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Final Glides: GPS or Pressure Altitude?
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chip.b...@gmail.com
Jul 6, 2016, 1:20:19 PM7/6/16
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Ignoring airspace and start/finish gate implications, which is better for managing final glides: GPS or pressure altitude?
At the Nephi Nationals, I was using a Dell Streak 5 running Top Hat glide computer software. I'd built a wiring harness for my PowerFLARM to successfully input traffic and pressure altitude date to a Kobo Mini but I couldn't get the Dell to work using the same set up. So all of the Dell's final glide info was based on GPS altitude.
GPS altitude at this contest differed dramatically from pressure altitude, from negligible at field elevation (5,000 ft.) to 1,000+ ft. at the top of our operating band (over 17,000 ft.). This was new to me. I just hadn't seen much difference between the two in my flying back east.
I assumed that forces on the glider airfoil were more likely based on density altitude, so I used my old Cambridge GPS/NAV driving GNII/Compaq 1550 for glide path control, since the Dell was telling me I had 1,000 ft. more altitude to play with when starting a long final glide.
The difference was also a factor at the start, where I had to configure the Dell with a 700 ft. higher start gate ceiling than the specified 12,000 MSL limit in order to get reasonably accurate automatic climb-thru-the-top starts.
Now that I've done some reading, I'm not so sure I should have ignored the Dell's GPS-based glide path recommendations, however. I've found several recommendations from knowledgeable authorities that GPS altitude is better for managing final glides. Obviously the GPS altitude is more accurate in an absolute sense, but my question relates to performance gliding through a non-standard atmosphere. Of course, I'm ignoring the IAS/TAS issues in all of this.
Comments?
Chip Bearden
ASW 24 "JB"
At the Nephi Nationals, I was using a Dell Streak 5 running Top Hat glide computer software. I'd built a wiring harness for my PowerFLARM to successfully input traffic and pressure altitude date to a Kobo Mini but I couldn't get the Dell to work using the same set up. So all of the Dell's final glide info was based on GPS altitude.
GPS altitude at this contest differed dramatically from pressure altitude, from negligible at field elevation (5,000 ft.) to 1,000+ ft. at the top of our operating band (over 17,000 ft.). This was new to me. I just hadn't seen much difference between the two in my flying back east.
I assumed that forces on the glider airfoil were more likely based on density altitude, so I used my old Cambridge GPS/NAV driving GNII/Compaq 1550 for glide path control, since the Dell was telling me I had 1,000 ft. more altitude to play with when starting a long final glide.
The difference was also a factor at the start, where I had to configure the Dell with a 700 ft. higher start gate ceiling than the specified 12,000 MSL limit in order to get reasonably accurate automatic climb-thru-the-top starts.
Now that I've done some reading, I'm not so sure I should have ignored the Dell's GPS-based glide path recommendations, however. I've found several recommendations from knowledgeable authorities that GPS altitude is better for managing final glides. Obviously the GPS altitude is more accurate in an absolute sense, but my question relates to performance gliding through a non-standard atmosphere. Of course, I'm ignoring the IAS/TAS issues in all of this.
Comments?
Chip Bearden
ASW 24 "JB"
Sean
Jul 6, 2016, 3:49:13 PM7/6/16
to
Unless the PDA is coupled with a reliable pressure altitude instrument upstream (such as a certified, calibrated logger), the PDAs own pressure altitude is going to be pretty poor in terms of accuracy. PDA or mobile device altimeters, especially and old dell streak, are about as reliable as a cheap altimeter watch. In that case (I also had a dell streak and XCSoar for awhile), I do use GPS as a reference and relied on GPS more than pressure, often cross referencing with my steam gauge altimeter to see how far it's off. In other words, I flew the "worst" of the two in terms of final glide. If one altitude says I have final glide by 300 feet and the other says 200 below, I assumed the worst. I also turn up the fudge factor (add safety buffer). Also, the best final glide profile is generally to build up the margin early in the final glide and then begin diving it off at 5-10 miles depending on the glider performance and conditions. So when using a low accuracy PDA pressure altitude I would hold off longer to burn off any excess altitude and be weary of the potential benefit of risking a low finish. In other words I often finished fairly high.
Obviously, in US and FAI contests, the scoring software adjusts pressure changes and compensates. So the altimeter setting you launch with is very accurate at the finish assuming you have a fairly reliable, calibrated pressure altitude. A PDA without reliable pressure altitude is going to be fairly poor, to almost useless.
Even with a quality pressure altitude instrument, altitude considerations and learning to trust your final glide accuracy, and to understand and trust exactly what your logger is logging as you get close to min altitude, takes some practice. John Godfrey is excellent at explaining this as is John Good.
Post my XCSoar experiences, I have had an SN10/Oudie and now a ClearNAV2 and LXNAV S10. These instruments are all rock solid in terms of pressure altitude and I have quickly grown to trust them completely. They are both very accurate and reliable in terms of contest final glides and accuracy. I can usually take myself down to within 15 feet of the minimum finish altitude in total confidence. Also, the pressure altitude displayed on the screen is (generally) almost exactly what the logger is showing at that moment. So, if you see 1003 and the min is 1000, your OK, generally. Yes, I know the CN2 is expensive but the SN10 is now pretty cheap and very easy to install. And the SN10 NMEAs nicely to the PDA of your choice if you want a moving map to "supplement" the core SN10 data.
Not sure that really helps, but this is a quick summary of my experience.
S-
Obviously, in US and FAI contests, the scoring software adjusts pressure changes and compensates. So the altimeter setting you launch with is very accurate at the finish assuming you have a fairly reliable, calibrated pressure altitude. A PDA without reliable pressure altitude is going to be fairly poor, to almost useless.
