Flying Clog,
BFL by definition is where V1 is chosen so that ASD = AGD
In order to understand the concept, you need to look at the effect that V1 has on each of these aspects.
1. Increase V1 means increased ASD(higher abort speed = more runway)
2. Increase in V1 decreased AGD(this is where lots of people have a problem....what it boils down to is that it is easier to continue the take off with a higher V1)
Go and draw the relationships as a graph with distance on the vertical scale and V1 on the horizontal scale.
When you put the 2 graphs together on one graph, the point where they intersect is your balanced field length.
Note that ANY variation of V1 results in a higher distance required to do the take off. If you increase the V1, the ASD increases and if you reduce V1 the AGD increases.
The BFL represents the minimum runway length that you need for a given weight(or more importantly the greatest weight you can carry off a given field length)
This brings us the the principle of an unbalanced field........that is where clearway is factored into the calculation. To contradict myself.....you can carry more weight off an unbalanced field(than a balanced filed), by reducing the V1. Unfortunately this calculation is very complex. You need to factor in Take off run(all engines), Take off run(OEI), Take off Distance(All engines), Take off distance(OEI), brake energy limits and Accelerate stop.
The main benefit of using BFL in flight planning boils down simplicity at a small weight penalty.
Ok. Just my bit. Feel free to ask questions.
Cheers
KTK