Aeronautical Decision Making Is Never One Size Fits All

ILS signals, which require transmitters and antennas on the ground, seem more susceptible to issues than GPS. For example, in March 2019, signal issues, caused by 2 to 5 feet of snow around the localizer antenna, led CommutAir Flight 4933 to touch down in snow to the right of the runway at Presque Isle, Maine (KPQI). Also, notes on ILS charts sometimes state that an autopilot cannot be used below some altitude on the approach. Those usually originate when problems are detected during routine test flights by an FAA flight check aircraft. 

Most of my teaching is in GA aircraft equipped with the Garmin G1000 and Cirrus Perspective glass cockpits, so my comments are slanted toward these systems and might not fully apply to what you fly. The software code base in these two avionics systems is similar, though there are differences between the two, and there have been changes over time.

A big gotcha when flying an ILS is switching the CDI needle, or HSI course pointer, from the magenta GPS needle to the green nav radio needle needed to fly an ILS. Although RNAV, typically GPS for GA aircraft, can substitute for several things, per the FAA’s AC 90-108, it still can’t be substituted for a localizer signal. So, when flying an ILS, the CDI must be switched to the green needle, and that’s where some hiccups occur.

In most installations I’ve seen, if the autopilot is engaged in NAV mode and the pilot switches the CDI manually, the autopilot will revert from NAV mode to ROL mode with no aural warning that this has occurred. While the “GPS” annunciator on the PDF will flash yellow for 10 seconds before turning to a green “ROL” annunciation, many pilots miss this subtle cue. So, they don’t realize the autopilot will no longer track a navigation signal but instead will just keep the wings level. With RNAV approaches, there’s no need to switch the CDI needle, so this gotcha is avoided.

These Garmin systems can switch the CDI needle automatically, which is convenient. However, in some older G1000 software versions, when this switch occurs, the autopilot switches from NAV to ROL mode. Pilots are even less likely to see the autopilot revert to ROL mode in this situation, as they may be looking away from the PFD when the change occurs. Fortunately, in the Cirrus Perspective, and in newer versions of the G1000, the autopilot now stays in NAV mode when the CDI needle switch occurs automatically. But it’s still an issue when a pilot changes the CDI manually. These gotchas led an airline pilot I was training in a Vision Jet to say, “I never understood why you general aviation pilots prefer RNAV approaches. Now I do.”

This aberrant autopilot behavior is one reason I prefer RNAV (GPS) approaches over ILS approaches. But one night, while flying a Vision Jet into Chicago O’Hare International Airport (KORD), I learned the hard way that there are times you do want an ILS approach loaded. One of those times is when flying a visual approach into a major airport. 

I was flying in the left seat, and I brought my friend Luke Fischer along, as long trips can be tiring, and I correctly anticipated that it might be helpful to have a second pilot on board when flying into O’Hare for my first time. Vision Jets are equipped with my all-time favorite avionics suite, the Garmin G3000. It’s immensely capable, and its touchscreen interface is easier to use than the G1000 or Perspective. 

It was nighttime and we were assigned the visual approach to Runway 27C. I love that many of the newer Garmin navigators can load a “visual approach” for any runway. A word of caution, though. There is no guarantee that these visual approaches won’t fly you through terrain, so don’t use them at night at unfamiliar airports.  

Since the approach was over water and terrain wasn’t an issue, I loaded the Visual Approach Runway 27C, thinking it was the perfect use case for Garmin visual approaches. That plan unraveled quickly on final, when we were given instructions to maintain a certain altitude until one fix and 170 knots to another fix. Neither of the fixes were loaded in our system, and I didn’t realize they were fixes on the ILS 27C. 

Fortunately, Fischer scrambled to find the fixes and let me know when we reached each one. It was only later that I learned that it’s a common procedure at most airlines to back up visual approaches with an ILS. Knowing this, ATC often refers to fixes on an ILS, even if an aircraft has been assigned a visual approach. So, when flying a visual approach into a major airport, you’d be wise to load an ILS approach, rather than an RNAV approach or a visual approach. 

Another time you might be better off using the ILS is if you have an emergency, perhaps a loss of engine power, and have requested to be vectored directly to the final approach fix (FAF). Under normal circumstances, ATC won’t do that as rules require them to bring you in at least 1-2 miles outside a “gate,” typically located about 2 miles outside the FAF. But if you’ve lost engine power and you’re in the clouds, heading straight to the FAF could save your bacon. 

However, if you have an RNAV (GPS) approach loaded, those approaches typically won’t give you vertical guidance, if you join the approach inside the FAF. RNAV vertical signals are just a series of computations made by your navigator, using GPS data. So there’s no physical radio signal in space providing the guidance. By contrast, the ILS uses radio signals, and you can receive them at any point along an approach, even inside the FAF.

Regardless of whether you prefer an ILS or RNAV, it pays to understand the trade-offs between them. The right answer is “it depends.”

This column first appeared in the January Issue 954 of the FLYING print edition.