density ht, pressure ht and landing charts.

Air density is criticial to aircraft performance, which is why it
surprises me that both Piper and Cessna Landing performance charts
show the climbout weight limit as a function of pressure height and
not density height. As I understand it ( at least from Australian
Regs) the aircraft needs to have a minimum climb gradient of 3.2% in
the event of a go around being required. Surely the achievable climb
out gradient must be density ht dependant. I dont think I have a
problem with the the understanding of what pressure and density hts
are and how to determine them, but I cant reconcile my understanding
of what they are with the performance chart. .Am I missing something
here?
Terry

On Dec 26, 9:37*pm, terry wrote:
Landing performance charts

and

climb gradient of 3.2% in
the event of a go around being required.

Hmm, landing chart for go around?

In any event, my Cessna manuals both show pressure altitude on the
Takeoff and Landing charts. The chart also has columns to make
corrections for temperature (ie an approximation of density altitude)
and gives a formula for wind correction; there's no way to standardize
density altitude, really, except by a standard altitude (pressure/
temp). And that's what they do.

On Dec 27, 4:28*pm, wrote:
On Dec 26, 9:37*pm, terry wrote:

Landing performance charts

and

climb gradient of 3.2% in
the event of a go around being required.

Hmm, landing chart for go around?

Why the question mark? A landing chart is for landing but we have to
be prepared for the event of a go around. It makes perfect sense to me
that you would want to be sure you can climb out at a reasonable
gradient if you have to abort the landing..

In any event, my Cessna manuals both show pressure altitude on the
Takeoff and Landing charts. The chart also has columns to make
corrections for temperature (ie an approximation of density altitude)
and gives a formula for wind correction; there's no way to standardize
density altitude, really, except by a standard altitude (pressure/
temp). And that's what they do.

My take off chart has pressure , temp and also density altitude. The
landing chart only has pressure ht with no temp correction for the
climb wt limit section. I just dont understand why it isnt density
altitude. ( or the combination of pressure altitude and temp -which is
basically density altitude minus the humidity effect) The air density
and thus climb performance would be markedly different at 2000 ft
pressure altitude at temps of say 32 and 80 F yet I am supposed to
set the wt limit purely on the pressure altitude.
terry

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.

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.

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?

And 4000 ft density alt on a standard day just also happens to be
Pressure altitude.

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

On Dec 27, 3:37 am, terry wrote:
Air density is criticial to aircraft performance, which is why it
surprises me that both Piper and Cessna Landing performance charts
show the climbout weight limit as a function of pressure height and
not density height. As I understand it ( at least from Australian
Regs) the aircraft needs to have a minimum climb gradient of 3.2% in
the event of a go around being required. Surely the achievable climb
out gradient must be density ht dependant. I dont think I have a
problem with the the understanding of what pressure and density hts
are and how to determine them, but I cant reconcile my understanding
of what they are with the performance chart. .Am I missing something
here?
Terry

The temp correction.. WAT will give you the info you need.

Also, the correct term is altitude, not height. height is how high you
are above the ground, which is kind of meaningless in calculating your
performance.

Bertie

On Dec 26, 7:37*pm, terry wrote:
Air density is criticial to aircraft performance, which is why it
surprises me that both Piper and Cessna Landing performance charts
show the climbout weight limit as a function of pressure height and
not density height. *As I understand it ( at least from Australian
Regs) the aircraft needs to have a minimum climb gradient of 3.2% in
the event of a go around being required. *Surely the achievable climb
out gradient must be density ht dependant. *I dont think I have a
problem with the the understanding of what pressure and density hts
are and how to determine them, but I cant reconcile my understanding
of what they are with the performance chart. *.Am I missing something
here?

The Cessna and Piper POH's I have include both pressure altitude and
then a correction for temp. Its probably easier that way (at least
before computers) since the student doens't need to computer density
altitude. You just follow the chart with the pressure altitude and
then move over to the temp on the cahrt.

