Pressure Altitude and performance calculations

Greg Weber

Greg Weber

Filing Flight Plan
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Im confusing myself with my flight planning calculations around pressure altitude. I think it’s way simpler then I’m making it and hoping someone could help explain.

If I’m planning a trip at a cruising level of 5500, do I calculate pressure altitude for that by adding or subtracting the conversation factor? Meaning, let’s say altimeter setting is 30.14, that equates to -201.6. So do I subtract 202 from 5500 to say my pressure altitude is 5,298 for the cruise?
 
Greg Weber said:
Im confusing myself with my flight planning calculations around pressure altitude. I think it’s way simpler then I’m making it and hoping someone could help explain.

If I’m planning a trip at a cruising level of 5500, do I calculate pressure altitude for that by adding or subtracting the conversation factor? Meaning, let’s say altimeter setting is 30.14, that equates to -201.6. So do I subtract 202 from 5500 to say my pressure altitude is 5,298 for the cruise?
Click to expand...
How about this, always add the conversion factor. So if you add a negative number what happens? Is that simpler?

Wait, is this a trick question?
 
Ha! No trick! I've been legit confused but I think by writing it out in the thread question I may have sorted it out in my head.
 
You can get pressure altitude by setting your Kollsman window to 29.92. After a few times of setting it in the airplane, you'll get used to the effect it has on the altitude reading. Next time you're sitting in the airplane try playing with it. Pretend the AWOS says altimeter is 29.82, set it to that, read the altitude, then set it to 29.92 and read the altitude again. Repeat for an imaginary AWOS altimeter of 30.02.

(Spoiler alert, the readout goes up as the setting goes up; if the altimeter is 29.82 then PALT is 100ft higher)

Point is, the physical act of moving your fingers can be a mnemonic aid for many people.
 
One area I always get wrong on my written practice tests are the ones about moving from high or low pressure into the other and what your true vs indicated altitude should be. Confused the crap out of me every time.
 
Greg Weber said:
Im confusing myself with my flight planning calculations around pressure altitude. I think it’s way simpler then I’m making it and hoping someone could help explain.

If I’m planning a trip at a cruising level of 5500, do I calculate pressure altitude for that by adding or subtracting the conversation factor? Meaning, let’s say altimeter setting is 30.14, that equates to -201.6. So do I subtract 202 from 5500 to say my pressure altitude is 5,298 for the cruise?
Click to expand...
Technically that is correct. And taken a step further, for gnat’s butt accuracy, factor in the temperature to get density altitude, even if pressure altitude is all your charts show.
 
For me, the easiest way to remember in which direction to go is to imagine adjusting an altimeter from your current altimeter setting to standard (29.92).

- Alt setting 30.01. As you change the altimeter from 30.01 to 29.92 the indicated altitude decreases so pressure altitude is lower than actual.
- Alt setting 29.85. As you change from 29.85 to 29.92 the indicated altitude increase so pressure altitude is higher than actual.

If you want to calculate the change with math, add a zero to the end of the altimeter setting and drop the "." (x1000)-- 30.01 becomes 30010 --and subtract from 29920 (29.92*1000).

- Alt 30.01. 29920 - 30010 = -90. A negative number means subtract so {actual altitude} - 90 = Pressure Altitude
- Alt 29.85. 29920 - 29850 = 70. A positive numbers means add so {actual altitude} + 70 = Pressure Altitude
 
Why are you using pressure altitude? With little exception, aircraft performance is driven by DENSITY altitude which pressure considerations is but one part.
 
Greg Weber said:
If I’m planning a trip at a cruising level of 5500, do I calculate pressure altitude for that by adding or subtracting the conversation factor?
Click to expand...
Simple answer - No. You don't have do do anything to figure out your cruising altitude. Before you depart, you will set your altimeter based on the ATIS/AWOS/ASOS. Your altimeter will now read the correct elevation. Then you will depart and begin to climb to your cruise altitude. If you fly a slow airplane and are on with ATC, you may get handed off to another controller who may give you a new altimeter setting. You will dial that in to your altimeter and the altitude will change (very slightly), but you will still climb to 5,500' as read on your altimeter. Likewise, when you are in cruise and get handed off to another controller who gives you a new altimeter setting, you will dial that in. Your altimeter may change to show greater or less than 5,500' now. Adjust your altitude to so the altimeter reads 5,500'.

It's pretty simple in that regard and requires zero thought. Just put the altimeter setting in the Kollsman window and that is your altitude above sea level that you will use.

