I have noticed a potential anomaly in the fuel system from various descriptions and diagrams on pprune that I have not seen mentioned.
I would presume that there is something that I am not taking into account and that this has been considered by the manufacturers etc.
A diagram of the fuel system is in post 1026[1] by SilsoeSid and RVDT in post 1065 commented that the aircraft cruises left side low and that fuel spills over the "fence" back to the main tank when the left side is low.
After serial no 250 there is no fence and this aircraft is after no. 250.
According to the diagram in post 1026 there are three 50mm ducts that replace the "fence", two on the LHS supply tank and one on the RHS supply tank. Each tank has a center duct and the LHS tank has an additional duct on the extreme LHS.
Assuming that the ducts are all at the same level AND that the "Split Wall" (diagram in post 1026) does not 'leak' between the tanks and that the diagram is accurate there might be a problem here.
In order to achieve the goal of say 1m 30s between "fuel exhausted" engine flameouts it is imperative that if fuel is lost from the supply tanks due to aircraft attitude then it MUST not be the case that more fuel is lost from the LHS tank than from the RHS tank.
As far as I can see the design illustrated in post 1026 differs from this ideal. In a LHS low situation more fuel will be lost from the LHS tank due to the indicated placement of the ducts. This will reduce the 1m 30s margin. Think of it this way, the banking aircraft will lower the extreme left duct more then the center duct in each tank. Unless there is something I am unaware of or not considering this will lower the fuel level in the LHS supply tank more than the RHS supply tank.
Could this result in a near simultaneous engine failure on supply tank exhaustion?
There is a second issue concerning leaning (over) and fuel gauging. Do the fuel gauges maintain accuracy when the aircraft leans in the cruise as described? I mention this since pilots seem to report that the supply tank gauges show a difference between the supply tanks at all times. Is this necessarily a true difference in the cruise attitude?
I may try later to work out an estimate of how much extra fuel would be lost from the LHS tank but I bet someone beats me to it:-)
[1]
http://www.pprune.org/rotorheads/528...ml#post8197673