On Dec 27, 8:51*pm, Dave S wrote:
terry wrote:

* *I am supposed to

set the wt limit purely on the pressure altitude.
terry

No.. You calculate (or obtain) density altitude for your given condition
and day, then refer to THAT altitude on the pressure charts for
performance.

If we do not have the known conditions for the airfield, here in Aus
we are supposed to use published charts which have a declared density
altitude for different seasons for any given lat/long. But if you
were supposed to use density altitude why in heck do they label the
graph pressure altitude?, the 2 are very different things unless it
just happens to be a standard ISA atmosphere which of course doesnt
happen.

Density altitude can come from your airspeed indicator (if it has a
bezel ring), your GPS (if you input the other data or have an ADC), your
Dynon (or other glass cockpit AHRS if available) or your trusty E6B. You
might even just tune to ATIS/AWOS/ASOS to get it.

I have no idea what a bezel ring is ( but I will do some research)
but I know how to calculate density altitude if I am on the ground,
simply by correcting the pressure altitude ( determined by setting the
altimeter subscale to 1013.2 HPa ( 29.92 inches for most of you) and
then correcting for temp as measured by the outside temp gauge), but
the situation I am interested in is when I am flying to another field,
where I do not know the exact conditions and I am supposed to use
declared density altitude to determine before I go whether it will be
safe to fly out.


If your density altitude is 4000 feet, your performance charts for 4000
(Std day) apply. If your density altitude is 5000 feet, the chart or
interpolation for 5000 ft pressure alt/standard day apply.

performance at 4000 ft density altitude on a standard day is the same as
performance at 4000 ft density altitude on a nonstandard day (presumably
somewhere else). Make sense?

Sorry but I am not really following. I understand that 4000 ft
density altitude corresponds to a specific air density,
(1.088 kg/m3 to be precise ) which will result in specific performance
parameters for the aircraft. But a 4000 ft density altitude can occur
with an infinite combination of pressure altitude and temp, one of
which would be 4000 ft pressure altitude and 7 degrees Celcius ( ie
the standard day). The aircraft of course only cares about the
density.
.
And 4000 ft density alt on a standard day just also happens to be
Pressure altitude.
agreed, but its never a standard day. Are you saying I should just
simply substitute the declared density altitude for the pressure
altitude on the climb wt limit graph? if so it still doesnt make a
lot of sense to me, but thanks for trying Dave.

ps
Even my text book ( Basic Aeronautical Knowledge for the student pilot
- Trevor Thom manual) gives a specific example as follows using a
Cessna landing chart the same as mine.
field elevation 5050 ft
QNH 998 Hpa
Temp 0 deg C

doing the calcs on these figures gives a pressure ht of 5500 ft and a
density ht of 5007 ft. He uses the 5500 ft figure on the climb wt
limit graph.

Terry