During the ground component of a flight test, I was told by testing officer that 15% factor is added for each unknown component on the take off charts. E.g The scenario was: if wind info not available, surface is soaked long wet grass (and your take off chart has no window for surface), slope goes down but you don't now by how much, then "how would you account for this in your calculations?"

His response in the end was that 15% x 3 (for the 3 unknown factors of wind, surface, slope will be the factor amoutn. Not just one 15% as normal. 

I've looked everywhere for a legal reference. Is anyone aware of this? thanks

I've never heard of that one. In fact, from your description of a take-off chart with no allowance for wind or slope would make the chart quite useless. Perhaps John might like to comment on this one.

I was told by testing officer that 15% factor is added for each unknown component on the take off charts

Utter nonsense and no sensible engineering basis for such a view ..

"how would you account for this in your calculations?"

For a line pilot, you can't. You go look for some useful data.

Not just one 15% as normal.

What are we talking about here ?

I've looked everywhere for a legal reference. Is anyone aware of this

Do let us know when you find one ...

If your Testing Officer was a Kiwi he may be alluding to the NZ CAA Advisory Circular (AC) at www.caa.govt.nz/assets/legacy/Advisory_Circulars/AC091_3.pdf. Reading through this document the 15% factors you mentioned appear many times.

That being said, I see no legal reference for an Australian aircraft. I did find an old draft AC (dated 2003) at www.casa.gov.au/files/ac091-225pdf. This document lists some correction factors on pages 20 and 21 respectively.

FYI only.

Not wishing to engage in argument at all, but a critical reading of the two cited references indicates -

(a) Reading through this document the 15% factors you mentioned appear many times. However, on each occasion cited, the citation is inconsistent with the OP's testing officer's broad brush view.

(b) This document lists some correction factors on pages 20 and 21 respectively. Again the references are inconsistent with the testing officer's view.

My view is that the testing officer concerned needs to be taken to the bar for a lengthy discussion over a few beers ...

Agreed!

Just pointing out that there is some published documentation supporting the use of factors for aircraft with basic performance data - nothing more.

Indeed.

The use of fudge factors in certification and operational requirements has a long and illustrious history. 15%, certainly, figures in a number of takeoff and landing requirements. However, the present testing officer's global waffle should be noted to be just that, I fear.

An educational session at the bar definitely is called for.