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Pressure Altitude Clarifications


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#1 polywogkev

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Posted 27 November 2017 - 18:41

I had a question regarding pressure altitude and flight planning that I was hoping to get help with:

 

I understand how to calculate pressure altitude and that the standard pressure lapse rate is -1" per +1,000' altitude. On an R44's performance charts (OGE hover, for example), the left y-axis is pressure altitude. My question is, how would you calculate expected "pressure altitude" for an off-airport destination at a significantly higher altitude when there is no way to get a pressure reading from that location prior to arrival?

 

Here is a scenario:

We are departing an airport at 1,000' MSL with a pressure reading of 28.92" and therefore the pressure altitude is 2,000'. We are flying into the mountains and will need to do an OGE hover at 5,000' MSL (but relatively close distance to the airport). Would it be best practice to assume that spread of 1,000' between MSL and pressure altitude persists at the 5,000' location so for consulting performance charts we would look at 6,000' PA?

 

Thanks in advance.


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#2 Nearly Retired

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Posted 28 November 2017 - 10:28

Wait...what?

 

Here is a scenario:

We are departing an airport at 1,000' MSL with a pressure reading of 28.92" and therefore the pressure altitude is 2,000'...

 



#3 r22butters

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Posted 28 November 2017 - 11:18

Get the temperature at your airport and subtract 2° per 1,000' up to 5,000'. Then go to the OGE chart, start at 5,000' move over to that predicted temperature, then go down and it will tell you how much you can weigh if you want to HOGE at 5,000'
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#4 Hand_Grenade_Pilot

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Posted 28 November 2017 - 11:41

I had a question regarding pressure altitude and flight planning that I was hoping to get help with:
 
I understand how to calculate pressure altitude and that the standard pressure lapse rate is -1" per +1,000' altitude. On an R44's performance charts (OGE hover, for example), the left y-axis is pressure altitude. My question is, how would you calculate expected "pressure altitude" for an off-airport destination at a significantly higher altitude when there is no way to get a pressure reading from that location prior to arrival?
 
Here is a scenario:
We are departing an airport at 1,000' MSL with a pressure reading of 28.92" and therefore the pressure altitude is 2,000'. We are flying into the mountains and will need to do an OGE hover at 5,000' MSL (but relatively close distance to the airport). Would it be best practice to assume that spread of 1,000' between MSL and pressure altitude persists at the 5,000' location so for consulting performance charts we would look at 6,000' PA?
 
Thanks in advance.

Your best bet is to use the nearest station pressure, even if its not in the immediate area. Another option is to set the aircraft altimeter to standard pressure (29.92 hg) and note the difference between the reading and true elevation. Apply the difference to the true altitude that you are planning on hovering at.

For example:
Field elevation (departure point): 2,000MSL. Altimeter at 29.92 reads 2,400. There is a low pressure system over the region and you are experiencing a performance penalty of +400.

Your intended OGE hover point is 50 miles away and over terrain at 3,500. You plan to hover 500AGL. You will reference the y-axis of the performance chart at 4,400PA.

Fortunately, changes in atmospheric pressure due to high/low pressure systems are a relatively small part of aircraft performance. For example, lets look at the record low and high for New Orleans.

Record high: 30.83 hg (approx. -830)
Record low: 28.11 hg (approx. +1,717)

Those are extremes; for all practical purposes non-standard atmospheric pressure will not have a huge impact on performance.

Temperature is a much more important variable to keep track of as it will have a significant impact on your ability to hover OGE. Another very important consideration is wind/turbulence; while your calculations may look good on paper, turbulent air or wind coming from an unfavorable direction can prevent you from hovering OGE.

While OGE performance planning is useful for having a general understanding of whether youll be able to hover for the expected conditions, there are many variables that can ruin your plan. Always have a backup plan. For example, a staging area nearby where you can offload weight.

Also keep in mind that operating in a performance limited environment requires a lot of improvising and seat of the pants flying; the performance chart should never be relied on.
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#5 Hand_Grenade_Pilot

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Posted 28 November 2017 - 11:52

Some other considerations for performance that you can’t necessarily calculate are humidity, condition of the engine air intake filter (wet vs dry, clean vs dirty), condition of the rotor blades (dirty vs. waxed), and whether or not the engine is producing optimal power. Can’t stress enough that the performance charts are nothing more than a basic guide, not to be taken for granted.
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#6 polywogkev

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Posted 28 November 2017 - 17:01

Some other considerations for performance that you can’t necessarily calculate are humidity, condition of the engine air intake filter (wet vs dry, clean vs dirty), condition of the rotor blades (dirty vs. waxed), and whether or not the engine is producing optimal power. Can’t stress enough that the performance charts are nothing more than a basic guide, not to be taken for granted.

 

Thanks for both of the thoughtful responses - your point on pressure variance records (max / min) makes a ton of sense and I understand all the other variables. I would disagree with the other commenter on this thread about using the temperature lapse rate for the final density altitude calcs. I've found from personal experience that when mountain flying, albeit at higher altitude MSL, if your altitude AGL is still <1,000' then the temperature does not actually lapse at 2 degrees per 1,000' (better to be conservative).



