Once you understand how dead reckoning and VOR concepts relate to the world of GPS, you’re ready to move on to waypoint sequencing and turn anticipation. Understanding these two concepts is fundamental to the effective use GPS flight plans, the handling of ATC routing changes, and to reducing the likelihood that you’ll find yourself asking “Why’s it doing that?”
One goal of RNAV was to simplify navigation from the pilot's point of view and GPS generally offers significant improvements in navigational accuracy and flexibility. Okay, the equipment manufacturers have made things difficult with questionable user interface designs. The FAA and its contractors have created complicated procedures that expose levels of detail that could (and should) be opaque to pilots, but we'll go down that rabbit hole later. For now consider that there are really just two conceptual steps to waypoint navigation: Enter a waypoint into the GPS receiver, then navigate to the waypoint using the desired track and current track provided by the GPS. Don't forget to set your course pointer or OBS to the DTK supplied by the GPS or you'll see a message like the one shown above.
In its most primitive form, waypoint navigation means pressing the GPS receiver's Direct-to button, but you can also enter a sequence of waypoints (aka GPS Flight Plan) and this is where waypoint sequencing and turn anticipation come into play. The mechanics of creating, storing and activating GPS flight plans vary by brand and model of GPS, but for now we’re just talking about concepts.
Shake a Leg
Navigating the desired track (think magnetic course) between two waypoints in your flight plan is referred to as leg flying, which is different from just proceeding to a waypoint from your current position using the direct-to button. Proficiency training with a specific GPS unit should include demonstrating that you understand the difference by proceeding direct to a waypoint as well as activating a flight plan leg, then interceping and flying that leg.
The active leg is usually shown as a magenta line on the moving map whereas previous and subsequent legs to be flown are shown as white lines.
Best Laid Plans
For VFR flying, you can use any waypoints you want in your GPS flight plan; VORs, airway intersections, airports, visual reporting points, or anything else contained in the GPS database. You’ll still want to ensure you're steering clear of restricted airspace, active MOAs, and terrain or weather that might make you wish you were doing something else.
For IFR, your GPS flight plan should ideally match your IFR clearance. This is where the perfect, sequential, tidy world of GPS meets the rough and tumble, real-world of ATC vectors and amended clearances. Start by entering your departure airport and your destination airport. It would be nice if you could enter an alternate airport, but amazingly, most GPS units (save the discontinued G480/CNX80) still don’t support this concept!
IFR clearances may specify a SID (Standard Instrument Departure) or they may specify a heading to fly until ATC vectors you to join the en route structure. Most IFR-certified GPS allow you to load a SID based on your departure airport (the first airport in your flight plan). If the SID has more than one transition to the en route structure, the GPS will prompt you to specify the transition that ATC assigned. Loading a SID often requires you specify the departure runway and this can be especially critical for RNAV SIDs. Loading a SID will insert all the necessary waypoints into your flight plan.
Some SIDs are pilot-nav, the theory being that you fly the procedure with minimal input and guidance from ATC. Other SIDs are vectored, with the assumption that ATC will guide you to the en route structure. Many older GPS units don’t provide a way to load a vectored SID, so just enter the waypoint (usually a VOR) that defines the transition to which ATC is going to guide you. Either way, it's not uncommon for ATC to assign you headings to fly in order to separate air traffic, eventually telling you something like “when able, resume the Wild Goose Chase departure ...” This is where activating the appropriate leg from your GPS flight plan is a need-to-know skill.
Anytime you are being vectored, the heading you are flying may not correspond to the magenta line that the GPS is depicting. This often causes concern for an RNAV tenderfoot, but remember that the compass and heading indicator are also IFR instruments and you won’t always be following the GPS’s magic, magenta line.
Defining Victor or Jet Airways for the en route portion of a GPS flight plan is fairly straightforward: Just enter the name of each VOR and any changeover points in between. Some newer GPS units provide a way to load an airway and will fill in the necessary waypoints for you, but the process can be convoluted. The idea is you start with a waypoint on the airway that ATC has assigned, then select the airway you want to load from a list of possibilities, and then specify where you want to get off the airway. This will add the necessary waypoints to your GPS flight plan.
Loading a STAR (Standard Terminal ARrival) or IAP (Instrument Approach Procedure) for your destination will require you to specify the transition (where you want to enter the procedure). Loading these procedures will add waypoints to your flight plan. And like a SID, you may get vectored around by ATC and you’ll need to know how to proceed direct to a waypoint or how to activate and join a leg of the procedure.