Even with a quality pressure altitude instrument, altitude considerations and learning to trust your final glide accuracy, and to understand and trust exactly what your logger is logging as you get close to min altitude, takes some practice. John Godfrey is excellent at explaining this as is John Good.
Post my XCSoar experiences, I have had an SN10/Oudie and now a ClearNAV2 and LXNAV S10. These instruments are all rock solid in terms of pressure altitude and I have quickly grown to trust them completely. They are both very accurate and reliable in terms of contest final glides and accuracy. I can usually take myself down to within 15 feet of the minimum finish altitude in total confidence. Also, the pressure altitude displayed on the screen is (generally) almost exactly what the logger is showing at that moment. So, if you see 1003 and the min is 1000, your OK, generally. Yes, I know the CN2 is expensive but the SN10 is now pretty cheap and very easy to install. And the SN10 NMEAs nicely to the PDA of your choice if you want a moving map to "supplement" the core SN10 data.
Not sure that really helps, but this is a quick summary of my experience.
S-
kirk.stant
Jul 6, 2016, 4:03:55 PM7/6/16
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On Wednesday, July 6, 2016 at 12:20:19 PM UTC-5, chip.b...@gmail.com wrote:
> Ignoring airspace and start/finish gate implications, which is better for managing final glides: GPS or pressure altitude?
I guess the question is how the glide computer figures final glides? If it is purely geometric altitude divided by the glide angle that matches the Mc you set in (adjusted for wind, wingloading, etc), then theoretically, pure GPS elevation (not altitude) should be more accurate - since it is not affected by pressure or temperature differences, and the GPS error is usually pretty small.
> Ignoring airspace and start/finish gate implications, which is better for managing final glides: GPS or pressure altitude?
I'm guessing that GPS is better...
Kirk
roel.b...@gmail.com
Jul 6, 2016, 4:49:43 PM7/6/16
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I'm going with GPS. Static pressure is the hardest pressure to measure, and it is almost never without bias. Dick Johnson has evidence of this too.
GPS has more random errors,_I think_. Especially since your final glide is long. Static pressure might differ along the glide.
GPS has more random errors,_I think_. Especially since your final glide is long. Static pressure might differ along the glide.
chip.b...@gmail.com
Jul 6, 2016, 6:11:33 PM7/6/16
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To clarify:
The Dell Streak doesn't have a pressure transducer, to my knowledge. So it's using GPS altitude to figure final glides. GPS altitude is quite accurate, AFAIK, certainly more accurate than pressure altitude ("accurate" referring to altitude as measured with an extremely long string, that is).
My old GPS/NAV, LNAV, Compaq Aero/GNII combo has always done a great job of helping manage final glides and continued to do so at Nephi. But...the pressure altitude was 1,000+ feet lower than the GPS altitude when up high. And I was gliding through air that was less dense than a standard atmosphere. So I decided I was better off trusting the pressure altitude (used by the Cambridge system) than GPS altitude (used by the Dell Streak/Top Hat system). At the extreme, if the density altitude were extremely high, the glider would hardly glide at all; it would plummet despite the robust GPS altitude because the wings wouldn't generate much lift. At least that was my reasoning.
So it's not which system I trust, or how to manage final glides, or which polar to input, or which computer calculation to rely upon, or even whether the static pressure is measured precisely or has changed slightly since launch. I've been doing final glides since 1968 using a variety of methods starting with a cardboard whiz wheel. The question is which altitude to use. From 50 miles out, 13,000 ft on the altimeter/Cambridge (which agreed closely) gave me a 33 glide ratio to the 5,000 ft Nephi airport (ignoring the finish gate floor). The GPS altitude of 14,000 ft. at the same position yielded a 29.3 glide ratio. My feeling was that the 29 glide ratio was overly ambitious. But I'm eager to hear the discussion, hoping to avoid digging out my old engineering books.
The Dell Streak doesn't have a pressure transducer, to my knowledge. So it's using GPS altitude to figure final glides. GPS altitude is quite accurate, AFAIK, certainly more accurate than pressure altitude ("accurate" referring to altitude as measured with an extremely long string, that is).
My old GPS/NAV, LNAV, Compaq Aero/GNII combo has always done a great job of helping manage final glides and continued to do so at Nephi. But...the pressure altitude was 1,000+ feet lower than the GPS altitude when up high. And I was gliding through air that was less dense than a standard atmosphere. So I decided I was better off trusting the pressure altitude (used by the Cambridge system) than GPS altitude (used by the Dell Streak/Top Hat system). At the extreme, if the density altitude were extremely high, the glider would hardly glide at all; it would plummet despite the robust GPS altitude because the wings wouldn't generate much lift. At least that was my reasoning.
So it's not which system I trust, or how to manage final glides, or which polar to input, or which computer calculation to rely upon, or even whether the static pressure is measured precisely or has changed slightly since launch. I've been doing final glides since 1968 using a variety of methods starting with a cardboard whiz wheel. The question is which altitude to use. From 50 miles out, 13,000 ft on the altimeter/Cambridge (which agreed closely) gave me a 33 glide ratio to the 5,000 ft Nephi airport (ignoring the finish gate floor). The GPS altitude of 14,000 ft. at the same position yielded a 29.3 glide ratio. My feeling was that the 29 glide ratio was overly ambitious. But I'm eager to hear the discussion, hoping to avoid digging out my old engineering books.
Craig Funston
Jul 6, 2016, 6:47:00 PM7/6/16
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https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwjkysam89_NAhWJipQKHejhCj8QFggcMAA&url=http%3A%2F%2Fwww.borgeltinstruments.com%2FGPSvsPressurealtitude.pdf&usg=AFQjCNHmTCye6j3agWjcD224pGMG8yDxdg&sig2=HBx0S9FDZsbGZ0EI4fykrA
Craig
7Q
markm...@gmail.com
Jul 6, 2016, 8:26:58 PM7/6/16
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It is not unusual at Moriarty to set the altimeter (mechanical) and LX Nav S80 to known field altitude at the launch and have them display altitude after landing as much as 400 ft. higher, and occasionally lower, than field altitude.
