VAF Forums - Baro adjustment on 396/496 ?

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Baro adjustment on 396/496 ?

I have the 296 and I have not found a way to correct altitude for baro, hence it is alway wrong compared to my altimeter. I do not believe this exists on the 296.

Does the 396/496 allow for this correction or does it show (true?) altitude ? On my last trip my 296 was ~250ft high of the report altitude for the conditions.

Any hints or comments ? I am ready to be enlightened :D

Quote:

Originally Posted by speed (Post 166944)

Neither the 296, 396 or 496 has any kind of barometric altitude indication so they don't have an adjustment. All these units show a geometric altitude (quite accurate) that may be useful for terrain clearance but will not normally agree with barometric altitude.

Fly by your altimeter for anything involving ATC or cruising altitudes.

A little light reading on this

For those of you with pocket protectors and possibly a bandaid holding your eyeglasses together over the bridge of your nose, well you might like reading this. If you do read it and also understand it could you please break it down to something that the rest of us can grasp and elighten us?

http://www.avionicswest.com/PDFiles/alt2.pdf

Thanks,

Here is what the Garmin FAQ has to say on the subject:

Quote:

Why is the altitude on my GPS different than the altitude on my altimeter?

Regarding the altitude discrepancy, keep in mind that your altimeter does not measure altitude. What the altimeter measures is air pressure. Your altimeter is calibrated to display a certain change in altitude based on a change in air pressure. For every inch of mercury pressure change, the altimeter displays a change of approx. 1000 ft. of altitude. Think of it this way. If the plane is sitting on the ramp and shows the correct altitude when the outside air pressure is 30.00, it will show an altitude of ramp+250 if the outside air pressure drops to 29.75. It is still on the ramp, but the altimeter changed because the pressure changed. The standard lapse rate (SLR) for air pressure is 1-inch pressure = 1,000-ft altitude and this is how your altimeter is calibrated (approximately). However, when is anything in nature a perfect standard? What the SLR implies is that 1-inch will USUALLY equal 1,000-ft. depending on altitude. The higher you go, the less accurate this is. However, this is not a problem since everyone uses the same system. If everyone's altimeter is set to the same pressure setting, then they will all be off the same amount and vertical separation will be ensured. The GPS does not use air pressure to determine altitude. As such, it is not as apt to have pressure errors in determining altitude. What this means is that your GPS derived altitude is almost always more accurate than your altimeter. However, since everyone uses a pressure-based system to determine altitude, you need to fly using the pressure altimeter to ensure the required vertical separation.

mcb

Quote:

Originally Posted by Bryan Wood (Post 166961)

That was a great article. Thanks for posting. BTW, I don't have the pocket protector nor the glasses.

The 496 has an E-6B function where you can enter your temperature and and it calculates density altitude because it knows the nearest reporting station barometric pressure when the XM antenna is connected.

Regards,