I was reading of the following case in ARINC 424-17 spec. Unfortunately the image is copyrighted and I can't paste it here, so here is a low(er) quality image of mine replicating the situation:
The text says that the path after the
VI has to be coded as a
CF if the
XYZ is a VORDME but if the navaid is VOR only then it should be an
TF sequence. My question is not why is that sequence by how does the collocated DME affect the decision? The document mentions
This will allow a segment to be constructed, from one fix to the next fix, using an “intercept” where coding would otherwise not be possible.
But it doesn't explain why is it "not possible" to code this path as a
CF if the station doesn't have DME. How does DME help here? In my mind,
FOOBR is on a radial, let's say 55 from XYZ so if the airplane intercepts and follows this radial it will arrive to
FOOBR, no DME involved. Is there something that I'm missing here?
VI vector to intercept
CF course to fix
IF initial fix
TF track to fix
Ralph J asked "what's the difference between
TF paths?" That's actually a good question. The spec is quite laconic:
Course to a Fix or CF Leg. Defines a specified course to a specific database fix.
Track to a Fix or TF Leg. Defines a great circle track over ground between two known databases fixes.
To my understanding, TF is more accurate as the course is derived from the previous fix while CF requires the course and recommended navaid to be coded in the aircraft's database, hence making the TF a simpler choice when available.