Final Glides: GPS or Pressure Altitude?

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


On Wednesday, July 6, 2016 at 1:20:19 PM UTC-4, wrote:
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"