#7 polywogkev

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Posted 28 November 2017 - 17:03

Wait...what?

 

 

 

I should have said "altimeter setting is 28.92" not "pressure reading". My bad.



#8 r22butters

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Posted 28 November 2017 - 17:59

Thanks for both of the thoughtful responses - your point on pressure variance records (max / min) makes a ton of sense and I understand all the other variables. I would disagree with the other commenter on this thread about using the temperature lapse rate for the final density altitude calcs. I've found from personal experience that when mountain flying, albeit at higher altitude MSL, if your altitude AGL is still <1,000' then the temperature does not actually lapse at 2 degrees per 1,000' (better to be conservative).


Sorry, thought you were just going to HOGE 4,000' higher? As for AGL effecting the lapse rate, I have never heard of that?

,...but then again I don't fly in the mountains!
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#9 polywogkev

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Posted 28 November 2017 - 19:38

Sorry, thought you were just going to HOGE 4,000' higher? As for AGL effecting the lapse rate, I have never heard of that?

,...but then again I don't fly in the mountains!

 

I agree you're right on with the technical calculation. I've just noticed in So-Cal that the ground in the mountains seems to keep in the heat or something. Either way, I'd bias to being conservative (with a higher temperature estimate for planning purposes). Thanks again for all the insight guys.


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#10 SBuzzkill

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Posted 06 December 2017 - 13:34

2 degrees per 1000 feet is the standard lapse rate and not really what you're going to see outside of text books.  It's a very basic model for temperature in the atmosphere and in reality there are many, many variables that will determine the lapse rate.  And even then you will probably only see it maintained for a few thousand feet before the lapse rate changes again.

 

In the mountains it's even more difficult to predict and can vary depending upon which valley you're in, shade vs sun, where the winds are, how steep the terrain is, whether the air is dry or moist, etc.

 

The best thing you can do is look at what the weather is being observed to do as close to your destination as possible and try to interpolate from there.  Leave yourself a good margin for error, have multiple "outs" if possible, and do your best to fly conservatively.  I have found that even in areas that I know well I am consistently surprised.


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#11 polywogkev

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Posted 06 December 2017 - 13:45

2 degrees per 1000 feet is the standard lapse rate and not really what you're going to see outside of text books.  It's a very basic model for temperature in the atmosphere and in reality there are many, many variables that will determine the lapse rate.  And even then you will probably only see it maintained for a few thousand feet before the lapse rate changes again.

 

In the mountains it's even more difficult to predict and can vary depending upon which valley you're in, shade vs sun, where the winds are, how steep the terrain is, whether the air is dry or moist, etc.

 

The best thing you can do is look at what the weather is being observed to do as close to your destination as possible and try to interpolate from there.  Leave yourself a good margin for error, have multiple "outs" if possible, and do your best to fly conservatively.  I have found that even in areas that I know well I am consistently surprised.

 

Good stuff, thanks!



#12 WolftalonID

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Posted Yesterday, 10:56

Being a full time mountain pilot, the one thing I can tell you is we flight plan like crazy. Then we toss it out and go fly. In reality my flight planning is simple. I look to confirm wx wont sock me in. I look to see if we can get there based on fuel needs first, then on cargo weights. Next is winds. Winds at the airport mean nothing. I look at winds from 9000 and up. This is what will tell me somewhat what to expect in the high country. Wind is my absolute friend up there...to a point. Last I want to know my temps now, and expected high for the day. I may look at the altimeter as a factor but it has so little to do with my absolutes that its like a small high five with high pressure, and a ehhh we will watch power if its low pressure.

In flight we do a power check in hover prior to take off to see what a solid surface IGE is giving us for margin. If we are concerned that we could struggle, then a max performance takeoff will show you further limits on power checks.

Backing up to winds. An above post said winds will not let you hover well OGE due to turbulence? .....ohhh kay, well stay off the leeward side of terrain and it wont be an issue. Wind is our friend, like high water flow is to a kayaker. Mountain winds and high waters work exactly the same way too. Winds allow the aircraft to stay in ETL, reducing required power needed and at times I have hovered at 11k ft due to winds helping. This has been done in both R44s and 206b3s. When your hovering completely still at 14 MAP in an R44 at 11k feet to get good film shots of high altitude wildlife....you cannot tell me winds are making it harder. ;)

You need to realize there are ways to work winds in mountains that help you, and there are ways it will kill you. Stay on the windward sides. Avoid leeward sides. Winds compress against terrain like water does on rocks in a river. The windward side is smoother, provides tremendous lift, and reduces power when you stay tight to the mountain.(within 300-500 ft AGL) when you get above that mountain, or beyond the ridge on the leeward side, winds begin to roll, tumble, churn. Just like water does on the backside of boulders in a white water river.

If I see dead calm winds in flight planning...high altitude work must be reconsidered or extremely well planned to ensure safe margins up there. Winds 10-40 kts make life easy up there. Winds above 40 kts make it a bit harder to manage, and in some cases may inhibit safe operations all together. Usually though the later is a call decided by the pilot based on their personal skill limitations. Time will expand your capabilities but always back off if your unsure, because wind will kill you fast if your not doing things right in high country.

