Pressure ALT

[VFR Transit]

Is there an easy way to work out Pressure ALT???

Thanks
VFR

[Contacttower]

Yes....set 1013/2992 on the altimeter and read off.....

[VFR Transit]

How does that work with high temperature and low temperature, will it still be the same?

Thanks
VFR

[Contacttower]

Temperature doesn't really come in to it as such. Say the pressure is 1025 or something (which may indicate it is hotter than a standard day but not always) and you are at an airfield 360ft AMSL. You set 1025 and you have the true altitude AMSL displayed. If you then to set 1013 you have wound the altimeter back 360ft and is now reading 0ft...which would be your pressure altitude. Temperature only comes into it in terms of its incidental effect on pressure.

On a slightly different point.... the altimeter itself is affected by temperature a bit....in very cold temperatures it will under read a bit and over read a bit when hot (there is altimeter correction table in FAR/AIM I think) but the effect is very small and generally disregarded for operations at normal temperatures.

[VFR Transit]

Excellent

Thanks for the info

VFR

[wrecker]

For A/C performance you need "Density Altitude"!

[VFR Transit]

I have flight planning and performance next and can't get my head around it.

VFR

[JohnGV]

Interesting and useful thread for me particularily at the moment. im just gettin to try and work the current pressure altitude for me at the moment, so i can then work out the density altitude. ( by the way, i want to do this via the mathematical calculations, not the flight computer)

I was wondering if i have this worked right- help please!!!

This is the formula i am using to get my Palt;

station elevation (2000ft) + (29.92 - current alt setting 30.92) x 1000

= 19.99

Is this correct?? pls assist

Thanks in advance

Johngv

[denhamflyer]

For denisty altitude look at:-

Equations - Air Density and Density Altitude (http://wahiduddin.net/calc/density_altitude.htm)

[JohnGV]

Denhamflyer,

Thanks for that, I know how to work out the density altitude equation, i have it written down. what i am trying to find out is if my PRESSURE Altitude equation is right. As you have to know your Palt before workin out the density.... but thanks anyway,

Johngv

[Contacttower]

John is it the pressure altitude itself you are looking for?

One doesn't really need a formula as such for it if one already knows the field elevation and local altimeter setting.....

to take your example:

Field elevation 2000ft

Altimeter 3092

So the altimeter reads 2000ft with 3092 set. If you then set 2992 or if you don't have the altimeter to hand just take off 1000ft for every inch of mercury above standard atmosphere one happens to be you then get the pressure altitude. So in your case it would be 1000ft.

[JohnGV]

oh.... i think my groundschool classes have confused me then because I have a formula written down to find pressure altitude???

Oh . . . .im confused.

But ok ill remember that... thanks contact tower!:ok:

Johngv

[JohnGV]

Everyone who is interested (not many probably, considering the low replys!)

My formula was wrong, this is correct as i now understand it! Maybe it can help someone else in the same boat oneday!

Example for calculating pressure altitude;


station elevation (2000ft) + [(29.92 - current alt setting 30.92) x 1000] = 1000ft

Thanks for anyone trying to help,

JohnGV

[VFR Transit]

Hi Gemma

How is this one worked out, as i can't get my head around it at all.

8) You are contemplating a take-off at a high aerodrome (1460 ft elevation) on a hot day (temperature 29 degrees Celsius). Approximately what is the current density altitude if the QNH is 1013 mb?
A 3500 ft
B 2600 ft
C 1460 ft
D 1200 ft

(Above Question Copyright Airquiz.com)

Thanks
VFR

[Final 3 Greens]

You are contemplating a take-off at a high aerodrome (1460 ft elevation)

This question amused me, 1460ft elevation = high

I am awaiting one of the cousins to mention Big Bear or similar:bored:

[gasax]

Ok it's not that high by US standards.

I was at Annecy a couple of weeks ago 1519ft according to the guide. I have to say the temperature had a much bigger effect than the height.

(thermals off the hot tarmac!)

[Final 3 Greens]

I have to say the temperature had a much bigger effect than the height.

You are quite right, density altitude is very sensitive to temperature.

This is more likley to be a factor in the UK.

Good catch.

[IO540]

What is the point of Pressure Altitude?

It seems just a definition. It is what the altimeter reads when set to 1013 i.e. the Flight Level.

I also find it odd that so few planes seem to come with one of those graphical diagrams which take in weight, temperature, wind, runway slope, runway-end obstacle height, and instantly you can get the required runway distance.

Working this out by hand is very error prone.

[JohnGV]

Hi Gemma!

Thanks, very helpful, i only wish i had that sooner yesterday lol!

yes, i did realise this after my girlfriend pointed it out to me. i was forgettin the basic principles of maths! haha

The two formulas are not the same actually, which is where i was going wrong!

in the first, i had no brackets [like this] only ones (like this) i was forgetting that i had to multiply standard pressure with 1000 and then multiply current altimeter setting by 1000, then do the subtraction and then add on the 2000 feet ground eevation! problem solved now.

Then i moved onto density altitudes, found it even more impossible but then realised (TAKE NOTE ANYONE READING FOR DENSITY)

BEFORE you can do the formula for density altitudes, you must first remeber that

1) with every thousand foot of pressure altitude, standard temperature decreases by 2 degrees

2) with every 1 degree of decrease from standard temperature, you must add 100ft (or subtract 100ft if actual temp is below standard temp) to your pressure altitude.

Armed with this you can then proceed to calculating your density altitude;

pressure altitude + [100 x ( outside temp - standard temp)]

so heres an example

Example - 1,500ft + [100 x (20 degrees - 15 degrees)]

so first do your deductions to temp and inrease in altitude (dont do what i did and ignore them lol) then you get this;

1,800 + [100 x (20 degrees - 12degrees)] = 2600ft

you have a density altitude of 2600ft

Hope that helps people like me understand it!!

Johngv

[JohnGV]

IO450,

Thought i might try to answer your question, the point of pressure altitude is, in canada particularily, it is your altimeter seetting when your in the middle of nowhere and cant get a current ground station altimeter settin:)

Also, its used for getting an array of other calculations done i.e density alts!

cheers
Johngv

[Contacttower]

What is the point of Pressure Altitude?

I also find it odd that so few planes seem to come with one of those graphical diagrams which take in weight, temperature, wind, runway slope, runway-end obstacle height, and instantly you can get the required runway distance.

I may have missed your subtly placed humor there IO540 but doesn't one need pressure altitude to use those graphs? :8

[IO540]

Yes, CT, you are quite right.

However, looking at some sample cases, the normal range of QNH (say 980-1030) seems to make only a small difference to takeoff performance. Temperature makes more of a difference.

Of course, loading and surface types make by far the biggest difference and this is where most accidents (where this kind of stuff is a factor) have gone wrong.