Final Glides: GPS or Pressure Altitude?

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.

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

On Thursday, July 21, 2016 at 10:34:36 PM UTC-7, wrote:
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.

Chip Bearden
ASW 24 "JB"

Chip,

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

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

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.

Chip Bearden
ASW 24 "JB"

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
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.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "A Guide to Self-Launching Sailplane Operation"

https://sites.google.com/site/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/...anes-2014A.pdf

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.

On 7/23/2016 10:40 AM, 2G wrote:
On Thursday, July 21, 2016 at 10:34:36 PM UTC-7, wrote:
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.

Chip Bearden
ASW 24 "JB"
Chip,

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

--
Dan, 5J

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.

Chip Bearden
ASW 24 "JB"

Dress to egress. I always wear long sleeves, pants and light weight walking boots.

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.

wrote on 7/25/2016 12:40 PM:
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.

I'm sure that's true in the low desert, but it's not in the high desert
(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.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "A Guide to Self-Launching Sailplane Operation"

https://sites.google.com/site/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/...anes-2014A.pdf