The amazing Garmin 430 came out in 1998, but my first view of one in a plane was May 12th, 2001 at a Saratoga crash site. This confused pilot had loaded the ILS at KITH but forgot to punch the CDI button and enable “green needles” for correct ILS guidance. In the 20+ years since this has become known as the “$500 button” in flight training for the 
number of checkride failures it has caused. In the case of this Saratoga, it almost cost the owner his life. He had accurate lateral guidance on the approach with the wrong “nav source.” The centered glideslope needle was very misleading and drove him into the ground. (A similar historical problem with steam gauge CDIs is the ambiguity of “centered needles;” either “perfect” or “off!”)
Fast forward to last week, flying the latest/greatest Garmin iteration; a G-3000 avionics package in a new Citation M2 jet, and the same “green needle” problem caused my learner to fly right through the localizer at Newark on the ILS 22L. Obviously, pilots have still not gotten the memo on this issue: still common in with the GTN architecture avionics and even in jets. Personally, I think the problem is the rote-level training instructions; “push this button here, that one there and the ‘vector to final’ button only in certain cases.” To be competent (and safe) using these units, you have to understand the bigger picture of the GPS logic.
Most pilots stumble into the “Garmin Green Needle Gotcha” in a classic manner. They dutifully load and activate the approach while still flying on GPS guidance during the transition to the terminal area. Most typically pilots are flying to a fix on the final approach with GPS guidance get a vectored procedural shortcut – “fly heading XXX/descend XXX/ cleared.” Unfortunately, they seem to frequently fail to activate ILS guidance (or “green needles”). Though this error is more obvious on a large pdf presentation than in the old GNS 430s, this is still easily missed. With only a rote understanding of what is going on some steps continually get missed. This is a “pilot problem.” It is essential to visualize the bigger picture of “how your GPS thinks!”
Start with the most basic “green needle” ILS approaches seen at all NYC airports. There are no legal “airway transitions to these approaches and radar is required for just about ILS. This is an obvious “load/activate” and “vectors to final.” Outside the busy Bravo airspace, there are charted terminal transitions into an ILS approach. And though these ILS approaches can be flown separately on a “green needle” system (without GPS), they are usually navigated using GPS nav inbound. Once you get the ATIS, you load and activate an ILS approach with your GPS, and the system will nicely define all the legs and transition into the approach – so far so good.
The newer GTN logic will fly GPS guidance right onto final and automatically switch to “green needles” (if enabled in settings). This occurs at an unseen “sensor fix” 2 nm outside the FAF. It is essential that every pilot anticipate and verify this switch to ILS guidance (green needles) every time. Most CFI-Is get lots of questions about these setup issues: sometimes the GPS works automatically enabling the switch to green needles, and sometimes the magic fails- how come?
This has to do with how the GPS “thinks.” GPS is a “go to” navigator and draws lines between “path terminators.” If you shortcut a charted procedure, you have to inform the GPS (like a crew member). When an ILS requires vectors (LGA13), or the controller issues vectors and you fly a “heading to intercept,” your Garmin does not know to switch automatically to green needles. This has to be manually initiated by the pilot. Activate “vectors to final” or your plane will fly right through the course (what my pilot did). Basically, the pilot has to inform the system about this “shortcut!” In autopilot operation, the mode controller “APR” function will not activate or annunciate on the pfd “scoreboard” in this case. Unfortunately, most pilots miss this in the heat of battle.
The best practice for all this button pushing is the excellent free Garmin GTN iPad simulators. Search for “Garmin GTNXi” (or TXi) Trainer on the app store. Super versatile trainer for practicing your “buttonology.”
When *Not* to Enable “Vectors to Final” (VtF)
There are many times when “vectors to final” is not the right button to push, and I think this creates doubt in a pilot and makes them reticent to activate the VtF function. If the approach is loaded and activated and you are flying a full procedure (still in NAV mode), activating “vectors to final” will draw a single straight line down the final approach. This will also remove your navigational guidance to that course.
Only when you are actually being issued “vectors to final ” – and you are in heading mode following radar control guidance – is it time to activate “vectors to final.” Basically, the controller is short-cutting the full procedure and that VtF button tells your GPS navigator to activate the ILS (green needle) guidance. (BTW, this can also be done “old school 430” with the CDI button on the PFD).
“Vectors to Final” Without ILS
And here is the real zinger that even experienced pilots miss. This same exact problem occurs on a totally GPS-guided approach (usually LPV at a busier terminal) when you are “vectored to final.” This is less obvious because it is all on GPS guidance (no green needles). If the controller issues a vector to shortcut a charted full procedure, you also must also activate “vectors to final” to inform your GPS the procedure has been truncated. Any time you vary the charted sequence of waypoints, the additional “VtF” button must be pushed to enable the “heading/intercept” logic. Simple! Fly safely out there (and often)!
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