Modern IFR Navigation

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On July 3d, David Megginson wrote to the mailing list,

Cquote1.png The airspace system is in the process of changing drastically, and I'm following it this summer by finally biting the bullet and installing an IFR GPS (Garmin GTN 650) and ADS-B transponder (Garmin GTX 345) in my Piper Warrior II. What this means that for the first time in the 15 years since I started flying in real life, I won't be able to use FlightGear to practice the IFR approaches I'm flying in real life.

This article is about the discussion which followed that message, which focused heavily on modern IFR navigation.

Advanced RNAV Approaches

In the modern world, RNAV approaches are a lot more complex than following a magenta line and manually calculating a vertical speed or using VNAV. Modern RNAV approaches involve things such as "radius-to-fix" segments, which allow curved paths around mountains, which the aircraft can follow. One example is the RNAV (RNP) Y to 05R at Auckland:

Cquote1.png One thing that is very important moving into the future is the RF approach (radius to fix). Please read this: This style of RNAV approach is becoming more and more common, and FlightGear cannot yet simulate it.
Cquote1.png DME arc transitions still exist, and I enjoy flying them both in FlightGear and in real life. There are two important differences for the radial-to-fix transitions (as I understand so far, still studying up the new-to-me RNAV procedures):
  • You can fly them around any fix, not just a DME/TACAN (typically colocated with a VOR). Granted, you could do that in FlightGear now simply by using the GPS distance, but ...
  • The CDI actually gives you (and the autopilot) guidance for where you should be flying, so you're not manually estimating your course based on the DME/GPS distance while flying 90° to the navaid bearing.
It's the second one that will trip us up for using FlightGear as a way to practice modern IFR procedures.
Cquote1.png Are you wondering about the math to give you a guidance target for flying around a given point? Or are you wondering how to render the 'fly-to' indicator?

The first basically is the same as the Shuttle HAC intercept where Rdot (the derivative of the radius to the center point) in combination with R can be used as error for guidance and the second depends on your particular instrument - the Shuttle HUD shows a corresponding indicator (and so does the PFD) - but I suspect the instrument of choice of a GA aircraft will display it differently.

So this should be possible to code by any aircraft maintainer without much fuss as far as I can see.
Cquote1.png The Route Manager should be able to handle most of the "magenta line" tasks, but it may be that the more complicated routing such as the RF approach, fly-by vs fly-over requires some new autopilot coding as you describe.