Thanks, Leadsled.
Happy to bow to your experience.
My understanding was that some aircraft still posessing the old CASA manuals were the entire reason for the performance factoring in 20.7.4.
Will have to research the current standings a little further, obviously!
Although most a/c I fly these days have the manufacturer's manual, (and I do encourage these owners to get the manufacturer's document "approved", I still get a few AFRs in old Cherokees that still have the CASA document. Didn't realise they were illegal.
CAO 20.7.4
The 15% was a 1.15 factor applied to MTOW 2000kg or less, 1.25 for MTOW 3500 kg and interpolation on between (another 0.01 per 150 kg over 2000kg) for T/O distances.
Landing was 1.15 (2000kg MTOW) and 1.43 (4500kg MTOW) again with interpolation (approx 0.01 per kg over 2000kg).
Any of the CASA AFM charts (which were actually suprisingly easy to use compared to "add x% to ground roll for wet grass runways etc...) already had the relevant factor built in and stated in the conditions box.
Bear in mind that although the manufacturer's data may be with brand new aircraft, engine, prop etc., they are required to be derived (by the FAR's) with a pilot of "average skill". How advanced/average skill is determined, I don't know.
Re wrong again, the "approval" I referred to, as stated, was limited to the approval page to be added to the flight manual, not CASA verifying the manufacturer is correct in terms of this information. Under no illusion about that aspect.