Pressure Altitude

[tattooed]

I am having some difficulty getting PA and barometric altimetry under control. Seems the more I study and practice performance problems, the more I confuse myself. I think I have gone past the point of no return!!!

 

So, 1st issue. Determining PA using current altimeter setting, say 30.12

OAT: 10 degrees C

Site elevation 2000 ft.

Edited May 30, 2008 by tattooed

[helonorth]

Pressure altitude is just the altitude displayed in the Kollsman(sp?) when you set the altimeter to

29.92. Sounds like you are talking about density altitude. Just get out the whiz wheel and plug in

the numbers. Or maybe I don't understand your question.

[Goldy]

I am having some difficulty getting PA and barometric altimetry under control. Seems the more I study and practice performance problems, the more I confuse myself. I think I have gone past the point of no return!!!

 

So, 1st issue. Determining PA using current altimeter setting, say 30.12

OAT: 10 degrees C

Site elevation 2000 ft.

 

Maybe if you play with this site you can make sense of it all ??

 

http://wahiduddin.net/calc/calc_da.htm

[Hovergirl]

I am having some difficulty getting PA and barometric altimetry under control. Seems the more I study and practice performance problems, the more I confuse myself. I think I have gone past the point of no return!!!

 

So, 1st issue. Determining PA using current altimeter setting, say 30.12

OAT: 10 degrees C

Site elevation 2000 ft.

 

Is this for the written exam (and so done without the kollsman window)? Or for a flight plan (again without the window)?

 

If you assume a standard lapse rate of 1000 feet for every 1 inch of pressure change, you can subtract 29.92 from your kollsman reading: 30.12-29.92 = .200, and multiply by 1000 (.200 * 1000 = 200). So here the difference is 200 feet, and since your reading is higher, pressure is higher, and pressure altitude is *lower*. 2000 - 200 = 1800. (If the reading was lower, say 29.72, that means lower pressure, which means higher elevation so add the difference to the altimeter reading. Clear as mud, right?)

 

Or imagine it. You're sitting in the helo looking at the gage, which says 2000 and 30.12. You turn the knob down so 30.12 goes down to 29.92. every .1 inch lost is 100 feet, and you're going down by 200 (0.2 in the Kollsman). So the altimeter reading is going down by 200 to 1800, which is your pressure altitude.

 

Sorry if I made things more confusing for you. Set your window to 29.92 and the altimeter reads your pressure altitude. I promise by the time I'm a CFI I won't be so confused OR so confusing.

 

HVG

[tattooed]

Yes, HV is correct with my issue....but it's not with the Kollsman....it's when I'm sitting at home planning or practcing (which, BTW, I'm preparing for my checkride, so I'm making sure I am solid with the e6b.

 

 

What I had trouble with is coming up with PA for remote site and no barometer was given. I spent the evening with a pilot friend who helped me and had a great way to explain things. ALso, what I had forgotten is that you can still use your station pressure for 100 SM from your departure airport.

 

Also, calculating PA was hard for me. What she told me was this.

 

30.00 is always 80 feet below 29.92.

So, if you have a current BAR of say 30.12, then drop the .12 and you have 80 feet below. Now add the .12 (which is actually 120 feet more).

 

So, 80 feet below MSL and 120 feet below MSL is now 200 feet below!

Deduct that from my station altitude, and I now have pressure altitude! (At my departure airport, anyway).

 

 

 

 

That I can tell, you cannot calculate PA on the e6b, or CAN you?

Edited May 31, 2008 by tattooed

[icqa]

I'll throw my .02 cents in.

 

First of all Pressure Altitude isn't an altitude per se. It's a starting point for determining performance much like the datum line is used for determining CG. In this case Standard PA is 29.92 "Hg. The Altimeter setting is what you see in the Kollsman window when set to field elevation is on most days different than 29.92.

 

As other have said the standard lapse rate of change is 1"Hg for every 1000'. So what I've found easy to determine PA is subtract the current altimeter setting (obtained from either an altimeter nearby or the nearest reporting station) from Standard Pressure of 29.92.

