of 4 fuel tanks, 2 in each wing. The 2 forward tanks are FEED tanks and provide fuel to each respective engine. The Two rear tanks are TRANSFER or reserve tanks they cannot feed the engines directly (see comment on the fuel cock levers later*). The TRANSFER tanks are used to top up the FEED tanks as required.
The Fuel cocks are on the left hand side as part of the throttle quadrant. Each lever has 3 settings. In normal use they are in the " P1 u P2" position. This then causes each FEED tank to feed its respective engine. These levers also serve as Priming pumps before start by way of wobble action. More research needed specifically to understand the diff between P1 and P2 and P1 u P2 positions. These are related I believe to the position of the Fuel transfer levers on the right sill panel. (*This may also allow a direct Engine feed from a TRANSFER tank... yet to find a good reference on this. It would make sense from a combat damage control point of view)
Fuel distribution is controlled by the pilot using 2 levers mounted on the rear right Sill panel. These levers are rotated to point to the desired tank. The Front lever selects the required FEED tank. The Default or normal position is straight ahead. In this case each engine feed tank feeds its respective engine. (note this position is not presently selectable in the Sim)
The levers can be considered in a natural sense as "directional" type control. The front levers are where you want things to go and the rear levers where you want the transfer fuel to come from.
The Rear lever selects the desired TRANSFER tank and turns on its transfer pump. The centre position is the default or Normal position. In this case this results in each rear TRANSFER tank feeding its respective engine FEED tank automatically as fuel is used from the forward FEED tanks.
By using these levers the Pilot can transfer fuel from any RESERVE tank to any FEED tank.
The FEED tanks have a quantity of 375 litres each. The TRANSFER tanks have a quantity of 260 litres each.Each tank has a Lo level light set at 100 litres. Each tank quantity can be viewed by rotating the selector switch to the desired tank.
FUEL USAGE
For engine start the pilot will put each Fuel cock lever forward to the "P1 U P2" position. On the right sill panel he puts the Forward and rear transfer switches to the centre position. This allows each engine to be fed from its respective FEED tank and each TRANSFER tank to feed their respective feed tanks... keeping them topped up
In flight the pilot will simply use fuel from the 2 FEED tanks simultaneously. As soon as the Fuel level in each FEED tank drops the respective rear TRANSFER tanks will keep topping up the forward FEED tanks. I presume this is done by a Float valve arrangement in each FEED tank set at slightly below Full. The rear TRANSFER tanks will be the first to empty.When they get down to 100ltrs its respective 100 Low level light on the main panel will illuminate
Once the TRANSFER tanks are empty Engine feed is now from the remaining fuel in each FEED tank. When A FEED tank gets below 100litres its respective Low level or 100 Litre light will illuminate.
It is possible to "Cross transfer" fuel from any FEED tank directly to the other FEED tank by using the transfer switches. This might be required to minimise fuel loss following a leak etc. For example if the pilot wants to cross feed from the right FEED tank to the left FEED tank. He would have the Front lever pointing at the Front left FEED tank and the rear lever pointing at the front right FEED tank .. in a \/ arrangement.
For Fuel balancing he can also control directly which TRANSFER tank feeds a specific FEED tank. For example if the pilot wants to use the right TRANSFER tank ONLY to feed the right FEED tank he would select the forward lever to the right position.He would then rotate the rear lever to point at the "donor" right TRANSFER tank So the lever arrangement would be <
In this situation the Left FEED tank continues to feed the left engine directly. I "think" that in this condition the left TRANSFER tank is now isolated.
So by using the levers the pilot can directly control where the TRANSFER tanks transfer fuel to or isolate pretty much any tank.
When external drop tanks are carried there operation is automatic. External tanks are pressurised and feed directly into their respective FEED tank. Once a Drop tank is empty the respective FEED tank level will initially start to drop (until the normal transfer starts from the TRANSFER tanks). At this stage the pilot can jettison the now empty drop tank. There is no fuel quantity indication for the Drop tanks. Pilots would have a rough idea of end of drop tank time from the fuel consumption data. Drop tanks would always be used first.
References:
RAF PIlots notes "How to fly the BF110"
The BF110C,D,E An illustrated study by John Vascoe
The BF110C Vultee aircraft Corporation BF110C5 Exploitation document