Figuring out density altitude and Pressure altitude at remote locations

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Raftthis101

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Raftthis101
Anyone have a good method for figuring out density altitude and pressure altitude at remote airports and or grass strips that do NOT have an AWOS/ASOS? Student pilot (helo) here...and I have a guess I am going to be quized on this soon...but I am very puzzled. I have no problem calculating it when I have the parameters to input (temp and pressure) from ASOS/ AWOS but if you are going into a remote airstrip with no reported weather, how do you begin to calucate this? (And yes, I did ask my instructor this question...he rushed through a calculation very quickly that I didn't understand and when I asked him to go through it again step by step, he said 'just input the numbers and use whatever comes out'....whatever that means...)
Thanks in advance...:)
 
You can't really. The best you can do is calculate it from the nearest location that has reported weather. Or interpolate it from two bracketing locations.
 
Do you know the field elevation? Does the helo have an OAT gauge?
 
The helo does have an OAT gauge...however I was asked to calculate the density alt and pressure alt prior to flying...
 
Raftthis101 said:
he rushed through a calculation very quickly that I didn't understand and when I asked him to go through it again step by step, he said 'just input the numbers and use whatever comes out'....whatever that means...)
Click to expand...

Time to find a better instructor! That is inexcusable, IMO.
 
If you're "going into" and not there yet, you can't get the data other than interpolation from nearby reporting points.

If you're already there, set the altimeter to 29.92 and read the OAT gauge.
 
Thanks for all the replies--)) I didn't even think of using two bracketed points and using interpolation.....
 
The ADDS Winds/Temps tab includes forecast surface temperatures at 1-hour intervals for like 12 hours ahead. Combine that with the forecast sea level pressure off the surface prog charts (or QNH in nearby military TAF's) and the field elevation, and you should have a pretty close estimate of DA at your arrival time.
 
Thanks!! to all who responded. I have a much better grasp on it now. POA is a great resource to all newbies such as myself...--)))
 
WAAS/GPS is more accurate at determining altitude than any barometric altimeter. When approaching your destination set your baro altimeter to the indicated WAAS/GPS reading.

José
 
Piloto said:
WAAS/GPS is more accurate at determining altitude than any barometric altimeter. When approaching your destination set your baro altimeter to the indicated WAAS/GPS reading.

José
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That is just flat wrong. GPS altitude is not corrected for non-standard pressure.

It is also only calculating the distance above a mathematical spheroid that may or may not match the bumpy/lumpy Earth below your feet.

Thus why it's necessary to set the Baro altimeter setting *into* any IFR certified GPS for flying approaches with vertical navigation.

You're going to kill yourself if you rely solely on GPS altitude data with no pressure correction mechanism.
 
denverpilot said:
That is just flat wrong. GPS altitude is not corrected for non-standard pressure.

It is also only calculating the distance above a mathematical spheroid that may or may not match the bumpy/lumpy Earth below your feet.

Thus why it's necessary to set the Baro altimeter setting *into* any IFR certified GPS for flying approaches with vertical navigation.

You're going to kill yourself if you rely solely on GPS altitude data with no pressure correction mechanism.
Click to expand...

Okay I'll leave it up for posterity, but my post makes no sense. WTF was I thinking?

GPS altitude is definitely from the spheroid, so it's not right, but pressure has nothing to do with it.

Sigh. I've misplaced some critical piece of information in my brain. I'll go hunt for it.
 
denverpilot said:
GPS altitude is definitely from the spheroid, so it's not right, but pressure has nothing to do with it.
Click to expand...

One thing for certain, if it's a matter of not hitting a mountain, I'll go with the GPS altitude, rather than pressure altitude. GPS with WAAS is very good at nailing the altitude of the earth beneath my feet.

If I get a current altimeter setting, my Garmin 696's panel page altitmeter & the aircraft's altimeter will exactly follow each other for a minute or two.......more than not. Sometimes they'll eventually be several hundred feet different.

I've been checking and verifying GPS/WAAS altitudes for several years in mountainous areas. It really works!

L.Adamson
 
Sigh. I've misplaced some critical piece of information in my brain.
Click to expand...


I haven't done that.

In the last 3 minutes.

Wait til you completely misplace your entire brain. For a large part of your adult life.