 

So your field elevation is 2000' with an altimeter setting of 30.12.

 

29.92-30.12= -0.2

 

Now multiple the result by 1000 gives you -200. Now subtract that from the field elevation gives you a Pressure Altitude of 1800'.

 

Works the other way as well. Just remember to subtract the current altimeter setting from Standard Pressure, this way you don't have to remember to whether you need to add or subtract to the to field elevation.

 

Now given that you have the PA you can then determine Density Altitude (where the aircraft thinks its at) with an E6B or calculator or chart. Density Altitude is PA corrected for nonstandard temperature. Standard Temp is 15 degrees C. The E6B isn't very accurate. The Calculator works the best. Also you can get an approximation with the following:

 

DA=PA + (120 x (15-OAT))

Edited May 31, 2008 by icqa

[captkirkyota]

This topic has been a disturbing reminder to me that I, when learning this part in my training, had a rather new instructor, and never really acquired a complete understanding of when I would need to figure any of this when in a real world scenario, and so I just memorized the rote answers to pass quizzes and tests. So, the obvious question now follows, why, when, whatever would I need to figure this everyday if in an area without a WX reporting station nearby?

Seems like, say if I was like the guy with a helipad on my property, that I could VFR it until I get close enough to a place to listen and then set my Kols window.

 

Would it be that if I was a pilot on a job that , say was camped at a base in the woods for firefighting or some such thing and every morning I read my pressure altitude, to figure what my heli is going to perform like that day, so I can know how much water or passengers I can take, and if so, then I'd need to figure this several times a day as temps rise and lower throughout the day?

 

It would seem that all I'd NEED to do is just on occaision, while at set down since this type of job is mostly VFR, that I just need to leave it set at 29.92 and note what the altitude reads the next time I'm above the trees and such.

 

As you can see, I am still a bit confused here, so any REAL world reasons, experience, especially learning experiences, or any other such info would be great.

OR am I just really that dense and not as smart as I'd like to believe I am, or am I just stupid enough to ask the question everyone else is afraid to so they don't look stupid?

Either way, I would really like something more than the way the books have explained it, cuz I learn better by auditory and doing it, than reading it in a book.

I also want to be a great instructor and take OWNERSHIP of this info and make it thoroughly understood for all my students as well.

[Goldy]

The real world reason is knowing whether or not you can safely get airborne!

 

One thing I wanted to add that has not been said yet. Your altimeter does NOT tell you your MSL altitude. It only tells you what the pressure reading outside is. That is why all of this is so important. Some of the newer GPS units actually display Density Altitude right on the screen....makes dummies of all of us.

 

Great thread-

[helonorth]

The real world reason is knowing whether or not you can safely get airborne!

 

One thing I wanted to add that has not been said yet. Your altimeter does NOT tell you your MSL altitude. It only tells you what the pressure reading outside is. That is why all of this is so important. Some of the newer GPS units actually display Density Altitude right on the screen....makes dummies of all of us.

 

Great thread-

Your altimeter doesn't give you your altitude? And all this time that's what i've been using it for!

Edited May 31, 2008 by helonorth

[Goldy]

Your altimeter doesn't give you your altitude? And all this time that's what i've been using it for!

 

 

Just trying to make a point that no, it doesnt. However it does interpret that pressure, and if you properly calibrate it to local pressures, it does give you a fun dial to read so you know what number to give to a controller when he asks. Even though that instrument has absolutely nothing to do with the altitude that the controller sees on his scope.....wow, did I distract this conversation or what !

 

Back to DA, plenty of accidents due to attempted landings with very high DA's where the pilot looked kinda funny when the NTSB guy asked him if he knew what the DA was at his landing zone. Its really not about whether you can hover, its about being able to stop the 500-1000 FPM descent coming in on final. Of course if you had calculated the DA, you might consider a very long approach at around 300FPM descent.

[captkirkyota]

Just trying to make a point that no, it doesnt. However it does interpret that pressure, and if you properly calibrate it to local pressures, it does give you a fun dial to read so you know what number to give to a controller when he asks. Even though that instrument has absolutely nothing to do with the altitude that the controller sees on his scope.....wow, did I distract this conversation or what !

