decides to make this note, in case there was no perception of the following situation in MSFS and related to the AGL (Above Ground Level).
In case you try to explain, I think there is an incongruity in the way an AGL is made.
For example: Having the QNH set to 1013 (to simplify my explanation) and existing on the ground at a given airport, the altitude that is shown on the aircraft instruments is in accordance with the airport altitude; all right.
After taking off, the distance to the terrain to be presented, the ground made between the aircraft and the ground made according to the terrain cartography, but, I see that what is happening is the following; The closer I get to a higher part of the terrain than I have previously flown, the displayed distance between the aircraft and the ground it also increases (instead of decreasing) always keeping the altitude constant on the altimeter (since the altitude chosen for the cruise flight is constant
and the variable is in relation to the aircraft with the ground).
Am I right in mine regarding this feature of the simulator?!..
NOTE: This situation will nullify the correct information from the GPWS since the closer to the ground is elevated and cannot be configured from an aircraft the ground can be done in an increasing way the reverse.
Thanks for listening.
I await observation regarding this behavior of the simulator.
An aircraft altimeter, when properly set, measures altitude above Mean Sea Level. It doesn’t, and can’t, measure height above the ground. In order to do that you need a radar altimeter.
A plane’s altimeter works by measuring the outside air pressure.
The higher you get, the less the external air pressure.
As such, it can’t tell if a mountain is in the way, it only knows how high in the atmosphere it is.
In order for terrain avoidance, as mentioned, the radar altimeter is used to accurately measure the distance to obstructions.
What they say is correct, but maybe I explained it wrong.
In fact, what happens is that the measurement by radio altimeter of the distance of the aircraft in relation to the ground must be smaller the closer the aircraft is to the ground.
What I identify with a log program of a company I fly for is that the closer to the ground, the greater the value of the difference that the simulator gives me in feet in relation to the ground, what should be the opposite, the closer to the ground, the smaller this distance would be.
I have other friends with other simulators, namely xplane and fsx and this count is correct in the log, only in MSFS this discrepancy is presented.
Example: I am 1000 feet off the ground, however, there is a mountain with 1200 feet that I intend to fly over. To fly over this same mountain I climb to 1400 feet, so my altitude difference in
relative to the ground over the mountain would be 200 feet. What happens later in the flight analysis is that my altitude above the ground on the mountain is 1600 feet and not 200 feet.
That is easy to explain.
MSFS takes other factors into consideration to determine the correct pressure altitude.
In real life, one of the biggest factors is humidity. It affects atmospheric pressure.
MSFS is the first and only sim to incorporate that into the altimeter. No other sim does that!
As a result they are not showing the correct altitude, because they only consider the theoretical pressure.
Indicated altitude and true altitude are two different things. The indicated altitude = true altitude ONLY in standard atmosphere, meaning 1013.25 hPa and 15C with a standard delta T of 1.98 C per 1000 ft. Otherwise you’ll need to correct, first for non-standard QNH, then for non-standard temperature. There is a whole formula for the latter, you could use 4ft per 1000ft per 1C ISA deviation as an estimate.
Humidity has such a big effect that we never take it into consideration. The effect of humidity on aircraft performance is in the order of a few percent. No aircraft manufacturer supplies tables to correct for this. Pressure altitude with temperature is used to determine density altitude in aircraft performance graphs / tables. In altimetry humidity is not taken into account either.
The only simulation variable that gives your actual altitude above the underlying terrain is RADIO HEIGHT. All of the other simvars relating to altitude are referenced to sea level.