:D
 
L.Adamson said:
One thing for certain, if it's a matter of not hitting a mountain, I'll go with the GPS altitude, rather than pressure altitude. GPS with WAAS is very good at nailing the altitude of the earth beneath my feet.

If I get a current altimeter setting, my Garmin 696's panel page altitmeter & the aircraft's altimeter will exactly follow each other for a minute or two.......more than not. Sometimes they'll eventually be several hundred feet different.

I've been checking and verifying GPS/WAAS altitudes for several years in mountainous areas. It really works!

L.Adamson
Click to expand...

If hitting a mountain is an issue, I will either add a thousand feet or turn back.
 
denverpilot said:
Okay I'll leave it up for posterity, but my post makes no sense. WTF was I thinking?

GPS altitude is definitely from the spheroid, so it's not right, but pressure has nothing to do with it.

Sigh. I've misplaced some critical piece of information in my brain. I'll go hunt for it.
Click to expand...

GPS, assuming a decent geoid model (fair assumption with modern units including all WAAS) does a great job of giving your actual geometric altitude above sea level. It's the altimeter that's subject to error, due to temperature and other effects. But these errors are (more or less) the same between different altimeters, and altimeter altitude is what's used to separate aircraft and airspace.

On the ground it's okay(ish) to set your altimeter by adjusting it until it reads the surveyed field elevation, or the GPS MSL altitude (which should be within 50ft or so). In the air, doing the same thing can give you a grossly incorrect altimeter setting depending on the temperature and how high you are. This shouldn't get you into trouble with terrain clearance, but could lead to an airspace or traffic separation bust.

The other day, which wasn't particularly nonstandard temperature-wise, I was cruising along at 12500ft according to my correctly-set altimeter, and my GPS indicated 14300ft. Nothing was broken. I was 14300ft geometrically above sea level, as was everyone else flying nearby at 12500ft indicated.

Alles klar?
 
pericynthion said:
GPS, assuming a decent geoid model (fair assumption with modern units including all WAAS) does a great job of giving your actual geometric altitude above sea level. It's the altimeter that's subject to error, due to temperature and other effects. But these errors are (more or less) the same between different altimeters, and altimeter altitude is what's used to separate aircraft and airspace.

On the ground it's okay(ish) to set your altimeter by adjusting it until it reads the surveyed field elevation, or the GPS MSL altitude (which should be within 50ft or so). In the air, doing the same thing can give you a grossly incorrect altimeter setting depending on the temperature and how high you are. This shouldn't get you into trouble with terrain clearance, but could lead to an airspace or traffic separation bust.

The other day, which wasn't particularly nonstandard temperature-wise, I was cruising along at 12500ft according to my correctly-set altimeter, and my GPS indicated 14300ft. Nothing was broken. I was 14300ft geometrically above sea level, as was everyone else flying nearby at 12500ft indicated.

Alles klar?
Click to expand...


AIM 1-1-19(a)(4) says that GPS altitude should not be relied upon. Your experience should not be extrapolated into the procedures of other pilots.

Bob Gardner

Bob Gardner
 
I was talking primarily from my experience as a professional GPS engineer, and secondarily as a pilot.
 
pericynthion said:
I was talking primarily from my experience as a professional GPS engineer, and secondarily as a pilot.
Click to expand...

And you are correct.


"AIM 1-1-19(a)(4) says that GPS altitude should not be relied upon."

When it comes to the AIM, it will be my GPS I'm relying on for altitude, if worse comes to worse. I'll only care about altimeter settings for "seperation & ATC".

Since I live and fly in a mountainous area, and have have used numerous WAAS GPS's for comparison sake,.... I know that the GPS model is much more accurate, that altimeter settings.... unless the alimeter setting is very current. At that point, they can be within mere feet of each other.

L.Adamson
 
Even if the altimeter setting is perfectly current, it'll only show the true geometric MSL altitude while on the ground at the airport where the altimeter setting was measured. 5000ft above the same airport, it will be in error.

MEAs, MDAs etc have considerable margins, partly in order to deal with this error for IFR terrain clearance purposes.
 
If I wanted to land at a remote strip and didn't have a GPS (hey, it could happen) I would probably make damn good and certain I had a lot more airplane than I needed to get into out of the strip. Better safe than sorry.
 
Raftthis101 said:
Anyone have a good method for figuring out density altitude and pressure altitude at remote airports and or grass strips that do NOT have an AWOS/ASOS? ...:)
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Hi, Raftthis!

