Setting the CDI on a Conventional Approach (The “Kill Switch”)

If you fly an aircraft with an IFR-approved GNSS, you probably use that “suitable RNAV system” to help you fly all types of approaches, including ILS, LOC, and VOR procedures. In fact, if your aircraft isn’t equipped with DME or ADF, using an IFR-approved GNSS system may be the only way for you to fly many conventional procedures.

“Suitable RNAV systems” based on GNSS are described in AIM 1−2−3. Use of Suitable Area Navigation (RNAV) Systems on Conventional Procedures and Routes, AC 90−100, AC 90-108, and other FAA references.

For more information about the use of GPS along the final approach course of a VOR or NDB approach, see Use of GPS on Conventional Approaches (Update)

A critical step in flying conventional approaches while using GNSS to fly transitions/feeder routes is ensuring that the proper guidance is shown on the PFD/HSI as you intercept and then fly the final approach course.

For example, as shown below, when flying the LOC RWY 17 approach at Aurora, OR (KUAO), you could use the GNSS for course guidance as you fly the charted transition from the Battleground (BTG) VOR.

Some systems can automatically switch the CDI from the “magenta line” shown when using GNSS signals to “green needles,” usually labeled VOR/LOC, as you intercept final. But you must always monitor the avionics and, if necessary, use the CDI button (or other switch for your system) to change to VOR/LOC “green needles” before you join the final approach course.

For more information about guidance along the final approach course, see Use of IFR GPS on Conventional Approaches here at BruceAir.

Unfortunately, many pilots fail to confirm this critical step, which often occurs during a high-workload phase of an approach. For example, just as you are about to intercept the final approach course, ATC may issue a rapid-fire vector and approach clearance (“Fly heading 130, maintain 2,000 until established…”), you can be distracted while making a late configuration change, or while switching frequencies to the tower or CTAF.

In fact, this error is so common that many pilots and instructors call the CDI switch the “kill button” (or a similarly ominious name) to emphasize its importance.

I use a graphical reminder to help me ensure that I switch course guidance in plenty of time for a smooth intercept.

Like many pilots, I use a tablet and an aviation app (in my case, primarily ForeFlight) to display charts. Those apps typically have an annotation feature that lets you mark up charts to emphasize important information.

For example, on this chart for another approach at KUAO, I’ve noted a temporary change in minimums.

To remind myself to switch the CDI to “green needles” on conventional approaches, I use the annotation feature to draw a transparent green line along the final approach course.

I mark up the charts for conventional approaches during my preflight planning as I review weather, NOTAMs, procedures that I might fly, and other details.

I organize approaches that I fly often into binders in ForeFlight (other apps have a similar feature), and the markups are preserved between flights, so I don’t have to repeat this process for most of the procedures that I fly.

Because RNAV (GPS) approaches don’t require changing from GNSS guidance, I don’t highlight the final approach segment on those procedures.

To avoid cluttering charts, I also don’t mark the intial steps of a missed approach in magenta to signify that I can return to GNSS guidance to fly the miss, regardless of the type of approach. But if you’re in IFR training or new to using GNSS under IFR, highlighting the miss in magenta might be a useful reminder.