I naturally (as a non-competition pilot) avoid the long high speed final glides that are designed to get me to the field just high enough to get the gear down. I like a safety margin, because where I fly, the powerful lift is often matched by heart stopping sink.
Using an altimeter based on differing pressures during the day, both mechanical or electronic instruments can vary widely from actual pressures found on approach. I always check our AWOS several miles out to readjust the Kohlsmann setting to actual field settings. I ignore GPS altitudes, as I have no way of evaluating their accuracy.
Looking out the window is still the best method for judging arrival altitudes, pattern flight and touchdown. We aren't equipped for ILS approaches to landing. Don't rely on non-certificated equipment as if you are a B-747 coming into Heathrow in rain at night.
I naturally (as a non-competition pilot) avoid the long high speed final glides that are designed to get me to the field just high enough to get the gear down. I like a safety margin, because where I fly, the powerful lift is often matched by heart stopping sink.
Using an altimeter based on differing pressures during the day, both mechanical or electronic instruments can vary widely from actual pressures found on approach. I always check our AWOS several miles out to readjust the Kohlsmann setting to actual field settings. I ignore GPS altitudes, as I have no way of evaluating their accuracy.
Looking out the window is still the best method for judging arrival altitudes, pattern flight and touchdown. We aren't equipped for ILS approaches to landing. Don't rely on non-certificated equipment as if you are a B-747 coming into Heathrow in rain at night.
K m
Jul 7, 2016, 12:05:28 AM7/7/16
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On Wednesday, July 6, 2016 at 6:26:58 PM UTC-6, markm...@gmail.com wrote:
> It is not unusual at Moriarty to set the altimeter (mechanical) and LX Nav S80 to known field altitude at the launch and have them display altitude after landing as much as 400 ft. higher, and occasionally lower, than field altitude.
Mark,
> It is not unusual at Moriarty to set the altimeter (mechanical) and LX Nav S80 to known field altitude at the launch and have them display altitude after landing as much as 400 ft. higher, and occasionally lower, than field altitude.
Just to clarify, You set the known field altitude Before TO on the S80 and find this variance? On many of the LxNav instruments setting the field elevation sets a baseline for the GPS altitude for final glide. When I do this the QNH comes up exactly as on the AWOS (This, I am assuming verifies a good level of accuracy) . If you have a 400FT variance something may be up with your instrument. I find my (New $900 Winter) Baro to be off by as much as 100FT at the field but almost never on the GPS.
Looking outside is good too. RE the 747; We could start another thread on uncompensated VNAV systems. That's what WAAS is for.
markm...@gmail.com
Jul 7, 2016, 8:29:49 AM7/7/16
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KM- Now that you mention it, I really was just looking at the mechanical altimeter. It often shows a variance from field elevation after a flight, but I guess I haven't checked the S80. I just assumed it would show a difference if the pressure altitude was not adjusted to the AWOS altimeter setting. I will check it on the next flight.
Per Carlin
Jul 7, 2016, 9:11:35 AM7/7/16
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Mechanical altimeters, especially the 57mm Winter are known for being lazy on a fast descend.
It can differ more than 200m compared to ground just at landing after a flight, wait a few minutes and the needle will smoothly be back at zero (or QNH).
The only explanations for a S80 to show different altitude before start and after landing is:
- Its broken (not likely)
- The pressure (QNH) has change during your flight.
I only have the mechanical altimeters as I have to, the flight computer is my primary indicator of height. I guess the majority of XC-pilots use the computer/vario as primary.
It can differ more than 200m compared to ground just at landing after a flight, wait a few minutes and the needle will smoothly be back at zero (or QNH).
The only explanations for a S80 to show different altitude before start and after landing is:
- Its broken (not likely)
- The pressure (QNH) has change during your flight.
I only have the mechanical altimeters as I have to, the flight computer is my primary indicator of height. I guess the majority of XC-pilots use the computer/vario as primary.
Papa3
Jul 7, 2016, 9:12:09 AM7/7/16
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Lot's of good comments upstream. I think the question itself needs to be clarified. If the issue is "which one will help keep me from running into big hard things that get in the way of my final glide (e.g. a ridgeline, mountaintop, etc.) " then the obvious answer would be GPS. After all, it's referenced to the stuff you're worried about hitting. Even relatively low-cost GPS engines are providing sub-100 foot vertical errors (more typically sub-50 foot).
Pressure altimetry is eferenced to the airmass which as you know is rarely "standard". Especially when flying over the course of a long task in changing conditions (remember - "from high to low or hot to cold look out below"), it's not uncommon to have several hundred feet of change in altitude indication. Unless you have a current altimeter setting from your target airport (or at least one nearby) you've got a big unknown to deal with.
If the question was really "how do the systems in my cockpit work", that's a whole other kettle of fish.
P3
Pressure altimetry is eferenced to the airmass which as you know is rarely "standard". Especially when flying over the course of a long task in changing conditions (remember - "from high to low or hot to cold look out below"), it's not uncommon to have several hundred feet of change in altitude indication. Unless you have a current altimeter setting from your target airport (or at least one nearby) you've got a big unknown to deal with.
If the question was really "how do the systems in my cockpit work", that's a whole other kettle of fish.
P3
chip.b...@gmail.com
Jul 7, 2016, 11:41:54 AM7/7/16
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Agree with P3. The question is: given one indicated altitude (based on pressure, whether from a mechanical altimeter or modern device employing a pressure transducer) that is 1,000 feet lower than another indicated altitude (GPS, which is arguably a lot closer to the actual geometric altitude), which one should I use for an aggressive, 50+ mile final glide into the blue where I may not get another chance to climb?
