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 6 of them yourself (IF, TF, DF, CF, FM, HM). The most common is the TF leg between two waypoints. An IF leg is usually the first waypoint in a plan, your departing airport, but is also the initial fix of an Arrival or Approach. Going Direct creates a DF leg, and a Course to a Fix makes a CF leg or an FM leg, made by an OBS course to or frm a waypoint. In the GNS480, Chelton, GTN 750, or Avidyne 540 you can also create a manual hold (HM leg). All other legs are only added by a Procedure, a canned sequence of legs that represent it.
The top 5 legs in the left column, beginning with a V, are heading legs. The second letter indicates the ending of that leg; Altitude (A), Intercept (I), Radial (R), DME Distance (D) or Manual (M) termination. On the top right column are 4 of their counterpoints (CA, CI, CR, CD), having the same terminations but following a course instead of a heading. A vector-to final for example is a CFleg. The VR and CR legs are rarely used, and can be recognized by their ending description, the VOR and radial that you intercept. You could instead make those legs a VI or CI leg, and follow it with a CF leg to the fix where these radials are going.
There are three hold legs; one is used to reverse course (HF leg) in lieu of a procedure turn (PI leg). The HF hold sequences automatically to the nest leg on completing the reversal. You cannot activate this leg but can go direct to the hold waypoint. The HM hold has a manual termination; you need to push SUSP to sequence to the next leg after the hold (if there is one). The HA hold is used to gain altitude on climb out for terrain clearance before continuing on. In some units it automatically sequences on reaching the baro-altitude that terminates the leg. Your GPS must know when you reach that altitude (you need to be able to put a baro-correction into your GPS or EFIS). The PI leg is actually the outbound plus the 45's outbound and inbound. On intercepting the CF leg to final it sequences.
An Arc-to-Fix AF leg is used on arc approach legs, normally creating a 90° turn to final. An RF leg (Radius-to-Fix) differs in that the entry and exit of the leg are on a tangent to the arc. These are used in RNP approaches which requires special authorization for the plane and pilot. They may soon appear in some RNAV (GPS) approaches, but the FAA has been resistant to that so far.
Legs that start from a Fix and go to some end conditon include FA, FC, and FD legs. These end in an altitude, distance from the fix, and DME distance from a VOR respetively. Note that the FD, VD, and CD legs all terminate at a DME distance from a VOR, but only the FD leg starts from a Fix.
To fully represent a procedure a GPS must be able to create all of these legs. Most GPS devices do that now, but early units did not. These 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.
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 (from a magnetometer) 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.