Real Answers from Real Pilots

Meteorology Math

Hello!

I’ve just entered the aviation world. Took my first flight in my life back in January from the east coast to Los Angles. Loved the experience so much I did research about becoming a pilot. Took an intro flight lesson to see if I still wanted to learn it. Still did. Signed up for my local community college’s aviation professional pilot program.

Had my first week of class and took my first flight lesson with my instructor just over last week and this week. I’m taking meteorology, flight theory, and a practical class.

I’m having trouble in my meteorology class with some practice problems. I talked to my teacher and he didn’t really clarify anything. In fact he confused the class. How do you figure out pressure altitude? Is there an equation? My class came up with a variety of answers and my teacher agreed with all of them being in the ballpark.

Holly,

Honestly I’m a little surprised by your post. Of course there’s A formula and there are not a variety of answers, there’s only one (sounds like your instructor may be confused?). Btw, most people use their electronic E6-B (or a phone app) regardless it’s a fairly simple formula and one you should know. Anyway here goes:

Pressure Altitude = (standard pressure(29.92) – your current pressure (altimeter setting) x 1,000 + field elevation.
Let’s say field elevation at my local airport is 835′, standard pressure is 29.92 and lets say the current pressure (alt setting) is 30.15.
29.92 - 30.15 = -0.23
-0.23 x 1000 = -230’
835’ - 230’ = 605’ Pressure Alt.

Make sense?

Adam

Holly,

If I may add one more thing to Adam’s example. That formula works for
finding pressure altitude for any altitude, not just field elevation. Say
you want to know what the pressure altitude is for the cruising altitude
that you’ve selected? If you selected 5,500 ft, the math is the same except
for the last step. Instead of adding field elevation, add 5,500 ft.

Tory

Thanks for clarifying that up, Adam. My instructor is a retired pilot and just an adjunct for the college. I have a feeling this semester is going to be something with him mixing topics up.

And thank you Tory for throwing that in there.

~Holly

I think there is a little nuance in the calculation that is missing from your example. 29.92 is standard atmospheric pressure for sea level only. The the above examples will work if the field elevation is sea level (or approximated to be sea level). If the the field elevation is say 2000ft, the standard atm pressure at that altitude will be 27.93 because of the estimated 1" decrease in pressure altitude per 1000ft. So at this field, you will subtract the given pressure from 27.92 not 29.92.

Nana,

I think you might want to check your resources. First off based on your statement on a “standard day” at a 5000’ airport you’re saying the altimeter setting would be 27.92. Not sure how that’s possible since aviation altimeters have a range of 28.0-31.0? While 29.92 IS standard pressure at field elevation it’s also the “standard” (baseline) for pressure calculations REGARDLESS of field elevation (which is why if you’re at an airport with a 2,000’ field elevation on a “standard day” (and set your altimeter to 29.92 the altimeter will read “2,000”). As Tory said, whether you’re at 5,500’ cruise OR a field elevation of 5,500’ the math is the same and you’ll be using 29.92 as your baseline.

Adam

Nana,

Absolutely not. Please do not perpetuate your theory. It is wrong, and
dangerous to those that use the formula incorrectly. Density altitude is
what pilot’s really care about, BUT for the sake of the argument I will
stick with pressure altitude.

Observed altimeter settings are adjusted for pressure at sea level. If an
airport at 2000 ft is reporting an altimeter setting of 27.92 in Hg, that’s
not the pressure altitude on the field. That’s the pressure altitude at sea
level. Using the formula Adam shared at an airport at 2000 ft produces a
pressure altitude of 4000 ft. That’s important to note. If one used your
method, one would think that their airplane is capable of producing
performance characteristics found at 2000 ft. Amplify this example at
airports close to 10,000 ft in a C152, fully loaded and you can see how
dangerous it would be to try to take off. Again, density altitude is more
important, but you can’t calculate density altitude correctly if you don’t
calculate pressure altitude correctly.

Tory

I will look it up and provide a link to my sources. But I think you misunderstood my point. We all know pressure altitude is altitude corrected for non-standard pressure. So the dial in the kolsman window is a correction factor in relation to the pressure at sea level. It is not the given pressure reading. If you took a barometer to sea level on a standard day you you will read 29.92" of mercury. If you took the same barometer to an elevation of 2000ft, the barometer will read 27.92" not 29.92 because pressure decreases with altitude. That’s why if don’t have the ATIS of an airport, you Adjust the kolsman window until the altimeter shows the known field elevation. You don’t just set it to 29.92 even if it was a standard day.

