Flight Legs

            Flight plans are central to using a GPS.  Having one allows you to see your course on the map, provides autopilot guidance to track it automatically, and organizes its waypoint database for quick information retrieval.

            A flight plan is an ordered sequence of flight legs using only those legs defined in the ARINC 424 standards, listed here in our Table.  You can create only a small fraction of them yourself, most commonly the TF leg between two waypoints, and an IF leg which is the first waypoint in a plan. Going Direct creates a DF leg, and Direct on a Course makes a CF leg.  In the GNS480 and Chelton you can also create a manual hold (HM leg), but not in other Garmin products. All other legs are only added by a Procedure, a canned sequence of legs that represent it. 

            To fully represent a procedure a GPS must be able to create all of these legs. A number of GPS devices now do that. They include the GNS 480, Chelton, GTN 650/750, and the G1000W. These same units also let you add airway segments (sequence of TF legs).  The GNS 430W/530W navigators are missing (among others) the heading legs, which normally begin a Missed Approach or Departure, the VM leg (vector) that often ends an Arrival, and various others. These units may not create the RF leg, used in RNAV (RNP) procedures. For RNAV (GPS) procedures you don't need it.

            There are two things you need to know about each of the flight legs, do they sequence automatically and what autopilot commands are there on each of them? If you have a digital autopilot or a GPSS converter, you can accept roll steering commands directly to the autopilot. On straight legs, but not HDG legs, NAV commands are sent to the CDI so you can track them in NAV or APR mode.

            All the legs in a Chelton, the GTN 650/750, and the G1000W issue roll commands and can be tracked in GPSS mode, or with a GPSS converter sent to the HDG input of your autopilot.  In the G1000W the commands are sent to its internal G700 digital autopilot, and leg tracking is done through proper selection of the autopilot mode.  To get roll commands from the 5 HDG legs a magnetic heading input to the GPS is required. The GNS 480 does not issue roll commands on HDG legs even though it accepts a magnetic heading input and uses it for wind calculations.

            It's important to know which legs sequence automatically in your GPS on reaching the end condition for the leg (a waypoint, intercept, altitude, DME distance, etc). Sequencing is automatic from the 3 altitude legs in the GNS 480, and in the GTN units when it gets a barometric altitude input. Otherwise it must be done manually. Vector legs do not sequence automatically since they have no end, and sequencing is interrupted at the missed approach point in all units except the GNS 480.