 

Back to DA, plenty of accidents due to attempted landings with very high DA's where the pilot looked kinda funny when the NTSB guy asked him if he knew what the DA was at his landing zone. Its really not about whether you can hover, its about being able to stop the 500-1000 FPM descent coming in on final. Of course if you had calculated the DA, you might consider a very long approach at around 300FPM descent.

 

DA, I get, but the topic is PA. So is the answer to my question this, take my CX-2 with me, check PA in the AM, then stick the numbers in to figure DA and repeat a few times a day as temps go up? All so I can figure my HOGA and HIGA etc?

[tattooed]

I completely understand the IMPORTANCE of PA and DA (and how it affects engine/airfoil performance), however, trying to calculate it and USE it is a bit more troublesome for me.

 

 

I have a stage check coming up (stage II PVT), and I'm practicing my performance problems, and using the e6b. I would like to be really proficient with it.

I think I have calculating PA more under control. Here's a sample problem and let's work thru it:

 

 

 

Departure airport:

Elevation 208'

Temperature: 10C

BAR: 30.12

Aircraft: R22 Beta II

Take-off weight: 1370

80 KIAS

 

 

Landing Site:

Elevation: 6,000' MSL

Distance from Departure airport: 70NM

 

 

 

1. What is your PA

2. What is your DA

3. What is the temp at site?

4. Can you land?

5. What is your VNE?

 

 

 

 

Show your work.

I will post mine, and I am currious how people come up with their answers. NO electronic computers. Use traditional e6b only.

Edited June 2, 2008 by tattooed

[tattooed]

oops

Edited June 1, 2008 by tattooed

[tattooed]

1. What is your PA

BAR is 30.12.

29.92 and 30.00 = 80 feet, then add .12 (120 feet) 80+120 = 200 feet BELOW sea level

Deduct 200 from 6,000' = 5,800 feet PA

 

 

2. What is your DA

Using e6b, take PA (5,800) and dial in temp (M02). = approx. 5,100' DA

 

 

3. What is the temp at site? (yes, this question should have been before DA, oh well)

M02

(standard lapse rate is 2 degrees per 1,000 feet, 2 x 6 = 12, subract 12 from 10 = M02)

 

4. Can you land?

**Calculating fuel burn**

Spot is 70 miles away, 140 total.

80 KIAS, CAS = 77 kts, TAS (using DA 5,100) = 83 kts

To get to site, I'll burn 50 minutes worth of fuel = 8.4 gallons = 50 lbs.

1370 total weight - 50 lbs of burned fuel = 1320

 

IGE hover = Yes, I can IGE hover up to 10,000 feet PA

OGE hover = yes, I can OGE hover up to about 8,200 feet PA

 

5. What is your VNE?

VNE is approx. 94 KIAS, interpolating from POH chart.

 

 

 

Anyone find any errors?

[justfly]

...Also, calculating PA was hard for me...

[C.R.O.]

1. What is your PA

BAR is 30.12.

29.92 and 30.00 = 80 feet, then add .12 (120 feet) 80+120 = 200 feet BELOW sea level

Deduct 200 from 6,000' = 5,800 feet PA

 

 

2. What is your DA

Using e6b, take PA (5,800) and dial in temp (M02). = approx. 5,100' DA

 

 

3. What is the temp at site? (yes, this question should have been before DA, oh well)

M02

(standard lapse rate is 2 degrees per 1,000 feet, 2 x 6 = 12, subract 12 from 10 = M02)

 

4. Can you land?

**Calculating fuel burn**

Spot is 70 miles away, 140 total.

80 KIAS, CAS = 77 kts, TAS (using DA 5,100) = 83 kts

To get to site, I'll burn 50 minutes worth of fuel = 8.4 gallons = 50 lbs.