Thanks to Craig Funston for the Mike Borgelt article. Mike's RAS postings are actually one of the sources I'd consulted before I posted but this article is pretty definitive: "your glider cares about GEOMETRIC [edit: GPS] altitude when it comes to the distance you can glide at a certain glide angle."
So...assuming that Mike is correct (and I have a lot of respect for his views) I must assume that the glider will still achieve the same glide ratio out west on a hot day at higher altitude (than back east in cooler temps at lower altitudes)...but at a higher true airspeed accounting for the much lower density of the air.
So it's OK to set off with the GPS altitude in hand and fly aggressively (guided by indicated airspeed) even though my trusted pressure-altitude-based glide computer says I'm 1,000 ft. lower when I start.
Does that make sense?
Thanks to Craig Funston for the Mike Borgelt article. Mike's RAS postings are actually one of the sources I'd consulted before I posted but this article is pretty definitive: "your glider cares about GEOMETRIC [edit: GPS] altitude when it comes to the distance you can glide at a certain glide angle."
So...assuming that Mike is correct (and I have a lot of respect for his views) I must assume that the glider will still achieve the same glide ratio out west on a hot day at higher altitude (than back east in cooler temps at lower altitudes)...but at a higher true airspeed accounting for the much lower density of the air.
So it's OK to set off with the GPS altitude in hand and fly aggressively (guided by indicated airspeed) even though my trusted pressure-altitude-based glide computer says I'm 1,000 ft. lower when I start.
Does that make sense?
marc....@gmail.com
Jul 7, 2016, 1:20:02 PM7/7/16
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On Thursday, July 7, 2016 at 8:41:54 AM UTC-7, chip.b...@gmail.com wrote:
> So...assuming that Mike is correct (and I have a lot of respect for his views) I must assume that the glider will still achieve the same glide ratio out west on a hot day at higher altitude (than back east in cooler temps at lower altitudes)...but at a higher true airspeed accounting for the much lower density of the air.
>
> So it's OK to set off with the GPS altitude in hand and fly aggressively (guided by indicated airspeed) even though my trusted pressure-altitude-based glide computer says I'm 1,000 ft. lower when I start.
>
> Does that make sense?
Yes, that is correct. Old-school glide computers used pressure altitude because that's all they had. My homebrew glide software always used GPS altitude, and it was as accurate as could be expected (allowing for localized lift/sink) in western US soaring conditions. With a proper polar, it nailed more than one 75+ mile final glide into still air.
> So...assuming that Mike is correct (and I have a lot of respect for his views) I must assume that the glider will still achieve the same glide ratio out west on a hot day at higher altitude (than back east in cooler temps at lower altitudes)...but at a higher true airspeed accounting for the much lower density of the air.
>
> So it's OK to set off with the GPS altitude in hand and fly aggressively (guided by indicated airspeed) even though my trusted pressure-altitude-based glide computer says I'm 1,000 ft. lower when I start.
>
> Does that make sense?
Just to reiterate, GPS altitude is a short term noisy, long term stable measure of geometric altitude (which is what you want). Pressure altitude is a long term noisy measure of pressure altitude (which you don't want). My software actually blends the two, essentially continuously recalibrating pressure altitude against a 2 minute moving average of GPS altitude, producing a stable short term measure of geometric altitude, which (to me) is the ideal. I assume most modern glide software and computers do something similar.
Marc
2G
Jul 18, 2016, 1:43:19 AM7/18/16
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http://www.xcmag.com/2011/07/gps-versus-barometric-altitude-the-definitive-answer/
The differences are substantial, as is discussed in:
http://www.icao.int/APAC/Meetings/2012_SEA_BOB_ADSB_WG8/WP06_HKG%20AI.%205%20-%20Use%20of%20Barometric%20Altitude.pdf
One tends to think that GPS, being digital, is more accurate, but this is misleading. Myself, I would take the more conservative number (pressure altitude) and consider it an insurance policy. Final glides from that altitude are, by the very nature, long and subject to much error.
Tom
Dave Nadler
Jul 18, 2016, 8:27:00 AM7/18/16
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On Monday, July 18, 2016 at 1:43:19 AM UTC-4, 2G wrote:
> There are errors in both methods of measuring altitude, as is discussed in:
>
> http://www.xcmag.com/2011/07/gps-versus-barometric-altitude-the-definitive-answer/
>
> The differences are substantial, as is discussed in:
>
> http://www.icao.int/APAC/Meetings/2012_SEA_BOB_ADSB_WG8/WP06_HKG%20AI.%205%20-%20Use%20of%20Barometric%20Altitude.pdf
>
> One tends to think that GPS, being digital, is more accurate, but this is misleading. Myself, I would take the more conservative number (pressure altitude) and consider it an insurance policy. Final glides from that altitude are, by the very nature, long and subject to much error.
>
> Tom
Excellent references; Thanks Tom!
> There are errors in both methods of measuring altitude, as is discussed in:
>
> http://www.xcmag.com/2011/07/gps-versus-barometric-altitude-the-definitive-answer/
>
> The differences are substantial, as is discussed in:
>
> http://www.icao.int/APAC/Meetings/2012_SEA_BOB_ADSB_WG8/WP06_HKG%20AI.%205%20-%20Use%20of%20Barometric%20Altitude.pdf
>
> One tends to think that GPS, being digital, is more accurate, but this is misleading. Myself, I would take the more conservative number (pressure altitude) and consider it an insurance policy. Final glides from that altitude are, by the very nature, long and subject to much error.
