8 min read

“Fuel, flaps and trim.”

My primary flight instructor taught me to always double check these things right before entering the runway for takeoff: fuel selector on the right tank, fuel pumps on (if required), flaps set for takeoff and trim(s) set for takeoff. These were “the killer items,” I was told, and having them set improperly on takeoff could lead to disaster. Almost anything else could be handled in the air. Tune the radio wrong? Easy to fix after takeoff. Have the trim set too far nose up? You might not have time to recover.

This quick checklist is meant to be used in addition to a printed checklist, but it has been a great aid for me (and probably thousands of other pilots). It’s been especially helpful as I’ve transitioned into different airplanes. Whether it’s a taildragger or a turbine airplane, these three items are nearly universal, and I find myself chanting “fuel, flaps and trim” every time I take off. More than once, it has caught something set wrong.

I’d like to suggest a similar killer items checklist for instrument pilots, and we have technology to thank for it.

Checklists are the norm for IFR flying, since mistakes are often punishable by death, but sometimes checklists can actually get in the way. When you’re trying to intercept the final approach course on a slam dunk approach and the frequency is jammed with traffic, it’s easy to skip a step or forget the checklist entirely. That’s why I always verbally confirm two quick things before starting an approach: CDI button and type of minimums.

CDI setting

Before GPS, the course deviation indicator (CDI) or horizontal situation indicator (HSI) was fairly simple. Turn the knob to set the correct inbound course and fly the approach. But with the addition of GPS receivers to many general aviation cockpits, things got more complicated. Now, the CDI or HSI can receive inputs from both the nav radio(s) and the GPS(s). It’s up to the pilot to select which source is driving the CDI.

Garmin 530

The CDI button is small, but it’s very important.

In some airplanes there is a separate button on the panel to select this, but on most newer equipment this is done right on the GPS. For Garmin 430/530 receivers, there is a dedicated CDI button below the screen; for Garmin GTN 650/750 receivers, there is a CDI button on the touch-screen below the map. It’s simple to do, but also very easy to forget. On top of this, some systems let you select GPS 1 or 2, which doubles the chance of messing something up.

Here’s the most likely scenario (and one that has trapped me before): you navigate for the entire trip based on GPS, so the CDI selector is set to GPS. As you approach your destination, you get cleared for an ILS approach. You turn the knob to the inbound course for the ILS, but you forget to change the CDI button from GPS to VLOC. So while everything looks normal, you’re actually tracking whatever the GPS is set to–not the localizer. On some approaches this isn’t a big deal (especially if you loaded the ILS into the GPS flight plan), but on others this is a critical mistake. As you chase the needle, you’re flying far away from the localizer. Plus, you likely won’t see a glideslope, which can cause even more confusion.

In fact, this exact scenario happened in 2005 when a Pilatus PC-12 crashed in Pennsylvania, killing all aboard. It’s not clear what the final cause of the accident was, but it is clear that the avionics were not properly set. The navigation system lights showed that the CDI was set to GPS, even though the airplane was cleared for an ILS. This easily could have caused the course and altitude variations that the radar track showed.

If you’re flying a glass cockpit like a G1000, the HSI does a pretty good job of alerting you. The screen will show either GPS or NAV, and the color will change from purple (GPS) to green (nav radio). But for most steam gauge airplanes, it isn’t obvious at all. That’s why a simple check of the CDI button before the final approach fix is a must. If you have the option of displaying GPS 1 or 2, verify that setting as well.

What kind of minimums?

Just like CDI settings, the minimums line on approach plates is getting more and more complicated. It’s not unusual to see four lines for an RNAV (GPS) approach: LPV, LNAV/VNAV, LNAV and Circling. Without getting into the details (you can read those here), the critical thing to remember is that the LPV minimum is a decision altitude (DA), while the LNAV minimum is a minimum descent altitude (MDA). That means when you hit MDA–unlike a DA–you must level off until you see the runway environment and can make a safe descent to land.

RNAV approach minimums

Remember that the LNAV minimum altitude is an MDA – so level off.

So far, so good. But things get confusing when you’re flying an LNAV approach with a WAAS-approved GPS. That fancy GPS will create an advisory glideslope (what the FAA calls a VDA) between the final approach fix and the runway. This is a great feature, allowing you to fly a constant descent instead of the “dive and drive” approach. If you’ve ever flown one of these LNAV+V (the +V indicates an advisory glideslope) approaches, you know how much better they are.