1370 total weight - 50 lbs of burned fuel = 1320

 

IGE hover = Yes, I can IGE hover up to 10,000 feet PA

OGE hover = yes, I can OGE hover up to about 8,200 feet PA

 

5. What is your VNE?

VNE is approx. 94 KIAS, interpolating from POH chart.

 

 

 

Anyone find any errors?

 

Just a quick check, and the only error was in your first post on it, you said the altitude at the destination is 6,500', but your calculations you used 6,000'

 

That changes the PA and the DA, as well as the temp. by 1 degree.

 

Other than that, everything else looks pretty good.

 

Conor

Edited June 2, 2008 by C.R.O.

[kodoz]

DA, I get, but the topic is PA. So is the answer to my question this, take my CX-2 with me, check PA in the AM, then stick the numbers in to figure DA and repeat a few times a day as temps go up? All so I can figure my HOGA and HIGA etc?

 

Hey Kirk,

the HIGE and HOGE charts adjust for DA. During start up, you can figure your PA by setting the altimeter to 2992, then use that altitude when you calculate your performance limits. As the day gets hotter, you go back to your charts and recalculate your HIGE/HOGE. If there's a significant change in pressure during the day, you'd also have to figure that in.

 

Now, to get from your altimeter to real-world, you have to take the field elevation (MSL) wherever you intend on landing and correct for non-standard pressure (1000' per 1 in Hg).

 

--c

[tattooed]

Just a quick check, and the only error was in your first post on it, you said the altitude at the destination is 6,500', but your calculations you used 6,000'

 

That changes the PA and the DA, as well as the temp. by 1 degree.

 

Other than that, everything else looks pretty good.

 

Conor

 

 

So I'm a complete spaz......but you knew that already, Conor. Tee hee hee

 

 

Hey, for a good laugh, go ask my CFI how I am with math. I'm VERY curious what his response will be.

I'd love to see his face when you ask that!!!! Yah, I got issues. Funny thing is, I went ALL the way in mathematics...even majored for awhile as I was deciding between pre-vet and Chemical Engineering.

 

 

sandy

[tattooed]

Hey Conor! I edited it! Now it says "6,000" for spot elevation! LOL

 

 

It's not cheating, is it?

[C.R.O.]

Hey Conor! I edited it! Now it says "6,000" for spot elevation! LOL

 

 

It's not cheating, is it?

 

Yes.

 

So I'm a complete spaz......but you knew that already, Conor. Tee hee hee

 

I prefer to think of you as "Caffeine enhanced".

 

Hey, for a good laugh, go ask my CFI how I am with math. I'm VERY curious what his response will be.

I'd love to see his face when you ask that!!!! Yah, I got issues. Funny thing is, I went ALL the way in mathematics...even majored for awhile as I was deciding between pre-vet and Chemical Engineering.

 

sandy

 

I would be willing to bet that he would find a way to put a positive spin on it.

 

Conor

[helonorth]

This topic has been a disturbing reminder to me that I, when learning this part in my training, had a rather new instructor, and never really acquired a complete understanding of when I would need to figure any of this when in a real world scenario, and so I just memorized the rote answers to pass quizzes and tests. So, the obvious question now follows, why, when, whatever would I need to figure this everyday if in an area without a WX reporting station nearby?

Seems like, say if I was like the guy with a helipad on my property, that I could VFR it until I get close enough to a place to listen and then set my Kols window.

 

Would it be that if I was a pilot on a job that , say was camped at a base in the woods for firefighting or some such thing and every morning I read my pressure altitude, to figure what my heli is going to perform like that day, so I can know how much water or passengers I can take, and if so, then I'd need to figure this several times a day as temps rise and lower throughout the day?

 

It would seem that all I'd NEED to do is just on occaision, while at set down since this type of job is mostly VFR, that I just need to leave it set at 29.92 and note what the altitude reads the next time I'm above the trees and such.

 

As you can see, I am still a bit confused here, so any REAL world reasons, experience, especially learning experiences, or any other such info would be great.

OR am I just really that dense and not as smart as I'd like to believe I am, or am I just stupid enough to ask the question everyone else is afraid to so they don't look stupid?