>
> Tom
Papa3
Jul 20, 2016, 9:07:28 AM7/20/16
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On Monday, July 18, 2016 at 1:43:19 AM UTC-4, 2G wrote:
> One tends to think that GPS, being digital, is more accurate, but this is misleading. Myself, I would take the more conservative number (pressure altitude) and consider it an insurance policy. Final glides from that altitude are, by the very nature, long and subject to much error.
>
> Tom
Tom,
>
> Tom
Having a little difficulty reconciling your point that using pressure altitude is "more conservative." Having wrestled extensively with this topic when we were introducing GPS Position Recorders as an accepted method for documenting badge flights, I can summarize the two papers even more succinctly:
- GPS Altitude and Pressure Altitude are two different frames of reference.
- Airspace and related navigation constructs have been based on Pressure Altitude since the beginning of aviation since that was the only technology available.
- Therefore, you can't mix-and-match and expect things to work out easily (e.g. penalizing pilots for airspace infractions using "just" GPS altitude).
- You need to know/understand what "altitude" your instruments are displaying and for GPS which earth model they are using.
So much for the background.
The big eye opener for me in looking at hundreds of log files was how much variability there was in Pressure Altitude recording over the course of a day. Especially for long contest and record flights, it's not uncommon to see 8 or even 10 millibar changes in pressure and 20 degrees F in airmass temperature (common during CFROPA). This translates into several hundred feet of change in altitude indication RELATIVE TO THE GROUND over the course of a flight.
"From high to low or hot to cold, look out below" came about for a reason.
So, for my money, GPS Final Glides are more repeatable since they are consistently aligned to the frame of reference I care about most - the hard stuff I'm going to run into when I run out of air underneath me.
My 0.02
Erik Mann (P3)
chip.b...@gmail.com
Jul 20, 2016, 10:53:48 AM7/20/16
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My take on this is that there are three issues (at least):
1. Accuracy of GPS altitude. Good discussion in the first paper. Thanks, Tom!
2. Remaining clear of controlled/restricted airspace. U.S. FARs are clear in spec'ing pressure altitude.
3. Managing final glides. This was my original focus, in particular in the Western U.S. where both altitudes and temperatures can be high (leading to greater differences between observed pressure altitudes and GPS altitudes) and final glides are often much longer. The latter can increase the odds that, owing to both distance and time, a pilot may transition from an area with lift to an area where there is less or no lift, making it difficult to salvage a final glide that is falling short at the end.
Pressure altitude is conservative in the sense that it often under reads the geometric altitude and--at least out West--will therefore provide a cushion against unforeseen sink. But I already have an explicit arrival height safety margin. Layering that with an uncertain additional cushion isn't where I wanted to go.
That said, the GPS vs. pressure altitude cushion has the virtue that it tends to increase with altitude. Final glide computers I've used allow entering an arrival (safety) altitude of X feet, without regard to MC setting or altitude or length of glide. There are ways to make it proportional (e.g., % bugs or % risk) but using pressure vs. GPS altitude can do the same thing. Of course, the differences will vary from day to day so that introduces more uncertainty into an area that already has plenty of it. I'd rather nail the altitude (subject to the uncertainties of GPS determination) and factor in the safety factor(s) explicitly myself.
Thanks for all the input. I learned some interesting things.
1. Accuracy of GPS altitude. Good discussion in the first paper. Thanks, Tom!
2. Remaining clear of controlled/restricted airspace. U.S. FARs are clear in spec'ing pressure altitude.
3. Managing final glides. This was my original focus, in particular in the Western U.S. where both altitudes and temperatures can be high (leading to greater differences between observed pressure altitudes and GPS altitudes) and final glides are often much longer. The latter can increase the odds that, owing to both distance and time, a pilot may transition from an area with lift to an area where there is less or no lift, making it difficult to salvage a final glide that is falling short at the end.
Pressure altitude is conservative in the sense that it often under reads the geometric altitude and--at least out West--will therefore provide a cushion against unforeseen sink. But I already have an explicit arrival height safety margin. Layering that with an uncertain additional cushion isn't where I wanted to go.
That said, the GPS vs. pressure altitude cushion has the virtue that it tends to increase with altitude. Final glide computers I've used allow entering an arrival (safety) altitude of X feet, without regard to MC setting or altitude or length of glide. There are ways to make it proportional (e.g., % bugs or % risk) but using pressure vs. GPS altitude can do the same thing. Of course, the differences will vary from day to day so that introduces more uncertainty into an area that already has plenty of it. I'd rather nail the altitude (subject to the uncertainties of GPS determination) and factor in the safety factor(s) explicitly myself.
Thanks for all the input. I learned some interesting things.
Eric Greenwell
Jul 20, 2016, 1:51:24 PM7/20/16
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chip.b...@gmail.com wrote on 7/20/2016 7:53 AM:
> 3. Managing final glides. This was my original focus, in particular
> in the Western U.S. where both altitudes and temperatures can be high
> (leading to greater differences between observed pressure altitudes
> and GPS altitudes) and final glides are often much longer. The latter
> can increase the odds that, owing to both distance and time, a pilot
> may transition from an area with lift to an area where there is less
> or no lift, making it difficult to salvage a final glide that is
> falling short at the end.
>
> Pressure altitude is conservative in the sense that it often under
> reads the geometric altitude and--at least out West--will therefore
> provide a cushion against unforeseen sink. But I already have an
> explicit arrival height safety margin. Layering that with an
> uncertain additional cushion isn't where I wanted to go.
>
> That said, the GPS vs. pressure altitude cushion has the virtue that
> it tends to increase with altitude. Final glide computers I've used
> allow entering an arrival (safety) altitude of X feet, without regard
> to MC setting or altitude or length of glide. There are ways to make
> it proportional (e.g., % bugs or % risk) but using pressure vs. GPS
> altitude can do the same thing. Of course, the differences will vary
> from day to day so that introduces more uncertainty into an area that
> already has plenty of it. I'd rather nail the altitude (subject to
> the uncertainties of GPS determination) and factor in the safety
> factor(s) explicitly myself.
