Shrike Commander flown by Erick Teeters and Brian Rikeman over Lake Powell, Arizona. Some things bear repeating, and for understanding GPS navigation, the topic of flight legs is one of those “things.” In my training classes over the last 20 years and with many conversations with pilots, I’ve found that problems they encounter with their GPS often come from an incomplete understanding of this fundamental area. It’s been 13 years since my first article on flight legs but given the ongoing confusion I’ve observed I believe the topic is worth another look.
A flight plan is an ordered set of flight legs, of which there are 23, shown in their flight order on the FPL list in your GPS. Some VFR pilots who never fly procedures might think a flight plan is a series of waypoints along your route, but that’s not a proper description. Such a plan only uses two of these legs (IF, TF). The complete list of 23 legs is defined in the ARINC 424 standards, and shown in Figure 1, organized by type. A flight leg has two attributes; the path it takes and how it ends. The descriptions in Figure 1 generally indicate the path and ending.
Figure 1. A complete list of the 23 flight legs specified in the ARINC 424 standards, with their two-letter identifier, organized by types.
For example, the path for five of them is a heading, while another five follow a specific course. Endings for these include an altitude, the intercept of the next leg, a VOR radial, or a DME distance.
Seven of these ends in a fix (waypoint) and five of them start at a fix (if you include the IF point leg in each case). Three of them are holds, one is a point, and another is a procedure turn (the outbound and the two 45-degree parts, but not the inbound CF leg). A vector (without an end) is the FM leg. The D-> operation creates the DF leg from present position, and an OBS operation at a waypoint creates a CF leg to the waypoint and an FM leg beyond it. An OBS course can only be created for the seven legs ending in a fix. So, does that include the IF leg? Yes, it does!
Two of these have arc paths, the AF and RF legs. The RF leg is distinguished from the AF leg by its tangent arrival to the arc and tangent departure. The AF leg found on DME arc approaches has a 90-degree change in direction at the end, onto the final approach course. Also, the RF leg is generally found on RNP procedures that require both pilot and aircraft to be certified for the operation.
A point leg can be your departure airport, the initial approach fix of an approach, or an arrival transition. If you add a Departure to your flight plan the location of the IF leg changes from the airport (the spot somewhere on the field chosen to represent the airport) to the end of runway from which you’re departing. A graphical depiction of each leg is in Figure 2.
Figure 2. A graphical representation of the 23 flight legs.
There are leg-specific issues with some of the 23 legs. Will they sequence automatically through the ordered list on your flight plan, or must you intervene either by unsuspending or activating the next leg manually? Sequencing means that when you reach the end of the active leg the next one on your ordered list automatically becomes active.
Generally, to sequence your GPS must know you reached the end; legs ending in a fix always sequence, with one exception. At the missed approach point, by FAA fiat, sequencing must be suspended, and you must restore it manually to proceed to the missed approach legs. This is true for all GPS units except the GNS 480, certified before the FAA ruled on this. Note that you can always activate any leg you want and at any time by selecting it with a cursor in the flight plan and activating it through a menu choice. You can also activate a missed approach early (before reaching the missed approach point) by a menu choice.
The FM leg has no end, so you need to restore sequencing when ready. The HM leg, found either at the end of the missed approach legs or inserted manually in the flight plan, must be sequenced manually. The HF leg, the one-turn hold in lieu of a PT leg, initially suspends but automatically restores sequencing inbound. For those legs ending in an altitude, your GPS must get the baro-corrected altitude from a source like a PFD to know when you’re there. Those ending in an intercept require computations by your GPS to figure that out; there is no specific point where the intercept happens. Some GPS units will do that, and others will not.
Sequencing is both a leg issue and an equipment issue. Also, older GPS units may not create some of these legs. The GNS 400/500 series were missing heading legs, and many other legs, as were the early G1000 units that were based on the 400/500 logic (menu structure) and software. However, modern GPS units can create all 23 leg types.
A good practice after creating your flight plan is to review the listing of it on the FPL page, particularly after adding a procedure. That’s where many of these “strange” legs appear. You can only create six legs by your knob turning and button pushing, so the remaining 17 only appear in procedures. The six you can create are the IF, TF, DF, HM, CF, and FM legs. The last two come from creating an OBS course.
Adding a procedure inserts the sequence of legs that represent it somewhere in your flight plan. A big question is where, and you can answer that by reading your flight plan. Look at each leg and their order on the list. Departures are simple since those legs are inserted after the IF leg representing your departure airport. That IF point changes to your takeoff point at the end of the departure runway. It also eliminates the TF leg to your first enroute waypoint.