So a GPS flight plan is just a collection of waypoints that you either entered yourself or that were added when you loaded a departure, arrival, or approach procedure.
Fly-Over vs. Fly-By
Now that you know (at least a bit) about GPS flight plans, let’s revisit waypoint sequencing. GPS keeps track of where you are and as you reach the current waypoint, it usually sequences to the next waypoint, calculating the new desired track, distance, time en route and other stuff in the process. Yet there are times when a GPS won’t automatically sequence to the next waypoint and it’s important to understand the when and why this will occur.
The majority of the waypoints in your GPS flight plan are called fly-by waypoints, usually depicted on charts as a four-pointed star. As the name implies, you don’t have to fly directly over these waypoints. Since the GPS knows your groundspeed, current track, and the number of degrees of change between the current DTK and the upcoming DTK, it can provide turn anticipation for fly-by waypoints. If you pay attention, follow the GPS’s advance warning and turn when it says to turn, you’ll end up on the new DTK. Old school pilots who were taught to fly past a VOR, verify station passage, then turn to the new course may find they have trouble letting go of their old habits. Just keep reminding yourself that turn anticipation is a good thing.
Another type of waypoint is called fly-over. As the name implies, you must fly right over a fly-over waypoint. Try as I might, I couldn't find a definition of just how close you must pass to a fly-over waypoint but my personal experience is that it's pretty damn close. One example of a fly-over waypoint is the missed approach waypoint on an IAP: Fly over the MAP and you’ll have to push a button to re-enable waypoint sequencing to the missed approach segment (on most GPS units). Same holds true for most missed approach holding waypoints. Which button you push to re-enable waypoint sequencing depends on the brand and model of GPS.
Some RNAV SIDs and STARs have fly-over waypoints, depicted as a four-pointed star enclosed in a circle, but waypoint sequencing is not suspended for these waypoints on a SID or STAR; The procedure designers just want you to navigate to these waypoints very precisely.
You can manually suspend waypoint sequencing for any waypoint at any time by pressing a button. Depending on the brand of GPS receiver you're using, the name of the button or softkey may be OBS or it may be SUSP, but press that button or softkey and the GPS won't sequence from the current waypoint until you intervene with another button push: Pretty handy when ATC tells you to fly an ad hoc holding pattern using the current waypoint as the holding fix.
Many pilots ask why some GPS receivers call this feature OBS and I the reason is that pressing that button lets you treat the current waypoint as though it were an old-fashioned VOR: You can turn your course selector or OBS knob and set whatever DTK to the waypoint you want. When you press OBS or SUSP, you’ll see a magenta line on your moving map leading to the waypoint on your selected DTK and a white course line extending outward on the opposite side of the waypoint.
GPS receivers allow pilots to load all sorts of instrument approaches, thereby adding the necessary waypoints that make up the approach to the GPS flight plan. Monitoring a moving map to maintain situational awareness during an approach is cool, but the waypoint sequencing get's messed up when you are being vectored to intercept the approach. When being vectored, the first task is to activated the correct leg of the approach so that the GPS will sequence through the waypoints in your flight plan in the correct order as you fly the approach. For a detailed discussion of this issue with Garmin units, go here.
Lastly, consider the location of the missed approach fly-over waypoint (MAP) for an ILS or an RNAV approach with vertical guidance (LPV or LNAV/VNAV). For approaches with vertical guidance, the MAP waypoint is defined by centered localizer/LNAV and glide slope/VNAV needles at the Decision Height (DH). Consider also that the FAA puts ILS and localizer-only approaches on a single approach chart as it does for RNAV LPV and LNAV/VNAV approaches. GPS designers had figure out how to handle this and they came up with this kludge: They consider the MAP waypoint to be at the runway threshold. The MAP waypoint is given a name like RW34 for “runway 34.”
In the example above, you reach the DH, see nothing but clag, and start the missed approach. The GPS will not have yet suspended waypoint sequencing since you haven’t arrived at the runway threshold, so you’ll need to fly over the runway threshold before you can activate the missed approach. Oddly, the Garmin G1000 provides an Activate Missed Approach feature, but every time I’ve tried to access that feature it had a grayed-out appearance and couldn't be selected.
Stay Ahead, Keep Ahead
Now you know a bit more about the crucial foundations of RNAV: The importance of automatic waypoint sequencing, the role of a GPS flight plan, how your GPS flight plan may not conform to the real world, and the important difference between direct-to and leg navigation. You've probably noticed that we're delving into increasingly complex RNAV concepts, so stay tuned for my next installment: Creeping RNAV complexity and what you can do to combat it.