-Robert

On Dec 28, 4:26*am, "Robert M. Gary" wrote:
On Dec 26, 7:37*pm, terry wrote:

Air density is criticial to aircraft performance, which is why it
surprises me that both Piper and Cessna Landing performance charts
show the climbout weight limit as a function of pressure height and
not density height. *As I understand it ( at least from Australian
Regs) the aircraft needs to have a minimum climb gradient of 3.2% in
the event of a go around being required. *Surely the achievable climb
out gradient must be density ht dependant. *I dont think I have a
problem with the the understanding of what pressure and density hts
are and how to determine them, but I cant reconcile my understanding
of what they are with the performance chart. *.Am I missing something
here?

The Cessna and Piper POH's I have include both pressure altitude and
then a correction for temp. Its probably easier that way (at least
before computers) since the student doens't need to computer density
altitude. You just follow the chart with the pressure altitude and
then move over to the temp on the cahrt.

Thanks Robert, I dont actually have the POH , only a copy of the
landing and take off charts which are copied directly from the flight
manual ( from the flight school, it is not my aircraft) . As I say my
text book has the same chart, It is has 4 graphs , The bottom left
square has as you say both pressure altitude on the y axis, which you
take across horizontally to intersect the temperature to go up to the
required runway distance ( makes sense) BUT you are then ( according
to my textbook example) supposed to take the pressure altitude line
horizontally across to the climb wt limit graph on the bottom right.
which is therefore not temp corrected. Does that make sense? ( a
picture would be useful here)

On Dec 28, 2:03*am, Bertie the Bunyip
wrote:
On Dec 27, 3:37 am, terry wrote:

Air density is criticial to aircraft performance, which is why it
surprises me that both Piper and Cessna Landing performance charts
show the climbout weight limit as a function of pressure height and
not density height. *As I understand it ( at least from Australian
Regs) the aircraft needs to have a minimum climb gradient of 3.2% in
the event of a go around being required. *Surely the achievable climb
out gradient must be density ht dependant. *I dont think I have a
problem with the the understanding of what pressure and density hts
are and how to determine them, but I cant reconcile my understanding
of what they are with the performance chart. *.Am I missing something
here?
Terry

The temp correction.. WAT will give you the info you need.

Well I can do the temp correction and calculate density ht ,then I
would have to forget all about about going across the graph
horizontally like it says to and just treat the climb wt limit graph
as totally separate to the others, and me thinks I will do that from
now on, but then it aint pressure altitude that I am plotting, so I
will also have to cross out the word pressure and replace with
density. I really think the author of the graphs ( and the text
book ) should be doing that, rather than a little old PPL from
downunder with barely 200 hrs of mostly puddle jumping to his name.

Also, the correct term is altitude, not height. height is how high you
are above the ground, which is kind of meaningless in calculating your
performance.

Yes , I was a bit sloppy with terms there, point taken ( I was
definately referring to altitude)
Thanks

terry wrote:
Are you saying I should just
simply substitute the declared density altitude for the pressure
altitude on the climb wt limit graph?


Yes.
That is EXACTLY what I am saying. Calculated is better than declared,
but if on field or regional weather is not available use what you have.
(sorry, I presumed US flying here, not aussie back country)

If your density altitude is X, your performance is always Y
(oversimplified).

A hot day on the coast may have the same density alt as a cold day in
the hills. If Density Alt is X, your performance will always be that
same Y.

Your performance at a density altitude of 4000 feet for example, on a
standard day (which also equals the pressure altitude chart, by
definition)..... is THE SAME at a density altitude of 4000 feet on a hot
day (presumably somewhere else, but dont let that confuse the issue).

All the plane cares about is that its a density alt of 4000 ft. Whether
its a standard day 4000 feet density alt or a hot day or a cold day or a
high pressure day or a low pressure day it does not matter. If your
CALCULATED density altitude is 4000 feet, your airplane will behave as
if it was at 4000 feet on a standard temp/pressure day. So you use the
chart for 4000 ft pressure altitude (which on a standard day, is 4000
feet density altitude).



As for the bezel ring comment, in the states, most of the rental planes
I've used have a little plastic ring on the airspeed indicator. I had a
momentary brain fart... that ring is used to calculate a rough true
airspeed. Not all ASI's have it. Sorry for the bad info.