Many pilots think of a flight plan as a list of waypoints that you follow, in order, from the first one to the last one. That’s how you create a plan. But in fact what you are doing is creating a sequence of flight legs that you follow one after another, with the GPS automatically sequencing at the end of each leg to the next one. This way of looking at the plan helps you understand procedures, which are a “canned set of legs” that represent that procedure. The only kind of leg you can enter in a plan is a TF leg (Track to Fix). It is described by the course, distance, and ending waypoint (this description does not include the starting waypoint). The Chelton and GNS480 also let you create HM legs, holds at any waypoint of your choosing, with a manual termination (and further choices of inbound course, time, or distance, and direction of turns).
So why is it important to think “flight legs” instead of a sequence of waypoints? GPS devices are designed to automatically sequence to the next leg if possible. However, on some legs that’s not possible and sequencing is “suspended”. The SUSP light comes on, and you must push the SUSP (or OBS) button at the right time to restore sequencing and make the next leg “active”. The active leg is the one colored magenta, and your cross track distance from that leg (or extensions on both ends) is what the unit feeds to the CDI or HSI, so your autopilot can track it (it may instead create roll commands for those who have GPSS autopilots (or converters). The 430/530’s don’t send signals to autopilots on heading legs (only the Chelton and G1000 do that), so in general you need to know which of the 23 legs can be flown by the autopilot, and how (from analog CDI signals or digital roll commands --- or both).
Procedures are added using a variety of legs; the ARINC 424 standards describe all 23 of them. Only legs from that list can be used to represent a procedure. For example, a common first leg of a departure or missed approach might be a VA leg (heading to an altitude). You fly the particular HDG until reaching a specified baro altitude (sequencing is automatic). Following that you may get a VI leg (heading to an intercept) to the next course (sequencing will be suspended on this leg). After the intercept, the next leg is often a CF leg (course to fix).
Some GPS units cannot create all 23 legs, so their procedures (departures, arrivals, or missed approaches) may be incomplete. The 430/530 is one such GPS. Their WAAS upgrade made some improvement by adding curved legs (which includes arcs, procedure turns, and holds). Roll commands (but not NAV commands) are generated for those legs. Among the many legs the 430/530 does not create is the VM “vector leg” (heading to a manual termination) often found at the end of an arrival. Without it your last arrival leg is erroneous (it goes to the airport). The original G1000 had the same shortcomings, but later software versions create all 23 legs (as do the Chelton and GNS480).
You can make any flight leg “active”, at any time. Simply identify the leg in the flight plan (put a on it cursor) and push the D-> button twice. Or, with the cursor on it, chose “Activate Leg” in the menu. There are reasons to do this (that’s another story) when procedures are added, but here is an example depicted above.
We are cleared to KSNS via the flight plan shown on the 530. The first (active) leg to MQO is displayed on the G600. We’re given a heading of 330° after takeoff from KSBP, entered on the PFD with the HDG bug, and told to intercept the airway between MQO and PRB. We put MQO in the flight plan to have a leg from there to PRB. Now, before takeoff, we want to “Activate” that leg, shown in the pictures here. Put the cursor on the second leg (ending at PRB), push D-> once to initiate, and again to choose to “Activate”. Then push ENT to make it active (leg is now in magenta).
It’s essential to know how to identify a specific leg in your flight plan (with a cursor). The trick is to find the line displaying the end of that leg (a waypoint, altitude, intercept, arc, or manual termination). Then, the action you take will be for that leg.