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gmcjetpilot(at)yahoo.com
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 Posted: Tue Sep 26, 2006 2:37 pm Post subject: Van's gauges vs Dynon (ALTIMETRY) |
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OH MY GOSH!
GPS has nothing to do with altimeter settings.
First if your altimeter is with in 20 feet of field with local QNH (Baro setting from tower, atis or approved weather observer) YOU ARE DOING GREAT.
IF you want to get rid of that altimeter send it to me, please, email me, I'll take it.
The FAR limit is +/- 75 feet or 150 feet total spread.
When comparing two altimeters, what ever temp or humidity errors there are, they are the same on both, so that washes out.
In part 135 and 121 there are dual altimeter limits and they are set by the manufacture, but typically they can be as much as 100 to 150 feet, e.g., one can be 75 feet low and one 75 feet high from field.
Small Part 91 GA planes don't have dual instrument requirement, unless fitted with an advance flight deck, for example RVSM (Reduced Vertical Separation Min from 2000' to 1000 from FL290-FL410).
Bottom line it's the +/- 75 rule.
ALSO where YOU really AT the field elevation ref point????? Airport elevation at the same field can be way different from one spot to another on the field. May be you where 20 feet lower than you thought and the mechanical altimeter was perfect and the dynon was off 22 feet or what ever?
ALSO altimeters have non-linear error with altitude. It must be accurate within 75 feet when set on the ground, but it must also be accurate with 75 feet throughout. The error can very as you climb. An altimeter may be off 2 feet at sea level and 100 feet at FL180. That is why altimeters are bench tested throughout their range, not just at one altitude.
Please, any one think GPS is used, read your flight manual on altimeters or write me. I am sure EFIS altimeters are just based just on corrected pressure, no magic.
The Dynon comment just means it does not compensate for non standard conditions. GPS altitude can be very inaccurate based on the angles and position of the satellites. All you have to do is look at the altitudes your gps gives you.
We fly INDICATED ALTITUDE. If it is a perfect standard day and atmosphere, than that is hight above MSL. However with non standard, pressures, lapse rate, temps and humidity we have errors that make use higher or lower from true MSL. That is why we have the old saying:
FLY from HOT to COLD, HIGH to LOW, LOOK out BELOW.
The biggest driver is pressure, than temp, than humidity. Humidity has the effect of making the air less dense.
You are lower than you think based on indicated altitude, which is reading higher than normal or standard conditions. Thus every 100 nm reset your Kollsman window.
Regardless all altimeters are set the same BARO or QNH (below FL180) or QNA (29.92 above FL180), so we are all on the same page. There are no special corrections by EFIS altimeters vs mechanical.
THERE are special ATC and IFR procedures for extreme temps and pressures, but that is beyond the topic at hand, simple altimetry is all we have, which can be confusing.
The original posters errors mean nothing. They are indeed both good and very acceptable. Their error may also change from day to day at different conditons, temps or field elevations. The electronic transducer in the dynon may have some temp compensation issues that might increase its error? Heep in mind the altimeter may be TSO'ed and the Dynon not. I have a Dynon BTW in a RV-7 so I like it and I do not use a back-up for VFR flight.
Cheers George CFI-II-MEI, ATP
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From: Gerry Filby <gerf(at)gerf.com>
--> RV-List message posted by: Gerry Filby <gerf(at)gerf.com>
At one point this weekend the barometer at the field where I'm
building was exactly 29.92 - the Dyon EFIS read 62 feet, the
official field elevation is 66 feet. I also installed one of
Van's "steam gauge" Altimeters - supposedly a fallback
instrument if my wiring/plumbing goes belly up on the first
flight. The steam gauge reads minus 20 feet :-\
I know which gauge will be junked out of my panel at the first
opportunity ...
__g__
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[quote][b]
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n5lp(at)warpdriveonline.c
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| Posted: Tue Sep 26, 2006 3:46 pm Post subject: Van's gauges vs Dynon (ALTIMETRY) |
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On Sep 26, 2006, at 4:35 PM, <gmcjetpilot(at)yahoo.com> wrote:
| Quote: |
The Dynon comment just means it does not compensate for non
standard conditions. GPS altitude can be very inaccurate based on
the angles and position of the satellites. All you have to do is
look at the altitudes your gps gives you.
Well if you compare your GPS altitude to altimeter altitude it does
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vary. If I am flying I am going to use the altimeter and if I want
to know the actual height of a mountain I am going to use GPS. These
are different missions and each method has its strengths.
Our aircraft altimeters are not good at all at telling us our actual
height above sea level. This is because the atmospheric profile is
rarely standard. If temperatures aloft are cold, we will be lower
than the indication. It doesn't matter too much as long as we don't
fly too close to the mountain. Note the required clearances for IFR
flight.
If for some reason I want to actually now how high I am, I will go
with GPS. I check the automobile one at every marked elevation and
rarely see an error of more than 20 feet. Really pretty extraordinary.
Larry Pardue
Carlsbad, NM
RV-6 N441LP Flying
http://n5lp.net
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