Tips and Tricks #26
Suspended Flight Legs

Keith Thomassen, PhD, CFII
www.avionicswest.com

A simple flight plan (no procedures added) shows the list of waypoints you’ll pass enroute to your destination, each one being the end of a flight leg (TF leg) except the first one, which is a point leg (IF leg). The Active leg is the one sending signals to a CDI indicating any deviation from course, and is shown in magenta on the map and there will be a magenta marker on your flight plan identifying it. Course deviation can be used by an autopilot to make course corrections, and when you reach the end of each leg the next leg becomes active. This is automatic leg sequencing, which is done by every GPS.

If you add a procedure to this simple flight plan you will likely add legs that are not so familiar to you but are described (http://www.avionicswest.com/Articles/Legs.html) on my website and shown here in Figure 1. That article lists the 6 legs you can create, and the other 17 that are only added in a leg set representing a procedure.

Some of these don’t automatically sequence, and a suspend light (SUSP) is announced somewhere on your GPS display. When it does you need to figure out what to do; nothing, or push the SUSP key (now, or later). So you should learn which legs sequence automatically, which ones do not, and which ones might, depending on your specific GPS and the things wired to it. Some legs always suspend, such as VM, FM, and HM. These must be manually terminated, that is you need to take action at a specific time to go on to

Figure 1

Figure 1. All of the 23 “official” flight legs defined in ARINC 424 standards.

the next leg. The manual step is to push the SUSP key (to un-suspend). It’s important to note that this isn’t the only way to get out of this leg. You could go to your flight plan list and activate the next leg, or go Direct to somewhere on or off your plan. In an IFD-540 you must do this, since they do not have a SUSP button.

A departure or missed approach may include a vector (heading), a VM leg. An example is in the Skyline Eight Departure from KOAK. It begins with a VD leg, ending at the 4 DME from OAK, but then has a vector leg, and the SUSP light will come on. You’ll leave the VM leg when instructed by ATC, so you then sequence to the next leg (by pushing the SUSP key). It’s very important to know what it will sequence to, and for that you simply look at your flight plan list and it will tell you. In this case you’ll get further vectors to join a radial from a VOR (CR leg), which is the same as a course (CF leg) to some downstream waypoint on that radial. Your GPS will likely choose the latter option.

At the end of an Arrival you’ll sometimes find an FM leg, like the Concord Two Arrival into Sacramento Metro in Figure 2. On arriving at the ELKOE intersection, the next and last leg of the Arrival is a vector (FM leg) on a 340° heading. This puts you west of the field, parallel to the runways. We plan to do an ILS approach to 16R, so this is putting us on a right downwind. We’ll illustrate what to do there, and when, with the GTN 750. Figure 2 shows us on the vector in the left picture. The faf is at JARNU so when it passes off our right side you can expect vectors from ATC to the final approach course. The right picture shows the flight plan, and the ILS approach following the FM leg.

Figure 2

Figure 2. Arrival at KSMF, ending with an FM leg. We’ll transition to the ILS 16R.

When you push the UNSUSP tile above the SUSP light you will sequence to the next leg in your flight plan, which shows TUDOR next. On the left screen, note that it shows you on the “Vectors” leg and that D–> TUDOR is next. This is strange, since we chose vectors and not TUDOR as the iaf for the approach. But recent software changes to the 750 now add waypoints on final if you choose vectors, so that they can be used for navigation.

Contrast this same arrival and approach using a GNS 480, which does not add waypoints along the VTF. As shown in Figure 3 on the flight plan in the left screen, after the FM leg (HDG 340°) is the CF vector-to-final leg, a 168° CRS to JARNU. The SUSP light is also on, since we are just beginning the FM leg (vector) shown on the right screen. The SUSP button on the lower left will be pushed at the appropriate time.

Figure 3

Figure 3. Same Arrival at KSMF, but using the GNS 480

What these two GPS units have in common is that FM legs suspend, and pushing to un-suspend activates the next leg. This is true for any modern GPS, except the IFD 540. There you put a cursor on the next leg in the flight plan and choose the Activate Leg option. Figure 4 shows the result of pushing SUSP after passing JARNU (the faf) on

Figure 4

Figure 4. Sequencing to the approach in the 480 (left) and 750 (right).

each unit. On the 480 (left screen) the VTF leg is now active and shown in magenta. It’s a CF leg, a 168° CRS to the final approach fix. On the right, when removing the suspension, we head direct to TUDOR. There is a sharp corner there, so turn anticipation has come into play and we’re going to intercept the TF leg that ends at FAPIN. JARNU is the leg after that, and nowhere is there a Vector leg (CF leg) in the flight plan list. Different GPS unit do the same things but in different ways.

