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I am planning a cross country and I wanted to know how would I calculate my PA for performance.

If the elevation of my airport is 16ft

and the altimeter setting for the day is 30.03

I subtract

30.03 -29.92


.11

.11*1000=110ft

Do I add the factor to Field elevation? or would I subtract?

and what determines if I add or subtract?

And if I am climbing to an altitude of 6000, would I use a Altitude OF 6000?

2.Should I add/subtract that PA factor TO to or from 6000 to get my PA?

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  • $\begingroup$ Alway subtract current altimeter setting from 29.92. 29.92 - 30.03 = -.11 * 1000 = -110 ft. If the pressure is lower, the subtraction would be positive, and you add the result to field elevation. $\endgroup$
    – wbeard52
    Commented Jul 31, 2023 at 3:45
  • $\begingroup$ @wbeard52 Ok I got that but what I am trying to understand is why would we subtract the conversion factor or add it if it needed to be added. Just because its a negative number we subtract and add if its positive? whats the reasoning behind that theory? $\endgroup$
    – youngpilot
    Commented Jul 31, 2023 at 17:10
  • $\begingroup$ If the altimeter setting is higher than 29.92 it would indicate that a given volume of air is at a lower altitude. so when we apply the formula above, we get a negative number so that the pressure altitude becomes lower than field elevation. $\endgroup$
    – wbeard52
    Commented Aug 1, 2023 at 0:05

1 Answer 1

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30.03 is larger than 29.92. Pressure gets lower as your altitude gets higher, so a larger pressure corresponds to a lower altitude.

So, since the pressure is lower than standard, your pressure altitude is lower than your indicated altitude, and you should subtract this factor. Your pressure altitude on the field is -94 feet.

If you subtract the altimeter setting this way every time, you can always subtract. This is because you'll get a negative result when the altimeter setting is lower than standard, and subtracting a negative number is the same as adding.

When you climb to 6000 feet, that is an indicated altitude telling you how far over the datum plane of 30.03 inHg you are. To convert that to a pressure altitude you will want to subtract the same 110 feet, so the pressure altitude will be 5890 feet.

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