Ok thanks for the clarification

Nana you can look all you like. Simply put, you’re wrong (and frankly it’s disturbing that you’re still arguing your point). The QUESTION was how do you calculate Pressure Altitude (not how do you determine altimeter setting without an ATIS, or taking a barometer to altitude or anything else). When calculating Pressure Altitude you use 29.92 as the “standard” and baseline in the formula. There were no nuances missed by me or Tory, we both CORRECTLY answered the question and if the person took your suggestion “So at this field, you will subtract the given pressure from 27.92 not 29.92” at best they would answer the question incorrectly, at worse they’d be dead.

As for setting altitudes I assure you that I’ve… ahhh forget it…

Adam

No need to exaggerate or respond in that manner. No one would be dead. And there’s nothing disturbing about an intellectual discussion to try and gain a better understanding of a subject. It’s not personal. That’s how knowledge is shared sometimes. Isn’t that what this forum is for?Yes you and Tory answered the question correctly and I was wrong In response to that question. IT was a misinterpretation on my part. My math was wrong. No problem in admitting that. You always subtract from 29.92 because the pressure given is in relation to sea level for that day and already includes the adjustments I mentioned.that is the correct way to do it. But to clarify the discussions that emerged from the earlier question, No, if t it’s a standard day the barometer measures 29.92 at sea level (like it should on a standard day), you will not get that same reading 2000ft above sea level. That would imply no pressure change with altitude which you and I know isn’t true.

Nana,

Really? An exaggeration? Sooooo you’re saying there haven’t been dozens of crashes caused by miss-set altimeters and pressure/density miscalculations. I suggest you do some Googling.

I have and always do welcome ALL questions and intellectual discussion which is why I participate on this forum and have been an instructor for 12 years. What I take exception to and the reason for my tone is the way the information was presented by you and your backpedaling on your response when shown that you were in fact wrong. You were mistaken and that’s fine but rather than say ok thanks you decided to educate me. While I’m very happy you read the chapter on Pressure Altitude I also have read a thing or 2 over the years. It is nothing personal but if you’re going to challenge and provide info for another student (or help with a question) I encourage you to do a) be correct unless you’re confused as well in which case you should ask and b) do so tactfully. Trust me this will serve you well in this industry.

Adam

Adam,

I appreciate your contribution to this page as a mentor. Again, it’s not personal. I haven’t been here that long but I can tell you that your responses sometimes come off as more of an attack than educational. I wasn’t trying to educate you. I wasn’t challenging you And I wasn’t back pedaling. I was admitting my mistake.i was admitting my misunderstanding. It’s called learning. There’s always room for continued learning regardless of experience level.All you needed to do was say, “no that’s not right” and that would have sufficed. All you needed to do was point out my mistake and provide clarifying explanation. I would expect that from anyone I was looking up to as a mentor. Tory did that perfectly so it was easy to see where the mix up was. You will notice that I responded to Tory’s explanation because that was what actually clarified it for me. So I said, thanks for the Clarification. That’s what you do when someone teaches you something. I didn’t see his post until after my second comment otherwise I wouldn’t have posted it.

And no, my misunderstanding even though it was wrong really wouldn’t lead to anyone’s death. You know why? Because when you’re giving an altimeter setting on the ATIS, you dial that exact number in the kolsman window. No math needed. There might be accidents due to incorrect alt settings but not for the reason you’re trying to project.

Have a good day Sir.

Nana

Nana,

Honestly I believe that’s what I did/said in my first response. I guess you took offense to me saying “you should check your resources” vs Tory’s “it is wrong and dangerous”? Sorry?

As for causal factors I stand by my statement as I have the NTSB reports to support it.

Peace Out

Adam

Can we please put this behind us? It’s not personal at all. For what it’s worth, there was absolutely nothing wrong with your first response. You said to go back and read my sources which I agreed to do. That’s one way to figure out if I misread/misinterpreted something. It was your second response I found problematic. And you admitted yourself it has a certain tone to it. That’s about it

This thread is now closed.

While the mentors on here are happy to share their aviation knowledge, the main purpose of this forum is for potential pilots to gain insight into the industry, let’s keep the discussion focused on that.