On Jan 15, 10:24*am, quietguy wrote:
Your PA-to-DA calculations clearly differ from Cessna's, probably
because they used a different standard atmosphere. *There are plenty
to choose from: International SA, U.S. SA, ICAO SA (all revised over
the years) and some others, some of which are no longer used. *You'd
need to find out which SA was used by Cessna when the 172N was built.
Good luck with that project. *I would just plot some points from the
POH and draw a smooth curve connecting them; I'd be conservative in my
choices of data points and call that good enough.

Just when I thought this was the correct reason. I have now further
analysed the data in the flight manual and looked at the landing
distance required data which was in exactly the same form , ie a table
of distance required as a function of different combinations of
pressure altitude and temperature. With this data table after
converting to density altitude, i get a nice smooth curve of landing
distance required vs density altitude ( as I would have expected with
the take off distance data). This would seem to eliminate the use of
a different standard atmosphere as the cause of the discrepancy.
Whilst I will certainly take your advice and use the conservative
line, my curiosity ( and stubboness) wont rest until I understand the
reason for this.

I am sure someone at Cessna would be able to explain it. Anybody know
who I should contact?

terry