Stuck at work--need takeoff/landing distances for a 172 please

On Wed, 13 Jul 2005 16:02:20 -0400, "Peter R."
wrote:

"Peter R." wrote:

Assuming we're talking about KUDD, 73' elevation, 40 deg C temp,
altimeter 29.79, I come up with pressure altitude of 190' and density
alt of 3050.

Hmmm, I keep arriving at a DA about 1200 feet higher. Cross checking this
with several DA calculator websites confirms about 4,200 ft or so.

Nevermind, I see the discrepancy. The OP was asking about 120 degrees F,
which is about 49 degrees C. That was the temp I was using.

Yea, my bad. They only print the chart to 40 so that number got stuck
in my head. 49C comes out with 4k density alt. So extrapolating from
the 40C column at 4K comes out with 1770/3085 for roll/50' obst.

Yea, my bad. They only print the chart to 40 so that number got stuck
in my head. 49C comes out with 4k density alt. So extrapolating from
the 40C column at 4K comes out with 1770/3085 for roll/50' obst.

You don't need to account for density altitude in this case since the POH is
already giving the performance numbers at the non-standard temperature. By
using the numbers for 40C and 4000ft you're essentially double counting the
effect of higher temperature and far underestimating the performance. Since
the chart only goes to 40C you still have to extrapolate to the higher
temperature and you have to make a guess about the pressure altitude but in
the end the distance to clear a 50' obstacle should still be under 2000 ft.

- Ray

On Wed, 13 Jul 2005 14:48:27 -0700, "Ray" wrote:

Yea, my bad. They only print the chart to 40 so that number got stuck
in my head. 49C comes out with 4k density alt. So extrapolating from
the 40C column at 4K comes out with 1770/3085 for roll/50' obst.

You don't need to account for density altitude in this case since the POH is
already giving the performance numbers at the non-standard temperature. By
using the numbers for 40C and 4000ft you're essentially double counting the
effect of higher temperature and far underestimating the performance. Since
the chart only goes to 40C you still have to extrapolate to the higher
temperature and you have to make a guess about the pressure altitude but in
the end the distance to clear a 50' obstacle should still be under 2000 ft.

Yea, I was thinking that since the chart lists pressure altitude on
the left column, but figured I'd throw out the number for the 4K
pressure 50C extrapolation and let the original poster do decide what
to do with the data.

"Ray" wrote in message
...
[...]
Since
the chart only goes to 40C you still have to extrapolate to the higher
temperature

Or, they can use the density altitude, but pick performance figures for
standard temp from the POH. No extrapolation necessary in that case.

Pete