Many of us remember the days before GPS, when navigation was primarily with VORs. These, coupled with a CDI to show your course, required that you tune in the VOR or localizer (VLOC), and set the OBS course on the CDI to track that course. In the age of glass panels, with modern GPS/VLOC receivers like the GTN 750, coupled with a primary flight display (PFD), most of the chores involved in shooting an ILS are automated. Here, we’ll walk through a typical scenario to illustrate the many ways in which that happens.
From my home base of Nevada County Airport in Grass Valley, California (KGOO), let’s plan to fly the ILS 14 practice approach to nearby Yuba County Airport (KMYV), which is loaded into the 750 from the PROC page. We enter the airport, the ILS 14, and the transition at CULUL. Choosing “Load Approach & Activate” creates a flight plan consisting solely of the approach. By activating it on the ground a flight leg is created from KGOO to CULUL, which is the initial approach fix and (after the procedure turn) the final approach fix. In this process, the ILS frequency is put into NAV standby. This is a quick way to make a flight plan for practice approaches without having to specify your home airport or destination in the plan.
Figure 1 shows the map page of the flight on the left, including the missed approach to YUBBA, while the right side has the flight plan with the VNAV altitudes listed for the step-downs. Note on the top right that the first leg is “Direct CULUL.” Climbing to 4,500 feet creates a Top-of-Decent (TOD), where the VNAV course to CULUL begins. CULUL has a crossing altitude of 2,100 feet. If VNAV is enabled in the 750, and you have a Garmin PFD and their recent digital autopilot, you can fly this VPTH (vertical path) with the autopilot in VNAV mode. If not, but you have an autopilot with VS mode, you can use that. First, check the VNAV descent rate on the Flight Plan page (Menu -> VNAV) to determine the vertical rate that will reach the desired VNAV altitude. If instead you want to descend at, say 500 feet per minute, select that here and the TOD point will adjust accordingly. On this same Menu page, you can enable VNAV.
Figure 1. A flight plan (right) consisting solely of the ILS 14 to KMYV, with CULUL as the initial approach fix. We are one min from the TOD (left), a reminder to enable VNAV and engage the VNAV mode on a GFC 500 or 600. Note the GPS altitude (GSL) of 4,500 feet on the map (upper right corner), and the VNAV altitudes of waypoints on the flight plan list.
After takeoff, with the autopilot in GPSS mode if digital, or on an analog autopilot in HDG or NAV if you have an external or internal GPSS convertor, you’ll keep the CDI on the 750 in GPS mode until switching to VLOC for the ILS. Note that while the S-TEC 3100 digital autopilot names this lateral mode GPSS, the GFC 500 autopilot only has a NAV button, but tracks these digital roll commands in that mode.
While enroute to the TOD, this is a good time to review the chart for the ILS course bearing, minimums, and missed approach procedure. You should review the waypoint altitudes too, but these are shown on your flight plan list, and on the map next to each waypoint (see the 2100 box next to CULUL on Figure 1). You can enter the minimum altitude for this approach (264 feet) into the PFD as a reminder on final to the RW14 map.
To automatically switch the CDI on the 750 from GPS to VLOC when intercepting the final approach course inbound, enable this on the System -> Setup page. That selection is shown in Figure 2. The center picture shows us tracking the GPS course outbound on the procedure turn, which has a TOD in the turn to start down the VNAV course to CULUL. On the right you see the CDI has switched to VLOC. This happens when you penetrate a box between two and 15 miles from the final approach fix, and within 1.2 nm of the final approach course, providing you first switch the NAV frequency for the ILS (110.50) into active (center picture). If you’re vectored onto the approach within two miles of the final approach fix it won’t happen. While on the 45-degree inbound to final, select the APR mode on the autopilot to arm, then capture, the glideslope at the right time.
Figure 2. Use Auto Switch for the ILS CDI Capture from GPS to VLOC (left). On the procedure turn outbound (center) there is a TOD at the start of a VNAV path that ends at a 1,600 feet bottom-of-decent (BOD) at the final approach fix. On the right, while inbound on the 45 degree, the CDI has just automatically switched to VLOC.