Using GPS would remove the altitude uncertainty, but still leaves the
> 3. Managing final glides. This was my original focus, in particular
> in the Western U.S. where both altitudes and temperatures can be high
> (leading to greater differences between observed pressure altitudes
> and GPS altitudes) and final glides are often much longer. The latter
> can increase the odds that, owing to both distance and time, a pilot
> may transition from an area with lift to an area where there is less
> or no lift, making it difficult to salvage a final glide that is
> falling short at the end.
>
> Pressure altitude is conservative in the sense that it often under
> reads the geometric altitude and--at least out West--will therefore
> provide a cushion against unforeseen sink. But I already have an
> explicit arrival height safety margin. Layering that with an
> uncertain additional cushion isn't where I wanted to go.
>
> That said, the GPS vs. pressure altitude cushion has the virtue that
> it tends to increase with altitude. Final glide computers I've used
> allow entering an arrival (safety) altitude of X feet, without regard
> to MC setting or altitude or length of glide. There are ways to make
> it proportional (e.g., % bugs or % risk) but using pressure vs. GPS
> altitude can do the same thing. Of course, the differences will vary
> from day to day so that introduces more uncertainty into an area that
> already has plenty of it. I'd rather nail the altitude (subject to
> the uncertainties of GPS determination) and factor in the safety
> factor(s) explicitly myself.
two most important ones: wind and lift/sink.
I really noticed the effect of wind while flying at Parowan in June: my
vario nav page used the vario's "live wind" (updated several times a
minute) it computes from it's sensors, while the flight computer used
the wind obtained while circling. When 30-50 NM from the goal, I saw
variations in "altitude required" of as much as 2000'.
--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm
http://soaringsafety.org/prevention/Guide-to-transponders-in-sailplanes-2014A.pdf
2G
Jul 22, 2016, 12:02:44 AM7/22/16
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I was only offering my opinion on which to use; you, of course, can chose whatever you feel comfortable with. You can always make your glide computer more conservative; I did it today by specifying 25% bugs when I had virtually none. Viola, the safety altitude dropped by 2,500 ft. (and it matched the margin my Oudie came up with)!
We have been used to using pressure altitude. Now, GPS gives us a new, higher altitude. What a blessing! Now you can arrive at the airport at a reasonable altitude and not waste all that time climbing another 2,000 ft. I think that if you have been getting back safely with pressure altitude why change? Remember, the ground doesn't care which method you use.
Tom
chip.b...@gmail.com
Jul 22, 2016, 1:34:36 AM7/22/16
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Tom,
Since I raised the original question, I'll jump in again to clarify. I'm used to using pressure altitude, too. And, frankly, back east where I usually fly, it doesn't make much difference. GPS and pressure altitude are normally almost identical at the altitudes where we operate.
Transported to Nephi, I found that GPS altitude was 1,000' or more higher than pressure altitude at 17,000'. What to believe? Which to use?
I played it conservatively and used pressure altitude. And my final glides were, as a result, conservative, perhaps overly so. At a contest, that conservatism can be costly in terms of points. And I like to know what the "raw" number is for required altitude and then adjust that for uncertainty, expected/unexpected sink or lift, changing winds, and other imponderables, including the risk/reward of cutting it too close after an otherwise good flight.
Since I raised the original question, I'll jump in again to clarify. I'm used to using pressure altitude, too. And, frankly, back east where I usually fly, it doesn't make much difference. GPS and pressure altitude are normally almost identical at the altitudes where we operate.
Transported to Nephi, I found that GPS altitude was 1,000' or more higher than pressure altitude at 17,000'. What to believe? Which to use?
I played it conservatively and used pressure altitude. And my final glides were, as a result, conservative, perhaps overly so. At a contest, that conservatism can be costly in terms of points. And I like to know what the "raw" number is for required altitude and then adjust that for uncertainty, expected/unexpected sink or lift, changing winds, and other imponderables, including the risk/reward of cutting it too close after an otherwise good flight.
Papa3
Jul 22, 2016, 2:12:18 PM7/22/16
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On Friday, July 22, 2016 at 12:02:44 AM UTC-4, 2G wrote:
>
> Erik,
>
> I was only offering my opinion on which to use; you, of course, can chose whatever you feel comfortable with. You can always make your glide computer more conservative; I did it today by specifying 25% bugs when I had virtually none. Viola, the safety altitude dropped by 2,500 ft. (and it matched the margin my Oudie came up with)!
>
Fair point. In the East, most of the time the difference between GPS and Pressure altitude is probably of second-order importance to a final glide "accuracy". For instance, the difference between an MC0 (very aggressive) and MC3 (slightly conservative) altitude buffer is probably one order of magnitude greater for most of our final glides.
>
> Erik,
>
> I was only offering my opinion on which to use; you, of course, can chose whatever you feel comfortable with. You can always make your glide computer more conservative; I did it today by specifying 25% bugs when I had virtually none. Viola, the safety altitude dropped by 2,500 ft. (and it matched the margin my Oudie came up with)!
>
Assuming I don't melt first tomorrow, I may play around with the instruments to see how much the displays differ (Clearnav, steam gauge altimeter, Tophat on my Samsung).
P3
2G
Jul 23, 2016, 12:40:58 PM7/23/16
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I have two glide computers; an Oudie 2 and a Air Avionics Display S (formerly called Butterfly Vario). The Oudie definitely uses GPS altitude. The Butterfly has both pressure and GPS altitude. Which one is used for glide computations? I don't know, but the Butterfly was always much more optimistic than the Oudie. The Butterfly has the distinct advantage of having a much more accurate measurement of real-time wind, which obviously has a direct impact on the glide computation.
