With regard to the increase in distances required for the CAA's "10% in crease in weight e.g. an extra passenger".........the intention was never to suggest that the take-off weight ever could be above MTOW...........that factor was designed to cater for aircraft such as the C172 which published performance figures for weights below MTOW and the CAA were trying to provide a simple factor to use if a last minute passenger arrives who will put the take-off weight above the planned one but not above the MTOW.
Many "light aircraft" such as the pipers have graphs that permit allowances for headwind, down slope, better than ISA, lower than max weight etc.
However, if you are taking the professional side of aviation as a model of safety, remember that we have to demonstrate that we can operate from the longest runway at an aerodrome in zero wind and from any other runway with an allowance for any headwind.................since most light aircraft operate from airfields where there is often only a single strip, then it must be capable of using that strip in zero wind.
When using wind for performance, only 50 percent of a reported headwind can be used........again many light aircraft flow privately give up when the wind is above 20Kt thus the best ever wind to be used in the calculations would be a 10Kt headwind.
How many small airfields used by light aircraft have accurate wind reporting, calibrated pressure reporting and temperature reporting and have had a survey showing exactly what the slopes are? The fields with all those tend to be plenty long for light aircraft!
The CAA factors ensure that take-off, never requires more than 75 percent of the runway available and landing never more than 70 percent........which may leave observers thinking that "hey, I can get away with a lot shorter than this".........yes one can but the safety margins are reduced.
I know some aircraft that I can land in under the book distance most days and match it on the rest..........but I know plenty of other pilots who fly the same aircraft and require more than the book distance because of lack of practice or the old adding on a few knots problem.
Any pilot who comes to me and says that he can match the book figures gets the chance to demonstrate their ability and few ever manage to repeat the performance consistently during a few circuits at a couple of aerodromes over an hour or so.
Just some thoughts.
Regards,
DFC