Let’s look at the 3 holds next. Most GPS devices (except the 430/530) allow you to create the HM hold at any waypoint in the database. When you enter these holds sequencing is suspended. When ready to exit the hold push SUSP inbound to the hold point. But what if you made that hold at a point not on your flight plan? In that case, you would instead go direct to some waypoint in your plan, or activate another flight plan leg.

The HF and HA holds are different. The HF hold is a pattern in lieu of a procedure turn. One purpose is to reverse course, so you’d think it’s simply another flight leg that will sequence automatically like the procedure turn. And it will, but it’s still a hold in case you needed to go around once more. So you need the ability to suspend sequencing. Figure 5 shows a “once hold” for the 750 (left) and GNS 530 (right) on a LPV approach to Mather Airport.

Figure 5

Figure 5. The “once hold” (HF leg) at Mather Airport (KMHR) in a 750 and 530.

For the 750 on the left screen, we’ve entered the hold and there is s SUSP tile, but no SUSP light (which would be in the box below it). That means sequencing will be automatic, but you have a tile to SUSP if you want to go around again. On the 530 in the right screen, there was no SUSP light when we entered the hold, but inbound the OBS key was pushed to suspend sequencing so we could go around again. In a 480, the SUSP light would come on when entering the hold, but when turning inbound to the hold the light goes out automatically and it will sequence to the final course unless you suspend.

An HA hold leg ends when you reach the specified altitude for the leg. Some units will sequence automatically at that altitude, but to do so your GPS needs to know when you’re there. It needs your baro-corrected altitude. If your unit has that input from an air data system it might do it, but it then depends on the software system for that unit; can it process that information and force sequencing.

Similarly, climbs on runway heading or course, or from a fix, to an altitude (VA, CA, or FA legs) may or may not sequence automatically depending on your unit and hookup to air data. An example of a VA leg is on the missed approach at KBFD that requires a climb to 4000’ on runway heading after takeoff. Another example (ahead) is on the Departure in Figure 6.

On some departures you find legs that terminate at an intercept or radial of the next course (VI, VR and CI, CR legs) and it is again dependent on your GPS as to whether it recognizes you are at that intercept or radial and will sequence automatically. The 480 will not, but the 750 will; it’s all a matter of what is programmed in their software. The same is true of legs terminating at a distance or DME distance from an initial fix (FC or FD legs), or of headings or courses ending in a DME distance (VD or CD legs). We referenced the VD leg on the Skyline Eight departure from KOAK. That leg will sequence in a 750, but not in the 480.

Figure 6

Figure 6. A Departure from KSMF, with a VA leg and 2 VI legs.

The VI leg is illustrated in Figure 6 in the DUDES 9 departure from KSMF on a 750. The preview and flight sequence is shown on the left, before Loading it, and begins with a VA leg to 1500’ before turning to a 140° heading (VI leg) to intercept the 156°course to LIAMM. The map on the right shows us on the doted magenta line that represents an active heading leg. After LIAMM there is a dotted white line (future leg), another VI leg on a 090° heading to an intercept of the 062° course to CROTI.

Lastly, there is leg suspension for missed approaches. The FAA requires this except in the 480, the first WAAS unit to be certified. There you sequence automatically to the MA legs from the missed approach point. The 430/530 shows a SUSP light there and you just push OBS key to continue. For the Chelton, shown in Figure 7 on the left screen, you will be presented with two choices on reaching the faf, Arm or Miss. Arming means the unit

Figure 7

Figure 7. Sequencing to the MA legs in a Chelton and the 750

will sequence at the missed approach point, but Miss forces sequencing at the moment you do it. It’s there so you can do an early miss. Note that SUSPEND is announced on the lower left corner. On the 750 in the right screen there is a SUSP light but no tile to UNSUSP. Instead there is the dialog box to remain suspended or activate the MA legs. If you want to start the missed approach early, go to the flight plan and activate the first leg of the MA procedure.

This completes the set of legs where leg suspension is in question; all others will sequence if they end at a waypoint on your flight plan. For example, a DF leg will sequence if you go direct to a flight plan waypoint, but otherwise it doesn’t know where to go next. Then you have to take more specific action, such as going direct to a point on your plan, or activate a leg in your plan. Same for an HM hold; if you put that hold at a waypoint in the flight plan you can sequence out of it to the next leg, by un-suspending the hold. But if the hold was created at a waypoint off of your plan it can’t sequence out of it and you have to decide what to do next. (Direct, or Activate Leg).

Leg sequencing is a complex subject, and remains mysterious even to long time users of their GPS. But this discussion should give you the information you need to figure it out on your GPS. As stated often here, there is no one rule for every GPS on sequencing legs. Each has it’s own capabilities and ways doing this. But one thing is always true for your GPS; it either does or does not sequence and you’ll know it if a SUSP light comes on. (The glaring exception is the IFD 540). Then, the question is what to do and when.

 

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