If you have a PFD with an HSI, you will be able to select the source driving the course needle. But when the 750 CDI switch occurs, the PFD will switch the source driving its HSI to VLOC as well, saving you that step. Common PFDs are the Aspen, Garmin G5 (HSI page), and the left half of a Garmin G500, all of which have this feature.
Another time saver is that the PFD can automatically set the course for the ILS (141 degrees here). On the Aspen, you must enable AutoCrs to do this. On the G500 Autoslew is automatic unless you have already selected NAV on the 500 before the 750 switches. Then, you must push the CDI soft key to select something else (GPS or the other NAV), then return to NAV. Now, it will Autoslew. Normally, you will have the CDI set to GPS on the G500, and then it will switch to LOC when the 750 switches, and the ILS course will automatically be selected. When in AutoCrs or Autoslew, you cannot change the CRS with that knob on the PFD.
Continuing the flight inbound to the RW 14, leg sequencing to the missed approach is suspended at that waypoint, as shown in Figure 3. Activate those missed approach legs by selecting “Activate GPS Missed Approach.” This activates the first CA (Course-to-Altitude) leg, on runway course, ending at 1,000 (center), when it will automatically sequence. It sequences to a leg that is a 309 degrees TRK (right) to intercept the 263 degree course to YUBBA, the missed approach hold point. Note that this intercept leg is not shown on the flight plan list in Figure 1; it’s a CI leg (Course-to-Intercept) that some GPS navigators will show. In the picture on the right it shows that our active leg is the course 263 degrees to YUBBA. Clearly it is not, since we’re on a 309° track. However, the unit will continue to this intercept point and then turn to 263 degrees, as if sequencing to the CF leg (Course-to-Fix) to YUBBA.
Figure 3. Arriving at the missed approach waypoint, sequencing is SUSP (left). So, “Activate GPS Missed Approach” to switch from VLOC to GPS and restore sequencing (center). Here, we are at 880 feet, and at 1,000 feet sequencing from this Course-to-Altitude (CA) leg will occur. The next leg (missing on the flight plan list) takes you to the course 263 degree Course-to Fix (CF) leg to the hold at YUBBA.
One final point; if you’re out practicing approaches and want to do another one nearby, you can add a second approach into the unit while on the missed approach legs of the current approach, but not earlier. With the autopilot flying the remainder of the missed approach and the hold, select another approach, say to nearby Oroville (KOVE). Touch the Home button, select PROC, select KOVE and RNAV 02 LPV, with the ESVOW initial approach fix, then choose “No” on flying the course reversal there, since your course to ESVOW from YUBBA is 346 degrees and the inbound to KOVE is 019 degrees, a 33-degree turn. Load this approach, and these new legs will be added after YUBBA. Only the missed approach legs from the ILS 14 to KMYV will remain in the plan ahead of this new approach.
There are many ways to begin this approach. When inbound to the hold waypoint at YUBBA, touch the title “Approach – KOVE RNAV 02 gps LPV”, then “Activate Approach” and you’ll proceed direct to ESVOW from present position. Alternatively, you can select ESVOW from the flight list and “Activate Leg” when you arrive at the hold point. Even more simple, just UnSusp while in the hold. This will restore sequencing and make a smooth transition at the hold point. However you sequence, adding another approach at this time is a very easy, stress free method to set up a new practice approach while the autopilot is flying the hold from the last one.
Suppose you wanted to depart the hold at YUBBA, which is on V23, to go northbound on that airway. You can’t select that airway from the hold leg, it has to be added to (cursor on) a leg ending at a waypoint on an airway. Here, the CF leg before the hold, ending at YUBBA, is such a leg. But if you touch that leg on the flight plan, Load Airway is shaded out, and you cannot select that airway. So, you add GRIDD to the flight plan after the hold, since it’s the first waypoint north of YUBBA on the airway. Touch that, and again the Load Airway key is shaded. It turns out that when you have an approach in the flight plan you cannot add an airway to the plan after the approach legs. The solution is to proceed D->GRIDD after the hold, then remove the approach from the flight plan, then add the airway at GRIDD.
I encourage you to follow this entire flight using the GTN Trainer on your iPad or tablet.
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 63 years, has the Wright Brothers Master Pilot Award, and is a current CFII.