Over time I came to trust the Oudie more than the Butterfly, even tho the Oudie suffered from bad wind estimates. I recently "adjusted" the Butterfly by bumping up the bug factor to 25%. Final glides in the mountains that I fly in the West are more problematic than over flat lands due to the strong influence of the terrain. The cost/benefit that I apply is not what you use, which I believe is your finish position in a contest. My cost/benefit is surviving the flight by having a high probability of making the final glide. And, unless you have done a retrieve out in the very bad lands of Nevada as I have done, I don't think you can appreciate my concerns.
This is a little off topic, but anyone flying cross country should be prepared to spend a night in the glider. This means having sufficient drinking water and clothing appropriate for the terrain you fly over. Carrying either a SPOT or an In Reach is a huge safety advantage. I also carry a handheld radio and a first aid kit. Do not expect to have cell phone coverage after a landout (actually expect to NOT have cell phone coverage, as happened to an Ely pilot two days ago in an area with cell coverage, but with a different provider).
Tom
Ramy
Jul 23, 2016, 2:32:09 PM7/23/16
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One conclusion is that our gliders perform worse than we normally measure by about 5% since the true loss of altitude is bigger than the loss of pressure altitude by about 5% on average.
Ramy
Ramy
chip.b...@gmail.com
Jul 23, 2016, 7:16:37 PM7/23/16
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Tom,
I suspect I was more conservative than some of my fellow pilots at Nephi. The point I was trying to make is that I want to be able to factor in that insurance myself, not have an unknown amount added because of using pressure vs. GPS altitude. Thanks to you and others for the insights about that subject.
For the record, yes, I've retrieved out of the badlands of NV...before mobile phones were in use. :) For years I've flown with lightweight hiking boots not just for a possible long walk out but to help protect my ankles in case I have to bail out. I carry extra drinking water, clothing that will take me thru a cold night, signal mirror, snakebite kit, an ELT, and recently (thanks to my brother, Mark), a SPOT tracker. I've also wired my ship's power so I can charge my mobile phone battery: the only thing worse than no cell coverage is having it but not an operable phone. I have made some dumb decisions over the years, including recently, but I don't intentionally take chances with safety. Contest points are not worth risking my life. Neither do I want to "buy" two or three insurance policies when one will do.
I suspect I was more conservative than some of my fellow pilots at Nephi. The point I was trying to make is that I want to be able to factor in that insurance myself, not have an unknown amount added because of using pressure vs. GPS altitude. Thanks to you and others for the insights about that subject.
For the record, yes, I've retrieved out of the badlands of NV...before mobile phones were in use. :) For years I've flown with lightweight hiking boots not just for a possible long walk out but to help protect my ankles in case I have to bail out. I carry extra drinking water, clothing that will take me thru a cold night, signal mirror, snakebite kit, an ELT, and recently (thanks to my brother, Mark), a SPOT tracker. I've also wired my ship's power so I can charge my mobile phone battery: the only thing worse than no cell coverage is having it but not an operable phone. I have made some dumb decisions over the years, including recently, but I don't intentionally take chances with safety. Contest points are not worth risking my life. Neither do I want to "buy" two or three insurance policies when one will do.
Eric Greenwell
Jul 24, 2016, 10:18:44 AM7/24/16
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2G wrote on 7/23/2016 9:40 AM:
> This is a little off topic, but anyone flying cross country should be
> prepared to spend a night in the glider. This means having sufficient
> drinking water and clothing appropriate for the terrain you fly over.
> Carrying either a SPOT or an In Reach is a huge safety advantage. I
> also carry a handheld radio and a first aid kit. Do not expect to
> have cell phone coverage after a landout (actually expect to NOT have
> cell phone coverage, as happened to an Ely pilot two days ago in an
> area with cell coverage, but with a different provider).
I agree with Tom. I'm always surprised by the pilots I see climbing into
> This is a little off topic, but anyone flying cross country should be
> prepared to spend a night in the glider. This means having sufficient
> drinking water and clothing appropriate for the terrain you fly over.
> Carrying either a SPOT or an In Reach is a huge safety advantage. I
> also carry a handheld radio and a first aid kit. Do not expect to
> have cell phone coverage after a landout (actually expect to NOT have
> cell phone coverage, as happened to an Ely pilot two days ago in an
> area with cell coverage, but with a different provider).
their gliders in sandals or shorts. Sure, it's hot during the day, but
even the desert can get uncomfortably cold at night. I know some of them
don't even have a jacket in the glider, either.
I don't routinely carry a transceiver, but my inReach does allow two-way
communication anywhere in the desert.
Dan Marotta
Jul 24, 2016, 11:28:05 AM7/24/16
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Very good points!
As to cell coverage, you might be interested in Republic Wireless. It
works with all cell providers and with wifi. Note, I'm not connected in
any way with them and I will realize no benefit (other than the
satisfaction of a possible save) should anyone switch.
--
Dan, 5J
As to cell coverage, you might be interested in Republic Wireless. It
works with all cell providers and with wifi. Note, I'm not connected in
any way with them and I will realize no benefit (other than the
satisfaction of a possible save) should anyone switch.
Dan, 5J
chip.b...@gmail.com
Jul 24, 2016, 11:41:07 AM7/24/16
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Another survival kit item I forgot to mention: a space blanket or heat sheet. Mine is 35 years old (!). But I have a closetful from marathons and they work just as well to stay warm, protect from rain, etc.
Message has been deleted
dbra...@gmail.com
Jul 25, 2016, 3:40:57 PM7/25/16
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Actually, It's not always that cold at night in the NV desert. On those high pressure days we fly this time of years where it's still 80 to 90 at 10pm.. It's very comfortable in the middle of the night. But if a low swings through, I'll grant you that's when it gets quite chilly.