When you talk about this said mountain at 3500 feet? Your still in flat country son. ;) just ribbin ya.
Seriously we take off daily from 4500 feet at home base, and operate over 10k very regularly. If you want to come do some serious training, call me sometime, we offer mountain courses for those who need some experience.

Edit: I forgot to talk about the power checks. A great way to check available power margins is to get to the altitude your headed for, out in the open where you have outs, and pull into an OGE hover. See if it will hang there or if it drops or rotor droops right away. If the ship hangs there a bit or settles very slowly, staying in ETL on appraoch to your LZ using updraft winds will give you a very safe margin. If she falls immediately due to lack of power, then the chances of settling into the terrain as you loose ETL are a pretty solid reality. Its basic power check steps like this combined with good wind awareness and temps that make you into a safe pilot. Cold windy days are far better than calm hot ones.

Edited by WolftalonID, Yesterday, 11:01.

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#13 TomPPL

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Posted Yesterday, 11:34

That was an excellent explanation there WolftalonID, thanks for posting, it was a great read.

 

 

Being a full time mountain pilot, the one thing I can tell you is we flight plan like crazy. Then we toss it out and go fly. In reality my flight planning is simple. I look to confirm wx wont sock me in. I look to see if we can get there based on fuel needs first, then on cargo weights. Next is winds. Winds at the airport mean nothing. I look at winds from 9000 and up. ............


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#14 Hand_Grenade_Pilot

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Posted Yesterday, 12:18

Being a full time mountain pilot, the one thing I can tell you is we flight plan like crazy. Then we toss it out and go fly. In reality my flight planning is simple. I look to confirm wx wont sock me in. I look to see if we can get there based on fuel needs first, then on cargo weights. Next is winds. Winds at the airport mean nothing. I look at winds from 9000 and up. This is what will tell me somewhat what to expect in the high country. Wind is my absolute friend up there...to a point. Last I want to know my temps now, and expected high for the day. I may look at the altimeter as a factor but it has so little to do with my absolutes that its like a small high five with high pressure, and a ehhh we will watch power if its low pressure.
In flight we do a power check in hover prior to take off to see what a solid surface IGE is giving us for margin. If we are concerned that we could struggle, then a max performance takeoff will show you further limits on power checks.
Backing up to winds. An above post said winds will not let you hover well OGE due to turbulence? .....ohhh kay, well stay off the leeward side of terrain and it wont be an issue. Wind is our friend, like high water flow is to a kayaker. Mountain winds and high waters work exactly the same way too. Winds allow the aircraft to stay in ETL, reducing required power needed and at times I have hovered at 11k ft due to winds helping. This has been done in both R44s and 206b3s. When your hovering completely still at 14 MAP in an R44 at 11k feet to get good film shots of high altitude wildlife....you cannot tell me winds are making it harder. ;)
You need to realize there are ways to work winds in mountains that help you, and there are ways it will kill you. Stay on the windward sides. Avoid leeward sides. Winds compress against terrain like water does on rocks in a river. The windward side is smoother, provides tremendous lift, and reduces power when you stay tight to the mountain.(within 300-500 ft AGL) when you get above that mountain, or beyond the ridge on the leeward side, winds begin to roll, tumble, churn. Just like water does on the backside of boulders in a white water river.
If I see dead calm winds in flight planning...high altitude work must be reconsidered or extremely well planned to ensure safe margins up there. Winds 10-40 kts make life easy up there. Winds above 40 kts make it a bit harder to manage, and in some cases may inhibit safe operations all together. Usually though the later is a call decided by the pilot based on their personal skill limitations. Time will expand your capabilities but always back off if your unsure, because wind will kill you fast if your not doing things right in high country.
When you talk about this said mountain at 3500 feet? Your still in flat country son. ;) just ribbin ya.
Seriously we take off daily from 4500 feet at home base, and operate over 10k very regularly. If you want to come do some serious training, call me sometime, we offer mountain courses for those who need some experience.
Edit: I forgot to talk about the power checks. A great way to check available power margins is to get to the altitude your headed for, out in the open where you have outs, and pull into an OGE hover. See if it will hang there or if it drops or rotor droops right away. If the ship hangs there a bit or settles very slowly, staying in ETL on appraoch to your LZ using updraft winds will give you a very safe margin. If she falls immediately due to lack of power, then the chances of settling into the terrain as you loose ETL are a pretty solid reality. Its basic power check steps like this combined with good wind awareness and temps that make you into a safe pilot. Cold windy days are far better than calm hot ones.

I suggest you read what I said a little more carefully. I said winds and turbulence MAY prevent you from hovering OGE. Your LZ or hover point is not going to always be on the windward side of the slope. Nor will the airflow always be smooth on the windward side. Where you operate, 40kts winds may be an advantage to you. When I was doing utility work in Hawaii in R44s and 500s, 40kts of wind hitting vertical cliffs will kick the sh%t out of you.

3,500 was an arbitrary number. Replace it with 8,500 if youd prefer; the main point was to demonstrate how to calculate PA and that a high/low pressure system wont affect performance much.
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