The altitude display on my vario seems to be the GPS altitude (inaccurate) instead of the barometric altitude (more accurate). I was able to prove this on my last flight: after topping out a thermal I looked at my vario and announced on the radio an altitude of 6350' and remembered seeing as much as 6400'. But when I downloaded the flight via Flychart I can clearly see that the highest I got was 6150' (barometric) and the GPS plot line seems to match what I was seeing/reporting.
I have read through the vario manual and have not seen any info on how to make sure the vario only reports/displays barometric altitude. Google searches have not yielded any help on this either.
Anybody have any ideas on this? Thanks, Chad
p.s. - looks like I actually left Middle T at 4285' instead of 4500' but the glide went well.
Flytec 5030 Displaying GPS Altitude
Moderator: Chip
Call Steve at Flytec
I'm sure there are people in the Club that can help but otherwise all Flytec.
I can't help you with the vario, but there is a misconception in your post.
GPS altitude is not less accurate than barometric altitude. Sometimes it is better, sometimes worse. More to the point, it doesn't "drift" as the weather changes, you move into different air-masses, or they move by you. It doesn't depend on inversions, non-standard days, etc. By definition, a good thermally day is a non-standard day, and pressure altitude will be more and more incorrect as you climb.
However, flight levels are determined by pressure altitude, not GPS altitude. So legal and safety concerns push towards using pressure altitude. Is it more important to know if you are at GPS altitude 10,000 feet, or if you are where the big, fast airplanes will be in your way Flight level 100?
I was forced to become familiar with this difference in order to defend the officialness of my longest flight. The GPS said I didn't break the law, the pressure altitude said I "may" have. The FAI (world body) accepted GPS, the NAA (U.S. body) wouldn't. It all worked out - Calibration at the factory showed my pressure altitude was reading a bit high - just enough to make me legal.
For more info on the GPS vs Pressure difference, try: http://www.xcmag.com/2011/07/gps-versus ... ve-answer/
BTW, I've seen pressure altitudes off by over 500 feet on occasion. That makes them much less accurate than GPS altutudes. That's not normal, but 100m, the standard "error" of a GPS is not normal either. Generally, the GPS is pretty darned close, and won't change more than 30 feet or so during the day. Pressure altitude routinely varies by quite a bit more than that. Of course, if you live (or fly) near one of the places where the calculated earth shape differs from the actual one, you'll get a consistently wrong GPS reading. But it will be consistent and correctable, unlike the drift error in pressure altitude.
GPS altitude is not less accurate than barometric altitude. Sometimes it is better, sometimes worse. More to the point, it doesn't "drift" as the weather changes, you move into different air-masses, or they move by you. It doesn't depend on inversions, non-standard days, etc. By definition, a good thermally day is a non-standard day, and pressure altitude will be more and more incorrect as you climb.
However, flight levels are determined by pressure altitude, not GPS altitude. So legal and safety concerns push towards using pressure altitude. Is it more important to know if you are at GPS altitude 10,000 feet, or if you are where the big, fast airplanes will be in your way Flight level 100?
I was forced to become familiar with this difference in order to defend the officialness of my longest flight. The GPS said I didn't break the law, the pressure altitude said I "may" have. The FAI (world body) accepted GPS, the NAA (U.S. body) wouldn't. It all worked out - Calibration at the factory showed my pressure altitude was reading a bit high - just enough to make me legal.
For more info on the GPS vs Pressure difference, try: http://www.xcmag.com/2011/07/gps-versus ... ve-answer/
BTW, I've seen pressure altitudes off by over 500 feet on occasion. That makes them much less accurate than GPS altutudes. That's not normal, but 100m, the standard "error" of a GPS is not normal either. Generally, the GPS is pretty darned close, and won't change more than 30 feet or so during the day. Pressure altitude routinely varies by quite a bit more than that. Of course, if you live (or fly) near one of the places where the calculated earth shape differs from the actual one, you'll get a consistently wrong GPS reading. But it will be consistent and correctable, unlike the drift error in pressure altitude.
Fly High; Fly Far; Fly Safe -- George