First off, since pressure altitude is defined as your altitude above the standard pressure datum (that place where an actual pressure of 29.92" Hg exists)...and since your altimeter indicates your altitude above the pressure datum you have set into the Kollsman window, simply set 29.92" Hg in your altimeter's Kollsman window and it will indicate your pressure altitude.

Then, read the temperature (in °C) from your outside air temperature gauge.

Get out your trusty standard issue E6b flight computer, set the temperature over the pressure altitude in the appropriate window and read your density altitude.

...and above all,
FLY SAFE! :)
 

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WileyP said:
Hi, Raftthis!

First off, since pressure altitude is defined as your altitude above the standard pressure datum (that place where an actual pressure of 29.92" Hg exists)...and since your altimeter indicates your altitude above the pressure datum you have set into the Kollsman window. simply set 29.92" Hg in your altimeter's Kollsman window and it will indicate your pssure altitude.

Then, read the temperature (in °C) from your outside air temperature gauge.

Get out your trusty standard issue E6b flight computer, set the temperature over the pressure altitude in the appropriate window and read your density altitude.

...and above all,
FLY SAFE! :)
Click to expand...

And if you can do all that with your hands full of airplane, you are a far better pilot than me. Good luck seeing and avoiding and all that while you're figuring this stuff out.
 
steingar said:
And if you can do all that with your hands full of airplane, you are a far better pilot than me. Good luck seeing and avoiding and all that while you're figuring this stuff out.
Click to expand...

Gee, we used to do it all the time before there were GPS's and all those fancy black boxes. We also determined our ground speed and estimated times of arrival at the next checkpoint and at the destination using an E6b and even (gasp!) the actual winds aloft to pass on with our pireps! And (please excuse my ignorance) I was not aware that a GPS could give you pressure altitude or outside air temperature, the two items from which density altitude are determined.

Takes less time to do it on an E6b than it does to set a waypoint into the black box or to set a radio frequency.

...and above all,
FLY SAFE! :)
 
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WileyP said:
Gee, we used to do it all the time before there were GPS's and all those fancy black boxes. We also determined our ground speed and estimated times of arrival at the next checkpoint and at the destination using an E6b and even (gasp!) the actual winds aloft to pass on with our pireps! And (please excuse my ignorance) I was not aware that a GPS could give you pressure altitude or outside air temperature, the two items from which density altitude are determined.

Takes less time to do it on an E6b than it does to set a waypoint into the black box or to set a radio frequency.

...and above all,
FLY SAFE! :)
Click to expand...

Used to be lots more accidents too. Yeah, fly safe.
 
steingar said:
Used to be lots more accidents too. Yeah, fly safe.
Click to expand...

Compiled by the Air Safety Foundation, the "Nall Report" is probably the most respected, accurate and complete analysis of general aviation accidents.

According to the "Nall Report" for 1997 (the earliest year for which I can find adequate statistics), 56.9% of general aviation accidents occurred during takeoff, climb, maneuvering, descent, approach and landing, phases of flight during which one would most certainly not be doing cross country computations. That means that 43.1% of the general aviation accidents occurred during other phases of flight (i.e. standing, cruise, "other" and "unknown"), when such computations might have been going on.

According to the "Nall Report" for 2009 (the most recent I have access to), 53.4% of the general aviation accidents occurred during takeoff, climb, maneuvering, descent, approach and landing. That meant that 46.6% of the accidents occurred during other phases of flight, when computations might have been going on.

Hmmmm:
1997, 43.1% of the accidents occurred when inflight computations might have been going on
2009 that number increased to 46.6%.

At best, one might consider that to be a statistical tie. To me, however, that certainly doesn't seem to indicate any noticeable decrease in accidents during phases of flight when computations might be going on.

...and above all,
FLY SAFE! :)
 
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There are some misconceptions here about GPS and Baro Altimeters. First all airports in the world are surveyed for location and altitude using datum WGS-84 which is the same one used by GPS. Baro altimeters has no datum reference. As altitude increase pressure gradients decrease at a non-lineal rate creating minute changes in pressure for large altitude changes. This creates large inaccuracies in baro altimeters at altitude to the point of no altitude reading beyond 60,000feet. While GPS will give you altitude readings way beyond 300,000feet all the way to space at one foot resolution.

