How can you determine pressure altitude without setting the altimeter?

Question

I already saw an answer about pressure altitude but for some reason, i just still don't get the relationships. How does the actual barometric pressure reading given relate to pressure altitude, is it the same thing? Can you figure out pressure altitude without using the altimeter setting in a plane? I'm missing something and just can't seem to get it straight.

Answer 1

The actual barometric pressure given directly correlates to the pressure altitude.  Let's say you are at an airport you know is 1000ft MSL (mean sea level).  That is, it is 1000' above sea level.  If the current baro setting is 29.92, then pressure altitude is 1000' .  Anything less than 29.92 will equal a pressure altitude HIGHER than the original 1000' and any baro setting above 29.92 will equal a lower pressure altitude. There are math tricks like the one posted above or there are charts that will give you an exact answer.  So, by setting the planes altimeter to 29.92 you will get the current pressure altitude, within limits of the altimeter's accuracy.

Answer 2

First, pressure altitude is used as an indicator for atmospheric pressure. As you probably know this and temp are used to calculate aircraft performance. If you want to know the pressure altitude without being in the plane or you just don't want to set it in the Kollsman window,then do the following: 1. Request the local barometric altimeter setting, let's say it's 28.00 "Hg.

2. Take 29.92 "Hg (standard atmosphere) and substract minus the 28.00"Hg. (29.92"Hg-28.00"Hg=1.92"Hg)

3.  1.92"Hg is the difference in pressure.

4. Take into account that each inch is worth 1000 ft.

5. Multiply 1.92"Hg X 1,000= 1,920 ft

6. The barometric setting (28.00) was less then 29.92, so we substract (we'd add if it was greater)1,920 from the present altitude of the aircraft.

7. Let's say we're at 10,000 ft;10,000-1,920=8,080 ft

8. 8,080 is your pressure altitude!!!

But what does this mean??

It means that on a standard day (15C-29.92 "Hg pres), you being at 10,000, really feels like 8,080 to the aircraft in regards to performance. It's basically just a way to put pressure into performace computations. I hope this answers it. Pressure altitude setting, 29.92"Hg is also used by aircraft above 18,000 ft at all times to provide aircraft separation.

Answer 3

Here's an illustration - You are standing at sea level holding a barometer in your hand, it reads 29.92,  then where you are standing would be considered standard pressure zero foot altitude at sea level.  For every 0.10 change from 29.92 would equal a 100 foot difference from sea level in this example; so if the pressure on your barometer read 30.02 then your sea level pressure altitude would be 100 feet above the ground.

Answer 4

Pressure altitude refers to the altitude recorded at standard atmospheric pressure, 29.92 hg. So setting your altimiter to that reading in the kolsman window will give you the altitude if it were a standard day or pressure altitude. A standard day is an atmospheric pressure of 29.92 hg at 15 degrees celcius at sea level with a -2 degree lapse rate per one thousand feet.