Perhaps I could offer a couple of thoughts.
Military aircraft fuel systems are complex and are getting more so. In my view, this is probably linked to the need for LO airframes, where more fuel has to be packed inside the mould line inside of dangling off the outside in a drop tank or two. As a result, fuel has to be spread around a number of tanks scattered around the airframe. On top of that, the aircraft C of G has to be maintained to close limits (this has always an issue for single rotor helicopters) and even required to help with cooling the aircraft. Add in AAR capability plus redundancy features and you have some seriously complex plumbing.
Harrier had some fairly challenging fuel system architecture to maintain fore and aft C of G, as well as some definitely 1960s type indication and control systems. Plus the UK's insistence on playing around with the way the indication systems worked. That led to the loss of at least two T12s due to pumps being selected off and not switched back on.
The RN had some issues with fuel management on Sea King Mk5, caused (as I remember it) by a substantial aft shift in C of G. Again, some fairly critical pumps had to be switched on and off at various times.
My view only, but any fuel system that needs to be actively managed by a single pilot is an aircraft loss waiting to happen. With two (or more) aircrew, the risk is reduced, but certainly not avoided. Similarly with fuel jettison systems that can be switched on and not always switched off. This is not always a recognised risk. It needs to be.
When the UK fitted RTM322s to Apache, I attended a CDR where we were told that the (really quite good) automatic fuel management system on the aircraft would have to be 'overridden' and 'actively managed' to cope with the much heavier engines aft of the C of G, as it would cost too much to change to software handling the management system. This was agreed to there and then by the then PM. Not a few of us in the audience disagreed. I'd be interested to know whether this caused any issues in service.
The F-35 has a complex fuel system, doing lots of stuff around the airframe. The head of the team designing it was Brit, very experienced and able to use all the lessons learned on Typhoon. He was very well regarded at Fort Worth, but he had to handle literally hundreds of charges to the system. His view was that the fuel system is frequently called upon to help solve problems all over the airframe. That can lead to complex results.
Best wishes as ever to all those poring over fuel system diagrams,
Engines
Last edited by Engines; 4th Jan 2017 at 08:53.