Garmin 430/530 TAS Calculator

[Michael]

Have any of you ever had strange results from your TAS calculator function on your Garmin GPS? I've been using it quite a bit to build an extensive database of altitude/TAS/GPH/power settings/temps. I've found 145 knots to be a good round figure for my TAS but have gotten some calculations to result in much higher speeds. First, as a rule I've been leaning to 100 ROP, but started experimenting with 125 ROP to see how much more HP I get and have had increased performance (in the 150 range). But on a few occasions I've had TAS reading of 160-165 knots! I checked all the numbers several times to be sure I had not made a mistake. On these occasions I was flying between 11,000 and 13,500; temps -5 to -10. Yes, my groundspeed was very fast due to a tailwind (groundspeeds were about 170). Somehow I don't think the GPS fully credited the tailwind!

Michael
97AV

[kwmoore]

I have had consistent results after my first few bungles (in 8367E). Make sure your units are configured properly (AUX pages): knots vs mph, feet vs meters, Fahrenheit vs centigrade, inches of mercury vs hectopascals (?) etc.

Altitude: in feet. Duh.

Airspeed: this should be the indicated airspeed from your ASI. Some ASI's have a "true airspeed ring," don't use this value.

Altimeter setting: another Duh.

Temperature: this is where I was getting messed up--in two ways--in 8367E. First, I was guestimating degrees centrigrade until after a few flights I changed the units to degrees F via the AUX page. Then it turned out that the OAT probe was positioned to catch warm air from the engine compartment and was reading 10-12 degrees F too high. I really thought I had a fast 260se, but the calculated winds aloft were always way off! After I compensated for the spurious OAT, 8367E flew as a 148-150 KTAS cruiser and the winds aloft were always closer to the forecast. At the low temperatures you reported, were you making sure to enter the "-" sign here?

The above is all you need for TAS. Your TAS will be independent of any winds aloft. If you then enter your compass heading it will tell you winds aloft and your headwind or tailwind component. It already knows your groundspeed, so you don't have to enter it.

[Michael]

Oh yes, I've become quite proficient with this page and have collected lots of good data. These data have just a few outliers that I had trouble accepting (but everything had been entered properly). Of course the fact that it has happened only 3 or 4 times (all on the same long X-country) it does lead to the very real possibility that I made the same error over and over; even when re-checking. Sort of like reviewing your own manuscript for mistakes over and over...some always get passed over repeatedly!

Michael
97AV