VLOC stays yellow

To trigger the transition from armed VLOC to active VLOC, all of the following capture criteria must be met:

·         The tuned frequency in the nav radio matches that of the approach navaid;

·         The Morse code decoded by the tuned nav radio matches the identifier of the approach navaid;

·         The aircraft track is within 15 degrees of the final approach course;

·         The course to the active waypoint is within 45 degrees of the final approach course;

·         The radio deviations are at most 50% of full scale for 5 consecutive seconds;

·         The active leg is part of the approach up to and including the final approach fix.

The IFD540 continuously uses the active nav frequency and the decoded Morse code in an attempt to find VHF navaids in the vicinity of the aircraft that have matching frequency and identifier.  The nav database makes a distinction between "VHF Navaids" and "ILS Navaids", though there are some ILS navaids included in the VHF navaid list.  That distinction is present in the source data that we get from Jeppesen.  For the ILS at CYHM (identifier IAB), the ILS is coded as an ILS navaid.  Therefore, the IFD540 doesn't find it in the list of VHF navaids.  The IFD540 will only turn the VLOC indication green when this process finds a matching navaid and since none was found in this case, VLOC remains yellow.

As I mentioned, there are several ILSs included in the VHF Navaid list.  One example in that same area exists at the CYZT airport (Toronto).  The ILS there is 110.15 and has an identifier of ITZ.  If you were to fly the ILS 08 there, the VLOC indication would indeed turn green.

Note that the yellow VLOC just indicates that the IFD540 isn't sure that the proper navaid is tuned, so you should verify that it's correct before using it.  The IFD540 will still use the signal that it's receiving, but the VLOC will remain yellow.  So, it's still perfectly safe to use VLOC guidance, it's just not as convenient because the IFD540 isn't doing the station verification for you.

I will be discussing this further with Jake and the rest of the engineering team.  Don't know if this can be added as a future feature in our 540 software or if it is better addressed at some other step (at Jeppesen and/or subsequent data processing).