These departure legs end at the departure transition, which may not have been the next waypoint (after the airport) in your enroute plan. If not, you have a discontinuity to deal with. If your unit adds discontinuities, sequencing is suspended there. Insert the desired route or accept a leg between the transition and that waypoint by deleting the discontinuity (Garmin does this automatically). You can avoid this disconnect if you know you’re going to fly a specific departure by making the transition waypoint the one after the IF leg you build into your enroute plan. Adding a Departure causes little difficulty. Approaches and Arrivals are another matter. Here, the different manufacturers each do things in their own way.
In the early days Garmin always added Approaches and Arrivals at the end of your flight plan, after your destination airport. This often creates the problem (if you follow normal sequencing) of backtracking from your destination before going to the approach IAF (if that’s how the geometry works out). They conceived of the “Activate Approach” idea, which sent you from present position to the IAF you chose for the approach. Note, however, that you never have to activate an approach, since you can always go direct to your chosen IAF or any other waypoint on the approach if necessary.
The GNS 480, Avidyne, and Chelton insert arrival and approach legs before your destination, creating different problems. For example, that insertion substitutes for the last leg you programmed into the flight plan, the one ending at your destination. If your flight plan was a simple one from your starting airport to your destination, adding the procedure eliminates the entire route to your destination and leaves you without guidance. Further, adding an approach eliminates the destination airport in favor of the missed approach waypoint. Now you can’t solve this problem by going direct to the airport. You can of course push D-> and select the airport, then go direct to an approach waypoint when cleared.
Even with more legs in your enroute plan, by the addition of a procedure you will wipe out that last leg and you might be tracking it, so beware. These insertions can also create discontinuities in your flight plan, which the GNS 480 and Avidyne will show (Avidyne calls it a Gap in route). On an approach the discontinuity is between your last enroute waypoint and the approach IAF. A gap in the flight plan means you have not specified how that connection should be made; either delete it to connect those two points or at the appropriate time, jump ahead of the discontinuity using D-> to a waypoint beyond it, typically an approach waypoint.
Garmin continued its practice (insertion after the destination) when launching the touchscreen 600/700 series, only to later switch to the “before the destination” method. But they made an exception for the one-leg plans between origin and destination (which is in fact two legs since there is an IF leg at the origin before adding the TF leg to your destination). Garmin simply added a D-> DEST on the one-leg plans so you didn’t lose guidance. Garmin has more recently switched back to their initial practice in certain cases (but not all). It’s complicated; you must look at your flight plan to see how your GPS does it. Where are procedure legs added? Picture in your mind the flow of legs and what problems you’ll encounter, and at what point. For example, when adding an Arrival, the initial legs may be overrun by legs in your enroute plan. To avoid backtracking to the transition you must use the D-> or Activate Leg options to jump ahead in the plan to one of the Arrival legs ahead (geographically) of you.
Finally, autopilot tracking is also leg specific. What signals does a particular leg send to the autopilot? These can be analog and digital. Some legs have analog signals to a CDI or HSI, displacing the needle. For those, an analog autopilot will send signals to the servos to zero-out the deviation. But some legs don’t send any analog signals; heading legs don’t, nor do the 45’s on a procedure turn or outbound legs on holds. However, for these lateral tracks, a modern GPS will generate digital roll steering commands that digital autopilots can track. Analog autopilots in HDG mode can track these as well if you have a roll steering convertor, either a separate unit or one built into many autopilots. You’ll also have a GPSS button and switch to select the converted roll commands (to heading commands) rather than HDG commands from a HDG bug. Most PFD units also have GPSS convertors.
With a digital autopilot you can also track analog VLOC signals from a VOR or LOC course, and the analog glideslope signal. This is done using the PFD that’s required with each digital autopilot (they must be certified as a pair). The PFD does the analog to digital conversion that allows the autopilot to drive the servos.
GPS Vertical commands (GPSV) are created for APV approaches (GPS approaches with vertical guidance) and for VNAV courses. You can create your own VNAV slopes on legs by setting altitudes on the flight plan at the start and end of a leg, or these slopes might be pre-programmed on Arrivals or Approach legs prior to the final approach fix. Only Chelton lets you create VNAV climbs; all the rest only do VNAV descents. Missed Approach legs and Departures (both with climb outs) don’t have GPS vertical guidance (except Chelton) so you need to use autopilot modes like FLC or VS.
As described here, the flight leg concept and associated issues for the various flight legs often lead to confusion as to “what’s going on,” or “what’s happening,” and “why is it doing this?” These can be resolved easily if you thoroughly understand the concept of flight legs and behavior associated with them.
Dr. Thomassen has a PhD from Stanford and had a career in teaching (MIT, Stanford, UC Berkeley) and research in fusion energy (National Labs at Los Alamos and Livermore). He has been flying for 65 years, has the Wright Brothers Master Pilot Award, and is a current CFII. See his website (www.avionicswest.com) on all his manuals plus numerous articles on GPS and other aviation topics.
