TCAS - REL vs ABS altitudes disagree

[amahran 608]

When flying at a Pressure altitude of FL390 in MSFS (geometric altitude of 40,630') , I crossed a plane flying at a pressure altitude of FL380 in P3D (geometric altitude of 38,005').

 

The ABS selection in the TCAS had their indication show "380", as it should.

The REL selection in the TCAS had their indication show "026" when it should have showed "-10"

 

The problem:

The REL altitude calculation doesn't use the user's own aircraft's pressure altitude; it uses its geometric altitude.

 

Ideal system:

ABS altitude calculations should utilize the AI aircraft's Pressure Altitude simvar and correct it using the user aircraft's altimeter setting.

REL altitude calculations should utilize the AI aircraft's Pressure-Altitude simvar, and subtract the user aircraft's Pressure-Altitude simvar

 

I wrote some pseudocode below, hopefully this makes some sense to the person who's actually responsible for the TCAS code on the MFD:

var TCASAltitudeTag;
var TCASUserAircraftAlt = simvar(user.PressureAltitude);
var TCASAITrafficAlt = simvar(AI.PressureAltitude);

if (TCASAbsRelSelection = ABS){
  TCASAITrafficAlt = TCASAITrafficAlt-1000*(altimeterSetting-29.92);
  if (TCASAITrafficAlt < 0){
    TCASAltitudeTag = concatenate("-",roundToInteger(-1*TCASAITrafficAlt/100));
  } else {
    TCASAltitudeTag = roundToInteger(TCASAITrafficAlt/100);
  }
} else {
  var AltDif = TCASUserAircraftAlt-TCASAITrafficAlt;
  if (AltDif <= -50){
    TCASAltitudeTag = concatenate("-",roundToInteger(-1*AltDif/100));
  } else if(AltDif >= 50) {
    TCASAltitudeTag = concatenate("+",roundToInteger(AltDif/100));
  } else {
    TCASAltitudeTag = "000";
  }
}

 

 

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[amahran 608]

This issue still exists today; the TCAS REL selection shows relative physical altitude between the user aircraft and the AI traffic (VATSIM, multiplayer, Live traffic, or offline AI), rather than the Pressure Altitude difference.

 

The Relative altitude doesn't show the Plus/Minus indicator either to show whether the other aircraft is above or below the user aircraft.

[Crabby 28]

You should be a detective!