Either way, I would really like something more than the way the books have explained it, cuz I learn better by auditory and doing it, than reading it in a book.

I also want to be a great instructor and take OWNERSHIP of this info and make it thoroughly understood for all my students as well.

I think this is the real question as I think the rest of it is pretty much academic. Maybe the fire

pilots sit down every morning looking at hovering charts, pressure altitude, density altitude, etc.,

but I doubt it. When you pick up the helicopter into a hover first thing in the morning and your

bouncing between 95-100% torque, you have a pretty good idea what's in store for you that

day. And as as it gets hotter and if you go higher, you better be on your toes. I fly at or close

to sea level most of the time, so I maybe talking out of my you know what. But depending on the

weather and weight, I have good idea how the helicopter is going to react, because I do it all

the time. By the time you have to worry about situations like this, you should have enough

experience to know what you and the helicopter are capable of in different situations. Maybe

there are some pilots that operate at high density altitudes that can say what they do.

[captkirkyota]

>>>>Maybe there are some pilots that operate at high density altitudes that can say what they do.

 

Hope so.

Thanks.

[choppedair]

1. What is your PA

BAR is 30.12.

29.92 and 30.00 = 80 feet, then add .12 (120 feet) 80+120 = 200 feet BELOW sea level

Deduct 200 from 6,000' = 5,800 feet PA

 

 

2. What is your DA

Using e6b, take PA (5,800) and dial in temp (M02). = approx. 5,100' DA

 

 

3. What is the temp at site? (yes, this question should have been before DA, oh well)

M02

(standard lapse rate is 2 degrees per 1,000 feet, 2 x 6 = 12, subract 12 from 10 = M02)

 

4. Can you land?

**Calculating fuel burn**

Spot is 70 miles away, 140 total.

80 KIAS, CAS = 77 kts, TAS (using DA 5,100) = 83 kts

To get to site, I'll burn 50 minutes worth of fuel = 8.4 gallons = 50 lbs.

1370 total weight - 50 lbs of burned fuel = 1320

 

IGE hover = Yes, I can IGE hover up to 10,000 feet PA

OGE hover = yes, I can OGE hover up to about 8,200 feet PA

 

5. What is your VNE?

VNE is approx. 94 KIAS, interpolating from POH chart.

 

 

 

Anyone find any errors?

 

Looks good just by eye-balling it, but you forgot one thing...

IGE hover = Yes, I can IGE hover up to 10,000 feet PA

OGE hover = yes, I can OGE hover up to about 8,200 feet PA

In reality, most of our helicopters can barely hover OGE at 1320lbs at sea level!

Oh, and I'm not really sure that you could even add enough weight to get an r22 up to max gross weight in the first place...?

[tattooed]

Looks good just by eye-balling it, but you forgot one thing...

IGE hover = Yes, I can IGE hover up to 10,000 feet PA

OGE hover = yes, I can OGE hover up to about 8,200 feet PA

In reality, most of our helicopters can barely hover OGE at 1320lbs at sea level!

Oh, and I'm not really sure that you could even add enough weight to get an r22 up to max gross weight in the first place...?

 

 

 

Bite me, Brian.

 

 

 

 

 

You're just jealous cuz I don't need fuel to fly. LOL

Sure saves me a TON of $$ on the "Fuel Surcharge"!

[tattooed]

ok, back to our original topic....PA.

 

 

I am having issues with the concept of "true altitude" and "calibrated" altitude, and how to calculate them.

 

Here's an actual performance problem my CFI had me work thru. I needed alot of "hand holding" thru true/calibrated altitude, but otherwise did fine with it. I am only including the data for PA, DA, and True/Cal altitude and leaving out everything else.

 

 

Departure site:

Elevation 2000 ft

ATIS: 285@20, Temp 12C, BAR 27.84"

 

 

Destination - To overfly a site (photo shoot) by 1,000'

Site elevation 7000'

Temp at site: M23C

No wx report, only temp.

 

 

Calculate (at altitude we are overflying site):

PA

DA

True Altitude

Calibrated Altitude

 

 

How would you work thru this problem?