There’s a trap here if you’re not careful. With the advisory glideslope, the LNAV approach looks an awful lot like an LPV approach. But if you keep flying the advisory glideslope after hitting MDA (as you would on an LPV approach), you might see some very large trees fill the windows. After all, one reason why an airport might only have an LNAV approach is obstacles on final don’t allow a more precise LPV approach. As the FAA says:

The published VDA is for information only, advisory in nature, and provides no additional obstacle protection below the MDA.

The rules haven’t changed here–an MDA still requires you to level off and wait to descend until your eyes tell you can make a safe landing–but it’s important to remember which type of approach you’re flying. If it’s an LNAV approach, it’s still a non-precision approach.

In the end, the killer item here is whether the minimum altitude is a DA (annunciated as LPV) or an MDA (annunicated as LNAV+V). Put another way, when you hit minimums, should you plan to level off or keep descending? Reminding yourself of this as you start the approach, in addition to the minimums, is a great practice.

Visual Descent Points

While it may not be a killer item, a natural question that arises from this discussion about LNAV minimums is how can you tell when to descend to the runway? As usual, your eyes are the key instrument here. But in addition to this old standby, more and more approaches now feature a Visual Descent Point (VDP), which is indicated by a bold V on the profile view. VDPs have been around for a while, and show where a standard descent angle intercepts MDA. VDPs often serve as a de facto missed approach point, since descents after the VDP will usually be quite steep.

Fortunately, FAA approach plates are now showing an additional piece of information.

For RNAV approaches only, the presence of a gray shaded line from the MDA to the runway symbol in the profile view, is an indication that the visual segment below the MDA is clear of obstructions on the 34:1 slope. Absence of the gray shaded area indicates the 34:1 OCS [obstacle clearance surface] is not free of obstructions.

The 34:1 slope happens to be a normal 3 degree angle, so if you see a gray shaded line after the VDP (see the image above), you know you can make a normal descent from VDP to the runway and remain clear of obstacles. If you don’t see that gray line, be alert for obstacles on short final.

At its most basic, a killer item checklist is meant to double check things right before a critical time, and takeoffs and instrument approaches certainly qualify. Descending from final approach fix (FAF) to the missed approach point (MAP) is one of of the few times you willingly fly close to terrain with no visual reference, so you’d better be certain that everything is just right. If you’re unsure or uncomfortable, it’s time to fly the missed approach and sort things out.

Making these two checks doesn’t take long at all. In fact, on the Garmin 530/430 these two settings are right next to each other. You can check that the CDI indicator is set to GPS or VLOC, then glance up slightly and see what kind of approach you have loaded (a green box should show either LPV or LNAV+V). This two second check may never find a problem, but if it does you’ll be happy you verified things.

Your Comments

Have you ever been caught by one of these traps? Do you have your own killer items checklist? Add a comment below–registration is not required.

John Zimmerman
25 replies
  1. Lawrence
    Lawrence says:

    I have been caught by the GPS to VLOC trap on an ILS.
    There’s a note in the manual for the Garmin 530W about this on p.104.
    Automatic switching is enabled with some autopilots.
    My experience is that sometimes it happens and sometimes it doesn’t.

    • Warren
      Warren says:

      The aircraft has to be within 1.2nm laterally, and within 2 to 15nm from the FAF to transition automatically. So if you are too wide it will not transition or if you are put on a tight base (less than 2nm from the FAF) it will not transition, according to one 430 manual. There is a diagram in most manuals I’ve seen depicting the area of automatic transition.

  2. Remington Box
    Remington Box says:

    At the flight school I went to, the VLOC/GPS button was referred to as the $500 button because of failed checkrides.

  3. Mauro Giacomet
    Mauro Giacomet says:

    >>That fancy GPS will create an advisory glideslope (what the FAA calls a VDA)

    I think the FAA made a big mistake there.

    At the DA the “glideslope” needle should indicate “fly up”, NOT continue all the way down to the runway. They tried to mimic the ILS glideslope, but the ILS is the way it is because of the technology.

    With the GPS it’s all in software, so for the sake of safety, I’d have made it indicate “fly up” at the DA.

  4. Cary Alburn
    Cary Alburn says:

    I’ve had a 430W in my airplane for a year, and I’m still (maybe always will be) in a learning mode. I knew about the CDI button trap but haven’t yet experienced a failure of the software between my ears–without an autopilot, it’s necessarily always a manual change for me. I did not know about the green box indicating the difference between LPV and LNAV+V approaches vs. non-precision advisory only versions–there is just so darned much to learn about the 430W! Very helpful–thanks!


