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Radio Altitude question


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I am finishing up the logic for my reverse thrust mechanism and need to read absolute altitude AGL a la a radio altimeter for one of the conditions (the 737-700 RT can engage if a/c is <10' AGL via radio altimeter).

Looking at the offsets in FSInterrogate while on the ground seems to indicate that all of the offsets pertaining to altitude are reading pressure altitude (MSL).

Did I miss one that gives AGL? If not, can it be simulated with the info that is available (including maintaining accuracy even if the pilot has the wrong pressure dialed into the altimeter)?

Thanks for your help!

Scott Fausel

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Looking at the offsets in FSInterrogate while on the ground seems to indicate that all of the offsets pertaining to altitude are reading pressure altitude (MSL).

No, I don't think any of them are pressure altitude -- that isn't the same as AMSL which is the true altitude above sea level. Pressure altitudes vary according to pressure and are used in aircraft performance calculations and conversions between TAS and Mach, things like that.

Did I miss one that gives AGL? If not, can it be simulated with the info that is available (including maintaining accuracy even if the pilot has the wrong pressure dialed into the altimeter)?

The ground altitude is provided in two places -- 0020 and more accurately at 0B4C. the aircraft's altitude is given at 0570. Both of these are AMSL and have no dependency whatsoever on any altimeter setting or pressure variations. Calculating the aircraft altitude AGL is simply a matter of subtracting one from the other, which is what all currently implemented radar altitude gauges do.

FSUIPC also calculates the altimeter reading based on the difference between its pressure setting and the QNH. This is at 3324, and is provided only to save those programming altimeters the slightly more complex calculations.

Regards,

Pete

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No, I don't think any of them are pressure altitude -- that isn't the same as AMSL which is the true altitude above sea level. Pressure altitudes vary according to pressure and are used in aircraft performance calculations and conversions between TAS and Mach, things like that.

Got it. I hadn't tried changing the pressure to check that.

The ground altitude is provided in two places -- 0020 and more accurately at 0B4C. the aircraft's altitude is given at 0570. Both of these are AMSL and have no dependency whatsoever on any altimeter setting or pressure variations. Calculating the aircraft altitude AGL is simply a matter of subtracting one from the other, which is what all currently implemented radar altitude gauges do.

Right. Knowing that it is true AMSL is the key. Thanks for the help!

Scott

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