flyrotary@lancaironline.net Mailing List Archive

On Thu, Sep 24, 2015 at 8:57 AM, Finn Lassen <flyrotary@lancaironline.net> wrote:

Off topic, except it's in a 13B Van's RV-3 ;-)

Again, after flying home from Sun'n'Fun, ATC saw me several hundred feet below what I saw on my altimeter.

Last week I finally got around to removing all the screws that holds the fuselage top over the instruments.

I hooked LEDs across the data lines that run from the altitude encoder to the transponder (gray code).
(note that 1 is actually 0 volts -- active low).

I checked METARs at nearby airports -- CTY, GNV... and I know my elevation exactly.
That matched what my Dynon D10A and steam altimeter showed.

I then spent several hours adjusting the high and low pots on the alt encoder until the codes changed within 10 to 20 feet of the 50 foot points when applying vacuum to the static system with a syringe. Can't get better that that, I though.

Alas, after replacing the fuselage top and multitude of screws, yesterday I hopped over to Cross City (CTY) a mere 15 miles away. I had to set the altimeter to 0.03 or 0.04 below what their METAR reported in order to get altimeter to show their field elevation -- 42 feet. Basically shows 30 to 50 feet too high altitude if I set the Dynon to the reported pressure. All that work for nothing!

Now I'm beginning to suspect that the pressures reported by the METARs are not all that precise.
Checking METARS right now at nearby airports, they range from 29.98 to 30.02.

Any suggestions on how to obtain an accurate air pressure reference?

Finn

Fly to a field that has atc & radar on the field, land, & get them to check your reporting while on the ground. (Fixed, known altitude)

Transponders are really finicky critters, and ATC's radars are even worse. Dirty contacts in the coax path from xponder to antenna, dirty/oily antenna, etc can all cause transmit errors, and atc is always a crap shoot, no matter what they tell you. Memphis Approach used to consistently report 'errors' of various types to a/c that had no issues in other atc areas.

Another issue is static port accuracy while in flight. It can introduce asignificant error at speed, if not set up right. Easiest way to see the error is to set your altimeter to your field elevation (or zero, if you prefer, but don't forget you did that), and fly a high speed low pass over the field. Steal a quick look at the altimeter while down low and fast. If it says you're lower (or higher) than you'd expect for your 10-20 foot altitude above field altitude, you have a static error problem. If it indicates lower altitude than expected, your airspeed indication is likely low as well. Correct the static and increase your airspeed by 10 knots, with no additional fuel burn. :-)

Charlie