For planning takeoff roll and landing distance, you will use density altitude. That is pressure altitude corrected for standard temp. Use a chart (probably in your POH, but you can download them from The Interwebs) or use an app.
 
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Deelee said:
Simple answer - No. You don't have do do anything to figure out your cruising altitude. Before you depart, you will set your altimeter based on the ATIS/AWOS/ASOS. Your altimeter will now read the correct elevation. Then you will depart and begin to climb to your cruise altitude. If you fly a slow airplane and are on with ATC, you may get handed off to another controller who may give you a new altimeter setting. You will dial that in to your altimeter and the altitude will change (very slightly), but you will still climb to 5,000' as read on your altimeter. Likewise, when you are in cruise and get handed off to another controller who gives you a new altimeter setting, you will dial that in. Your altimeter may change to show greater or less than 5,000' now. Adjust your altitude to so the altimeter reads 5,000'.

It's pretty simple in that regard and requires zero thought. Just put the altimeter setting in the Kollsman window and that is your altitude above sea level that you will use.

For planning takeoff roll and landing distance, you will use density altitude. That is pressure altitude corrected for standard temp. Use a chart (probably in your POH, but you can download them from The Interwebs) or use an app.
Click to expand...

And how does he find pressure altitude for calculations that require it?
 
Larry nailed that one.

((29.92 (std) - current pressure) * 1000) + elevation
Example:
From ATIS altimeter setting = 30.00
Field elevation = 192'

29.92 - 30.00 = -.08
-.08 * 1000 = -80
-80 + 192 = 112' so PA = 112'

Example 2:
From ATIS altimeter setting = 29.80
Field elevation = 192'

29.92 - 29.80 = 12
12 * 1000 = 120
120 + 192 = 312' so PA = 312'
 
Greg Weber said:
Im confusing myself with my flight planning calculations around pressure altitude. I think it’s way simpler then I’m making it and hoping someone could help explain.

If I’m planning a trip at a cruising level of 5500, do I calculate pressure altitude for that by adding or subtracting the conversation factor? Meaning, let’s say altimeter setting is 30.14, that equates to -201.6. So do I subtract 202 from 5500 to say my pressure altitude is 5,298 for the cruise?
Click to expand...

29.92
- 30.14
------------
- 0.22

-0.22 * 1000 = -220

-220 + 5500 = 5280

Not sure from where came the number "-201.6"??

Always subtract baro setting from 29.92 then, be the result be negative or positive add reference altitude to the result. That way you never need to worry about which way to go.
 
Greg Weber said:
One area I always get wrong on my written practice tests are the ones about moving from high or low pressure into the other and what your true vs indicated altitude should be. Confused the crap out of me every time.
Click to expand...

High to Low? Look out below!
Low to High? Clear the sky!

Works for pressure and temperature.
 
flyingron said:
Why are you using pressure altitude? With little exception, aircraft performance is driven by DENSITY altitude which pressure considerations is but one part.
Click to expand...

You have to calculate pressure altitude first to find density altitude.
 
dmspilot said:
That's not "calculating the change with math", that's a rule-of-thumb estimate. If you want an accurate answer for the FAA knowledge test you have to use the correction table.
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Sorry if I wasn't clear. I meant using math instead of using an altimeter. Subtracting one number from another is certainly math.
 
Dunno then. That’s always how I have done it and I only missed one on the private written.

I think I used the density altitude correction table but not anything for pressure altitude.
 
Larry in TN said:
Sorry if I wasn't clear. I meant using math instead of using an altimeter. Subtracting one number from another is certainly math.
Click to expand...

Maybe you think I was being pedantic with your word choice. That wasn't the point. We can call it "calculating a rule of thumb estimate with math", then. It is not accurate enough for some of the questions.
 
flyingron said:
Why are you using pressure altitude? With little exception, aircraft performance is driven by DENSITY altitude which pressure considerations is but one part.
Click to expand...

Can't speak for the OP, but my Piper performance charts for take-off distance, accelerate-stop, climb performance, etc are referenced to pressure altitude and have the temperature adjustments as part of the chart procedures.
Same for the Husky, although the chart section in the POH is rather skimpy compared to the twin. ;)
 
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GRG55 said:
Can't speak for the OP, but my Piper performance charts for take-off distance, accelerate-stop, climb performance, etc are referenced to pressure altitude and have the temperature adjustments as part of the chart procedures.
Click to expand...
:yeahthat:
 
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