How to determine Pressure Altitude

[Pilot12345]

Ok there is probably a simple answer to my question, but here it go's......

I know to determine the approximate pressure altitude at your departure or destination airport for performance charts, etc. you add 1,000 feet to field elevation for each 1.00 in. Hg that the reported altimeter setting value is below 29.92 in. Hg, and subtract 1,000 feet for each 1.00 in Hg above 29.92 in Hg., and that you always subtract the reported altimeter setting from 29.92 in. Hg., then multiply the answer by 1,000 to find the difference in feet between field elevation and pressure altitude....

Example:

29.92
- 30.07
0.15
0.15 X 1,000 feet = -150 feet

Ok so let's say the field elevation is: 3,750
Subtract the pressure altitude correction: -150 feet
= a field pressure altitude of 3,600

Here is my question.....
If the field elevation was 50 ft instead of 3,750
and using the same #'s above of 29.92 and 30.07
My pressure altitude would be -100 feet, that dosen't sound right. What am I doing wrong?
Thank's for your help!!!

 

[darkvw]

that's the right answer,and the easiest way to get it, I thought it was wrong the first time I did it too, didn't seem the right answer. WHen you correct for density altitude it comes out better.

 

[Fly_Chick]

You are correct with the math. A good starting point (and manner to visualize this) is to go out to an airplane on the ramp, and change the altimeter so that 29.92 reads in the Kollsman window.

Your pressure altitude will be the indicated altitude when the altimeter is set to 29.92.

 

[Singlecoil]

In some places in the winter (Alaska), you can have density altitudes 4000-5000ft below sea level. In that case, IAS is higher than TAS.

 

[Pilot12345]

Thank's everyone!

One more question. I notice on performance charts such as Takeoff and Landing the Pressure Altitude only go's down to sea level. So do I need to draw my own line below the the charts sea level line to get my correct Takeoff and Landing distances if I figure my Pressure Altitude to be below sea level?

Thank's Again!

 

[Singlecoil]

Thank's everyone!

One more question. I notice on performance charts such as Takeoff and Landing the Pressure Altitude only go's down to sea level. So do I need to draw my own line below the the charts sea level line to get my correct Takeoff and Landing distances if I figure my Pressure Altitude to be below sea level?

Thank's Again!

No, I wouldn't do that. If the chart doesn't go below sea level, then just use the sea level numbers. Your actual performance will be better than that so just consider it a built-in safety factor.

 

[midlifeflyer]

Thank's everyone!

One more question. I notice on performance charts such as Takeoff and Landing the Pressure Altitude only go's down to sea level. So do I need to draw my own line below the the charts sea level line to get my correct Takeoff and Landing distances if I figure my Pressure Altitude to be below sea level?

Singlecoil's answer is correct, but there is one thing you can do. Depending on the airplane you are talking about, the performance chart may actually go below sea level. Those combinations of pressure altitudes and temperatures are really calculating density altitude, even if they don't say so.

For example, a Cessna 172 Chart shows takeoff numbers for 0°C at sea level. That's a density altitude of about -1800'. So, for an "off chart" number, if you really needed to know what the expected performance would be, you can calculate your takeoff density altitude and find the equivalent number in the chart.

As singlecoil suggested, it's not that important for the negative numbers (unless your runway is shorter that what's needed at sea level) but it's a very useful tool when, as often happens where I fly, the altitudes are =above= the ones covered in the chart.