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When True Alt=Indicated Alt?

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When True Alt=Indicated Alt?

Old 08-16-2023, 09:52 PM
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Default When True Alt=Indicated Alt?

Hi to all. When would the true altitude be the same as the indicated altitude? Would it be when the sea level pressure is 29.92 and the temperature is standard for a given altitude, for example 9 degrees at 3000 ft? Is there other realistic scenario or condition where both altitudes are equal?

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Old 08-17-2023, 10:24 AM
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True alt should always be very, very close to indicated altitude in normal ops (below transition alt), assuming your altimeter setting is correct. That's how we avoid obstacles and comply with chart and ATC altitude requirements.

Temp, and to a lesser degree humidity, affect density altitude much much more than they affect indicated altitude. Those two factors can be considered negligible for indicated altitude. Indicated alt is all about air pressure.

Density altitude should equal indicated alt at standard temp for that elevation, with correct altimeter setting.
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Old 08-17-2023, 06:26 PM
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So long as everyone in a given geographic area is using the same altimeter setting, above or below the transition level, then the difference between actual altitude (true altitude) and indicated altitude does vary, sometimes considerably (anyone flying with a GPS can figure that out fairly rapidly), but it's not a concern relative to other traffic that's using the same altimeter setting, and not a concern relative to obstacles and terrain, until it gets cold. Why? Because the difference places us higher than obstacles. When it's cold, below freezing, the difference can place us at a lower true altitude than indicated, and at that point we may need to begin to apply temperature-based altitude corrections.

The transition altitude (18,000' in the US, above which all aircraft must be on 29.92--QNE for an altimeter setting, while aircraft below use a local altimeter setting--QNH) varies around the world; much of the world uses transition altitudes that range from a few thousand feet to forteen thousand or so; in Europe, one encounters transition frequently at 6,000' or less, and it various throughout Europe. Transition, therefore, isn't a good reference as to where altimetry issues, so far as a difference between true and indicated, might occur. These differences occur at all altitudes.

True altitude and indicated altitude will be the same when at field elevation, with the altimeter set to indicate field elevation. On aeronautical charts, obstacles and airport altitudes are given in true altitude. Land at a airport that's 4,572' and set your altimeter to that, and you're indicating true altitude. Now get the local altimeter setting and check yours; there may be a very slight difference. Set your altimeter to the local altimeter setting, and if your altimeter is calibrated legally, it should indicate within 75' of field elevation. Your density altitude may be higher, but your indicated altitude will be close, approximated to true altitude. As you takeoff and climb, your altimeter may continue to reflect true altitude, or it may not, given non-linear variations in temperature and pressure. In theory, so long as your altimeter is accurately baro-corrected, and so long as your altimeter is accurately calibrated and operates accurately at all altitudes, then your altimeter should be close to true altitude when set to the most accurate local altimeter setting.

In reality, the baro correction above the airport may not be the same as at the airport. In the standard world, temperature and pressure are consistent and make a linear change in a vertical column of air. In the real world, not so much. We normally don't care much until it gets cold and we're nearing obstacles or flying an approach; when the local pressure changes and local temperature changes, so does true altitude; hence, the saying "when flying from hot to cold, or high to low, look out below.

As mentioned above, while not altimetry, density altitude can be a very big factor in performance, and can create the air density of an altitude thats several thousand feet higher than our true altitude.
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