On Jan 15, 9:49*am, Dallas wrote:
On Mon, 14 Jan 2008 12:56:46 -0800 (PST), terry wrote:
So why would the takeoff distance required vary with temperature at
the same density altitude?

I confess that I'm lazy today and didn't read what you said for
comprehension, but I will address the above statement.

DA mantra:
Density altitude is pressure altitude corrected for non-standard
temperature.

I agree
Ok, so if DA has been corrected for temperature, then we're done. *

The "vary with temperature" part of your statement doesn't make any sense
if you are using DA, the temperature factor has been applied already. *You
wouldn't apply temperature again to come up with an answer.

No I havent corrected twice, the point of the post was that after
correcting pressure altitude for temperature to get density altitude,
different combinations of pressure altitude and temperature which
result in the same calculated density altitude give different
performance figures.

The statement above would make sense if you said: *The takeoff distance
required will vary with temperature at the same "pressure altitude".

yes it would make sense but then I would have had no need post the
problem, which is that the same density altitude calculated using
different combinations of pressure altitude and temperature do not
give the same performance. For example, Dallas if the presssure
altitude as 5000 ft and temp was 10 dec C, that would be a density
altitude of 5520 ft.
Now I could get the same density altitude of 5520 ft if I had a
pressure altitude of 3000 feet and a temperature of 30 C . Do we
agree on that? I I would then expect that the takeoff distance
required for either of these 2 scenarios would be the same. The point
of the post being that this not the case with the data set in the
table.

I could have explained it a lot better with graphs but I dont think
you can do that on usenet.

Terry