Garmin Tips and Tricks
GPS Fundamentals: Flight Legs
Another in our continuing series on getting the most from your Garmin flight management system, by Keith Thomassen, PhD, CFII
Learning to operate your GPS can be a daunting challenge, but having developed training classes for a number of panel-mounted IFR navigators, a simple truth emerged early on when I discovered that there were two types of problems that lead to confusion. The first is simply how to carry out specific operations on the device, and for this there are no shortcuts. Each device is different, and you just have to learn how to manipulate the buttons and knobs to carry out the operation in question on your brand.
My “Aha moment” on the second problem came fairly soon after I began offering classes in 2002 on the Garmin 430/530 (and later on the Garmin 480 and Chelton). I discovered that even when they became fairly proficient in “knobology,” students were still confused after completing an operation. The questions, “What happened?” and “Now what do I do?” started coming and soon dominated.
These questions revealed a lack of understanding of GPS fundamentals, so in this short article I want to explain by example (flight legs) what I mean. These fundamentals transcend a particular device, and apply to them all. This revelation also guided the writing of my four manuals (Garmin 400/500 series, 480, 1000, and Chelton), putting emphasis on “why” and not just “what and how.”
Flight legs are fundamental to creating and using flight plans. A flight plan is just an ordered sequence of flight legs that your GPS will follow from beginning to end, unless you need to interrupt that sequence. So what do you need to know about a flight leg? First, there are only 23 possible legs, and these are defined in the ARINC 424 standards and listed in the box here.
Early GPS devices could only create a few of these; the first to create them all were the Chelton and GNS 480. The Garmin 400/500 series only support a few of them, and curved legs were only added when WAAS was incorporated. In the 400W/500W series, many are still not available, such as heading legs. Consequently, the departures and missed approaches in those units are usually missing the first few legs, and you must refer to the charts for the full procedure. The VM leg that often ends an arrival is not available, resulting in an erroneous last leg (it goes instead to the destination airport). The new GTN 600/700 series corrects these deficiencies (it’s only missing the RF leg) and finally lets you add airway segments to a plan.
Flight plans begin with an IF leg, your first waypoint in the plan. Point legs are also used at the beginning of an approach—at an IAF or feeder. When you create an Enroute plan by selecting a series of waypoints, you are creating an IF leg and a sequence of TF legs. If you add procedures to the plan some of the other legs come into play, such as the five heading and five course legs, the four “Fix-to” legs, the procedure turn, or the three holds.
Only on the GNS480 and Chelton can you create a manual hold of arbitrary direction, turn, and inbound time or distance. This cannot be done on the other Garmin units (including the GTN 600/700 series).
When you make the plan “active” the first TF leg is usually your active leg, unless you are not at or near the IF leg. In that case, either the leg nearest to you is active or there is no active leg and you need to choose one to make active using Activate Leg (400/500/1000), or FlyLeg (480). The significance of the active leg is that navigation signals are sent out from that leg for autopilot guidance, so for each leg in your plan you need to know what those signals are and how to use them.
There are two kinds of navigation signals. The first is analog deviation commands to drive the needles of your CDI/HSI, based on the cross track error (XTE) from the active leg (or from extensions of that course). The second is digital commands sent directly to your autopilot for pitch and roll commands to your servos, to join or keep you on the active leg. For each leg type and GPS unit you need to know which signals are sent, and of course you need to understand your autopilot to know how to select a particular signal through your mode choice.
Analog signals are sent from straight legs, for use with NAV, APR, and GS modes, as are digital signals for GPSS and GPSV modes. On curved legs there are only roll commands to the autopilot (select GPSS mode)—no NAV commands to your CDI. So having GPSS is a major advantage in tracking arcs, the 45-degree portion of a procedure turn, and both the entry and a full hold leg.
Without a digital autopilot you can still use the roll commands by converting them (separate roll-steering converter and switch) to heading commands and operating your analog autopilot in the HDG mode. The switch is used to select those commands instead of the usual ones from the HDG bug on your DG or HSI. Such converters and switches are built into the Chelton, Aspen PFD, S-Tec 55X autopilot, etc.
All Garmin WAAS units create slopes (based on GPS altitude) on certain final approach legs, from FAF to MA, and send out both analog and digital commands to track them. For example, TruTrak digital autopilots are used to fly GPS approaches in the 400W/500W series and the 480 using these pitch commands (when wired properly). The Chelton unit creates baro-VNAV slopes on any leg based on target (pressure) altitude on procedure (pressure) altitudes for step-downs, climb-outs, and on all approaches (GPS and non-GPS). It sends pitch commands to a digital autopilot to track in GPSV mode. The latest Chelton GPS receiver and EFIS software also includes GPS altitude slopes for APV approaches—GPS approaches with vertical guidance.
Next to understand is leg sequencing from one to the next, which depends on both the leg type and which GPS unit you have. For the simple TF legs of an Enroute flight plan, sequencing is automatic when you reach the waypoint ending that leg. But what if the end of a leg is an altitude (VA, CA, HA) or an intercept (VI, CI)? Will sequencing be automatic, or must you push SUSP at the right time? In the 480, HA legs sequence automatically, but you must manually sequence with the 400/500. In the 480 you need to manually sequence at an intercept, but it is automatic in the 1000. Also, you need to manually sequence past the missed approach leg in the Garmin units (except for the 480, which is automatic).
Your CDI needle is your primary instrument for GPS navigation. It has a full-scale deflection (FSD) that depends on where you are. Legs that are in the Enroute or Oceanic phase have a FSD of 2 nm, so a half-needle width indicates you are 1 nm off course. That changes to 1 nm in Terminal phase (30 nm within the departing or arriving airport) for all recent Garmins.
In the approach phase between the FAF and MA, Garmin and Chelton both create localizer-type angular courses—2 degrees for Garmin compared to 3 degrees for a LOC course. In the new GTN 600/700 the FSD for the Departure (DPRT) and Missed Approach (MAPR) phases is 0.3 nm, instead of the old 1 nm for Terminal phase.
Finally, you need to know where procedure legs are inserted into your Enroute plan. Departures legs always begin the plan, but a new leg can be created between the Departure Transition waypoint and the first waypoint after the origin of your Enroute Plan. Arrival and Approach legs are added after your destination airport in the Garmin 400/500/1000 series, but just before your destination in the 480 and Chelton.
Each location creates its own “problems,” and each requires you to control sequencing (when, how) or to “jump ahead” in the plan to eliminate some legs or portions thereof. For the 400/500 series my Garmin Tips and Tricks in the Twin Commander Newsletter has covered these issues.
In conclusion, I’ve found that many of the questions from my students are in the “fundamentals” category rather than in the “cookbook knowledge” of which buttons to push. If you don’t know how to do an operation you know to go look it up. But if you’re missing a fundamental principle you don’t know what to do and need to learn these principles. If you explore how your GPS operates you will begin to solve such questions on your own. If you need help it is only an e-mail away: thomassen@avionicswest.com
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