On Wednesday, July 20, 2016 at 6:07:28 AM UTC-7, Papa3 wrote:
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,

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)

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)!

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