Eric Greenwell
Jul 25, 2016, 7:21:22 PM7/25/16
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(5000' and up). This link
http://www.ely-nevada.climatemps.com/temperatures.php
shows the average low temperature near Ely in Jul/Aug is in the mid-40s.
If it's breezy, that will feel cold, and very cold on a below average
night, especially in shorts and a tee shirt.
dbra...@gmail.com
Jul 25, 2016, 8:25:56 PM7/25/16
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One time at Ely when they had the fire on the mountain to the east, the firefighters also camped out with us. No tents, they just slept next to the truck on ground. I think that was early June.
dbra...@gmail.com
Jul 25, 2016, 8:29:22 PM7/25/16
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On Monday, July 25, 2016 at 5:25:56 PM UTC-7, dbra...@gmail.com wrote:
> One time at Ely when they had the fire on the mountain to the east, the firefighters also camped out with us. No tents, they just slept next to the truck on ground. I think that was early June.
sorry, I meant early July.
> One time at Ely when they had the fire on the mountain to the east, the firefighters also camped out with us. No tents, they just slept next to the truck on ground. I think that was early June.
Eric Greenwell
Jul 25, 2016, 9:35:41 PM7/25/16
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2G
Jul 25, 2016, 10:40:24 PM7/25/16
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The saying goes like this: plan for the worst and hope for the best, not the other way around.
Tom
kirk.stant
Jul 26, 2016, 9:27:35 AM7/26/16
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> I CAN'T BELIEVE some of these posts! Do you REALLY think that your are going to land out ONLY on the warm nights? One night this month in Nevada we had a FREEZE WARNING! That's right, a freeze warning.
>
> The saying goes like this: plan for the worst and hope for the best, not the other way around.
>
> Tom
Let's get real here. When you are taking off in over 100 temps, you are not going to be wearing clothes for a cold night in the mountains! And some cockpits simply don't have the room for all the stuff some of you say you carry (Ls6 & 8s come to mind).
>
> The saying goes like this: plan for the worst and hope for the best, not the other way around.
>
> Tom
So what. You should have everything you need to survive strapped to your butt - your chute. If it gets chilly, pop that expensive seat cushion and get some use out of it!
So carry a Spot, wear a chute with a mirror attached, and go fly...
Kirk
66
JS
Jul 26, 2016, 11:18:39 AM7/26/16
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All of this has a profound effect on final glide calculations.
Jim
Dan Marotta
Jul 26, 2016, 12:49:37 PM7/26/16
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I also carry a flint and steel which I acquired at survival school back
in '74. It still works and I haven't needed it (yet).
--
Dan, 5J
in '74. It still works and I haven't needed it (yet).
Dan, 5J
2G
Jul 27, 2016, 12:37:04 AM7/27/16
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On Tuesday, July 26, 2016 at 6:27:35 AM UTC-7, kirk.stant wrote:
Bottom line: there IS NO substitute for being prepared. You WILL have to deal with WHATEVER situation you encounter with the equipment that you launch with. Having done some serious expeditionary type mountaineering in my younger years I am somewhat more attuned to this issue than you seem to be.
Tom
kirk.stant
Jul 27, 2016, 12:40:19 PM7/27/16
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On Tuesday, July 26, 2016 at 11:37:04 PM UTC-5, 2G wrote:
> You can wear Jockey shorts and a tank top for all I care. Myself, I WILL wear full length pants and a long sleeved shirt and shoes that I can make a long hike in. Plus I will have a jacket, extra water, flash light, fire starter, whistle, first aid kit, extra food and a handheld radio. On top of that I have the chute option that you mentioned.
With the exception of a jacket and a handheld, I carry pretty much the same (stripped down to the bare minimum); if it's chilly I may even throw in a jacket. But if it's a hot, humid day I'll dress accordingly. I agree that you have to think about survival, but that doesn't mean carrying a tent with you!
> You can wear Jockey shorts and a tank top for all I care. Myself, I WILL wear full length pants and a long sleeved shirt and shoes that I can make a long hike in. Plus I will have a jacket, extra water, flash light, fire starter, whistle, first aid kit, extra food and a handheld radio. On top of that I have the chute option that you mentioned.
> Bottom line: there IS NO substitute for being prepared. You WILL have to deal with WHATEVER situation you encounter with the equipment that you launch with. Having done some serious expeditionary type mountaineering in my younger years I am somewhat more attuned to this issue than you seem to be.
Cheers,
Kirk
66
herbk...@gmail.com
Jul 27, 2016, 6:00:31 PM7/27/16
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Herb
kirk.stant
Jul 28, 2016, 11:37:50 AM7/28/16
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> Kirk, pry yourself away from the stinkin' prairie of IL (I know it well) and come fly with us out West occasionally. You'll get an appreciation for what Tom is saying. But wait, during your military career you must have spent some time over the high desert, right?
> Herb
Herb, I've got plenty of time over the high desert during 20 years flying out of Phoenix. Got my diamonds (all three - I hate paperwork) out of Turf to the Grand Canyon and back. Flew to Parowan one Sunday on a whim in my LS6, and took the first tow the following day and flew back to Turf. I've raced at Moriarty and El Tiro, and explored most of Arizona by glider.
> Herb
Lots of really scary tiger country out there - to go with the awesome lift (most of the time...). Planning for a landout was not something to take lightly!
And yes, BTDT in jets, too.
Sure, my use of O2 has gone down a lot since I've moved to StL, but while different, the flying is just as much fun...
cheers,
Kirk
66
2G
Jul 28, 2016, 4:50:32 PM7/28/16
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Yes, I DO fly over a lot of really scary terrain in the middle of Nevada (check it out with Google Earth) where you can be 50 or more miles from the next human. No, I didn't list a tent in my survival gear, but a space blanket could suffice.
Tom
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