Just the fact that the baro altimeter relies on an altimeter setting not at the same location of where the altimeter is located introduces significant different readings between a baro altimeter and GPS. A WAAS/GPS will always read MSL (WGS-84) (within 10ft) altitude at sea or at Denver irrelevant of air pressure or temperature.

All obstacle and terrain avoidance FAA approved equipment rely on GPS altitude not on baro altitude. Prior older versions relied on radar altimeters. RVSM aircraft calibration is done using GPS altitude. However baro pressure (29.92") relative altitude is used for traffic separation and TCAS, because it was established before GPS acceptance.

Jose
 
Piloto said:
There are some misconceptions here about GPS and Baro Altimeters. First all airports in the world are surveyed for location and altitude using datum WGS-84 which is the same one used by GPS. Baro altimeters has no datum reference. As altitude increase pressure gradients decrease at a non-lineal rate creating minute changes in pressure for large altitude changes. This creates large inaccuracies in baro altimeters at altitude to the point of no altitude reading beyond 60,000feet. While GPS will give you altitude readings way beyond 300,000feet all the way to space at one foot resolution.

Just the fact that the baro altimeter relies on an altimeter setting not at the same location of where the altimeter is located introduces significant different readings between a baro altimeter and GPS. A WAAS/GPS will always read MSL (WGS-84) (within 10ft) altitude at sea or at Denver irrelevant of air pressure or temperature.

All obstacle and terrain avoidance FAA approved equipment rely on GPS altitude not on baro altitude. Prior older versions relied on radar altimeters. RVSM aircraft calibration is done using GPS altitude. However baro pressure (29.92") relative altitude is used for traffic separation and TCAS, because it was established before GPS acceptance.

Jose
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The fact remains, piloto, that the AIM includes accepted methods of determining altitude and specifically warns against using GPS altitude. No amount of techno-speak will change that.

Bob Gardner
 
bobmrg said:
The fact remains, piloto, that the AIM includes accepted methods of determining altitude and specifically warns against using GPS altitude. No amount of techno-speak will change that.

Bob Gardner
Click to expand...

From AIM Section 4:

2. Other systems may be authorized to utilize these approaches. See the description in Section A of the U.S. Terminal Procedures books for details. Operational approval must also be obtained for Baro-VNAV systems to operate to the LNAV/VNAV minimums. Baro-VNAV may not be authorized on some approaches due to other factors, such as no local altimeter source being available. Baro-VNAV is not authorized on LPV procedures. Pilots are directed to their local Flight Standards District Office (FSDO) for additional information.

As we know LPV approaches use WAAS/GPS for altitude and vertical guidance.

Hurricane hunter crew maintain constant GPS altitude to measure the barometric pressure gradients at the center of the storm.

José
 
Piloto said:
From AIM Section 4:

2. Other systems may be authorized to utilize these approaches. See the description in Section A of the U.S. Terminal Procedures books for details. Operational approval must also be obtained for Baro-VNAV systems to operate to the LNAV/VNAV minimums. Baro-VNAV may not be authorized on some approaches due to other factors, such as no local altimeter source being available. Baro-VNAV is not authorized on LPV procedures. Pilots are directed to their local Flight Standards District Office (FSDO) for additional information.

As we know LPV approaches use WAAS/GPS for altitude and vertical guidance.

Hurricane hunter crew maintain constant GPS altitude to measure the barometric pressure gradients at the center of the storm.

José
Click to expand...

How did this thread get so far off track as to involve approach procedures? Go back to post #1.

Bob Gardner
 
We didn't have an altimeter setting the other day, so we checked the GPS. When we got a setting on approach to our home drome the thing was dead on.
 
When flying into remote fields with no AWOS or ATIS such as those in the Bahamas out island or South America GPS is essential specially at night. None of these fields have VASI or any visual guidance for the approach. Most GPS even the portables can provide some vertical guidance that is accurate and useful. But more important is figuring out if you are landing into the wind or with a tail wind into a 2000ft runway. Specially in mountain regions where winds could be opposite directions over a short distance. And what is left of the windsock is the ring. I first approach the likely runway. On final approach I check the indicated airspeed and GPS ground speed. If the ground speed is higher than airspeed I then step to the down wind side and land in the opposite direction after verifying I have a lower ground speed. The procedure is common in South America and in the Bahamas. Even in the US is useful. I found that these AWOS reports not to be as reliable as they claim and that many windsocks are not iluminated at night.

