Originally Posted by
Ixixly
Andrewr, I believe what he is referring to is that your FFR is meant to cover a few things such as Accuracy, Potential Fuel Contamination (Water), Mis-Reading (Not the same as accuracy) and a host of other potentials. If we did not have this then anytime someone slightly misreads their gauge, splashes it up the dipstick a bit high, has a bubble in their tank, puts the dipstick in on a bit of an angle by accident etc...etc... you're opening yourself up to getting overhead thinking you have 5-10mins spare (enough time to land) but actually not, or worse yet getting to that situation 5-10mins before arriving rather than in the circuit. I was always taught FFR is meant to be a final guard against a variety of potential mistakes/issues that could be easily made/missed. In other words you think you have that 30mins in the tank but you've should never assume you actually do.
This seems unlikely to me. More likely I would say is that for e.g. a 2 hour flight you must be sure you have 2 hours fuel, plus be sure you have 30 minutes reserve. If gauge inaccuracy, splashing or angle of the dipstick etc. mean you can't be sure, add more fuel until you are sure.
Your description sounds more like unusable fuel to me. Unusable fuel can often be used, but you can't be sure due to variations in aircraft attitude, turbulence etc that can affect feed. So it's fuel in the tank but you can't assume you can actually use it.
My understanding of FFR is that it is a final contingency for external events, e.g. the aircraft in front of you is disabled on the runway. It gives you some time to make decisions. Events that would trigger it's use are themselves likely to be considered emergencies. If you can't be sure the fuel is really there, you are back to square 1 where you have to get on the ground ASAP.