X-Virus-Scanned: clean according to Sophos on Logan.com From: "Charlie England" Received: from mail-yk0-f180.google.com ([209.85.160.180] verified) by logan.com (CommuniGate Pro SMTP 6.1.5) with ESMTPS id 8015071 for flyrotary@lancaironline.net; Thu, 24 Sep 2015 12:50:38 -0400 Received-SPF: pass receiver=logan.com; client-ip=209.85.160.180; envelope-from=ceengland7@gmail.com Received: by ykdz138 with SMTP id z138so89522871ykd.2 for ; Thu, 24 Sep 2015 09:50:02 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :content-type; bh=0h3V2gQViJ3rcfObtNAA7Cqed1RRYaEwcUuEKge77GM=; b=go4VsfCLbbil/MlaWO7MWywt3Wz586/yW+TD1mEt4IdxoBiixj3chIboeSxN/r0bq8 ZE+BBLWkmgiftXS8i85j1NGf3IrA5vjkpFvajqb/UsW/ntPzCES6mrjZhrMLwelBWe18 5GNWydGSF8KHyILT+MWALNCw1JapWj8eVYCr/wkLA/uES7q1wEjqg6ANrt/eM2s+p++z myvaRXtKY8TXroioGYHF3YmGB2Nnizw9cucldgN2zIeYO7eAnXq2FyaqGIUQONuSq7BW WHdxPctCKZ569VkdTw15dvvktxNlv3nxZRoeQ6oKmtmafck+C9QXL2wihsRR2fHu3D9V IhCA== MIME-Version: 1.0 X-Received: by 10.170.89.5 with SMTP id g5mr696992yka.68.1443113402498; Thu, 24 Sep 2015 09:50:02 -0700 (PDT) Received: by 10.129.155.15 with HTTP; Thu, 24 Sep 2015 09:50:02 -0700 (PDT) In-Reply-To: References: Date: Thu, 24 Sep 2015 11:50:02 -0500 Message-ID: Subject: Re: [FlyRotary] OT: Calibrating altimeter and alt encoder To: Rotary motors in aircraft Content-Type: multipart/alternative; boundary=001a113a0528ba044a052081056b --001a113a0528ba044a052081056b Content-Type: text/plain; charset=UTF-8 On Thu, Sep 24, 2015 at 8:57 AM, Finn Lassen 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 --001a113a0528ba044a052081056b Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable


On Thu, Sep 24, 2015 at 8:57 AM, Finn Lassen <flyrotary@lanc= aironline.net> wrote:
Off t= opic, 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 fu= selage top over the instruments.

I hooked LEDs across the data lines that run from the altitude encoder to t= he 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 encod= er 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 t= hat 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.=C2=A0 I had to set t= he altimeter to 0.03 or 0.04 below what their METAR reported in order to ge= t 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 30.0= 2.

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

Finn

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

Transponders= are really finicky critters, and ATC's radars are even worse. Dirty co= ntacts in the coax path from xponder to antenna, dirty/oily antenna, etc ca= n all cause transmit errors, and atc is always a crap shoot, no matter what= they tell you. Memphis Approach used to consistently report 'errors= 9; of various types to a/c that had no issues in other atc areas.

Another issue is static port accuracy while in flight. It c= an 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 lo= w pass over the field. Steal a quick look at the altimeter while down low a= nd 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 prob= lem. 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= =C2=A0

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