José
 
Piloto said:
From AIM Section 4:

2. Other systems may be authorized to utilize these approaches. See the description in Section A of the U.S. Terminal Procedures books for details. Operational approval must also be obtained for Baro-VNAV systems to operate to the LNAV/VNAV minimums. Baro-VNAV may not be authorized on some approaches due to other factors, such as no local altimeter source being available. Baro-VNAV is not authorized on LPV procedures. Pilots are directed to their local Flight Standards District Office (FSDO) for additional information.

As we know LPV approaches use WAAS/GPS for altitude and vertical guidance.

Hurricane hunter crew maintain constant GPS altitude to measure the barometric pressure gradients at the center of the storm.

José
Click to expand...

One final word on this thread that went from a student pilot's question about a basic procedure to a discussion of WAAS-based approaches without taking a breath:

We all know that every new issuance of the AIM includes changes, and these days there are changes to the GPS/WAAS sections in every issue. The language in the AIM relative to the use of GPS altitude vs barometric altitude has not changed from the first time GPS was mentioned in the AIM, long before WAAS existed, and the editors have had ample opportunity to remove or change that language if it was wrong or misleading; they have not done so.

In the front of the AIM there is a form with which users can contact the editors and point out perceived errors or omissions. Jose, I think that you should fill out that form, referring to the GPS altitude language, and ask that it be changed or modified together with your reasoning on why this action should be taken. Let us know what you find out.


Insofar as hurricane hunters and their need to adhere to anything in the AIM, Part 91, or Part 61 are concerned, I suggest that you acquaint yourself with the difference between civil aircraft and public aircraft.

Bob Gardner
 
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steingar said:
And if you can do all that with your hands full of airplane, you are a far better pilot than me. Good luck seeing and avoiding and all that while you're figuring this stuff out.
Click to expand...

~~~. I think the assumption is that he's on the ground at the airport getting ready to leave ... Or is he asking for when he's arriving at the un weather info airport? That'd be different :)
 
Bob

Since you appear so well versed in AIM how would you answer Raftthis 101 inquiry.

José
 
Piloto said:
Bob

Since you appear so well versed in AIM how would you answer Raftthis 101 inquiry.

José
Click to expand...

"If you are going in to this remote airport, and have kept your altimeter setting current by checking with an ATC facility within 100 miles (AIM 7-2-2), you should not need to calculate density altitude for landing. Caveat: I am not a helicopter pilot. You can get pressure altitude at any time by noting the reading in the Kollsman window....if you have an extra hand while flying a rotorcraft...quickly resetting it to 29.92 to get the PA reading, then re-setting the Kollsman to what it was before you started the exercise.

If you are departing a remote airport and know its elevation, set the needles to read field elevation, get the temperature off of your OAT, use your flight computer to figure out density altitude, and off you go."

Bob
 
An extra Giant "Thanks!" to all who responded. I have a 100% better handle on the whole concept and can now answer 'almost' all questions on it when quizzed by my fellow student pilots.--))
 
bobmrg said:
"If you are going in to this remote airport, and have kept your altimeter setting current by checking with an ATC facility within 100 miles (AIM 7-2-2), you should not need to calculate density altitude for landing. Caveat: I am not a helicopter pilot. You can get pressure altitude at any time by noting the reading in the Kollsman window....if you have an extra hand while flying a rotorcraft...quickly resetting it to 29.92 to get the PA reading, then re-setting the Kollsman to what it was before you started the exercise.

If you are departing a remote airport and know its elevation, set the needles to read field elevation, get the temperature off of your OAT, use your flight computer to figure out density altitude, and off you go."

Bob
Click to expand...

But what about if the field is more than 100nm from any ATC facility or if you are flying too low for VHF contact. Then tune in for VOLMET broadcast on HF.

http://en.wikipedia.org/wiki/VOLMET
http://www.dxinfocentre.com/volmet-wx.htm
http://www.youtube.com/watch?v=0V7qj8ZMxCs

Of course the VOLMET is only accurate at the indicated field. I found them pretty handy if you are not Inmarsat equipped. They also broadcast SIGMETS for their coverage area

José
 
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