  5. Craig
    Craig says:

    I am reminded of my first post-purchase, solo flight in that wonderful, slightly used 1984 M20J, with a nonIFR certified KLN-94 GPS. I navigated IFR direct via GPS for the three hour flight, VFR on top of the low cloud layer. Approaching St Louis, MO, I checked the weather (IMC) and started setting up for the ILS approach. Well bless my soul, I couldn’t remember where the d-damn switch was to change the nav display source from GPS to ILS. After a few turns, the ATC congroller changed from the initial voice to an obviously senior controler. You know you are putting on a show when that guy asks how you are doing on fuel. I finally gave up and requested a vector to the nearest VFR field. Once on deck, a quick phone call to the previous owner cleared my foggy memory on switch location. I don’t think I will EVER goof that up again, or confuse the nav source for the desired approach. Ugh!

  6. R. Fechter
    R. Fechter says:

    Somewhere I read about “Source, Force, Course”. It’s my pre-FAF quick checklist. Source, naturally, being VLOC or GPS. Force is what my GPS has decided for me on the type of minimums (LPV, LNAV+V etc). Course was the CDI course setting but now that I’m usually using one that automatically sets the CDI, I’ve been using “course” to remind me to check and make sure the autopilot is setup correctly.

  7. Paul
    Paul says:

    Been there done that during instrument training. Instructor in the right seat was sitting there laughing tears of joy as I struggled to get her down. I remember being totally confused about the missing glides slope. Eventually I realised what had happened which allowed my instructor to verbalise his euphoria… I hope I never forget that experience.



  8. Scott
    Scott says:

    My memory code for this and in fact when ever I’m making a navigation decision (driven into me by my instructor while training IFR) is IDENTIFY SOURCE so if I’m steering gps I look for that gps symbol above the CDI button, but if I’m going to a VOR or flying a radio signal approach I besides identifying the morse code, check that I’ve got the VLOC above the CDI button. It certainly helps me to do that.


  9. James Guldi
    James Guldi says:

    John’s blog

    This is a very good discussion. Thanks very much for all the insites.

    I teach Tailwheel and so not very exposed to this type of flying.

  10. RayPad
    RayPad says:

    CDI Setting- paragraph 5, “If you’re flying a glass cockpit like a G1000, the HSI does a pretty good job of alerting you. The screen will show either GPS or NAV, and the color will change from purple (GPS) to green (nav radio).
    Visual Descent point-Paragraph 5, “Making these two checks doesn’t take long at all. In fact, on the Garmin 530/430 these two settings are right next to each other. You can check that the CDI indicator is set to GPS or VLOC, then glance up slightly and see what kind of approach you have loaded (a green box should show either LPV or LNAV+V).
    Is the color change opposite for the G-1000 VS the 530?

    • John Zimmerman
      John Zimmerman says:

      Sort of. On the 530/430, the box that annunciates LPV/LNAV/etc is always green. Only the text changes.

      Above the CDI button, the word GPS is in green and if VLOC is selected it is in white.

  11. John
    John says:

    A 34:1 slope happens to actually be a -1.69° approach angle, almost half as shallow as the normal 3.00° glidepath.

      • Robert Chapin
        Robert Chapin says:

        Thanks for the link. The probable cause is described as, “pilot’s failure to maintain sufficient airspeed.” I also noticed the report doesn’t adequately describe the avionics involved. Does the GPS Select Mode in that airplane turn off the glide slope indicator, or does it only drive the EFIS controller? In other words, what was the pilot looking at if not the airspeed?

        • John Zimmerman
          John Zimmerman says:

          It’s confusingly written, but having flown PC-12s with these avionics, it’s easy to see what (probably) happened:
          – the EFIS controller sets the nav source for the HSI. In this case, he had GPS set in here with OBS. Pretty common in my opinion–he wanted to visualize the ILS while he was still a ways out.
          – the autopilot controller is either NAV, APR, or HDG. If it’s on NAV or APR it’s tracking whatever you selected above.
          – my guess is he never switched from GPS to the ILS on the EFIS controller. So he kept seeing a valid localizer needle (or at least the GPS overlay), but he never saw a glideslope. I’ve made this mistake myself, hitting the outer marker and not seeing a glideslope, I looked right at the mode selector.

          Obviously the airplane eventually stalled and crashed. It’s not real helpful for the NTSB to point out that the pilot didn’t maintain airspeed. What’s useful for learning is why. In my opinion, he has his avionics mis-set, got confused momentarily, probably clicked off the autopilot to try to fix it, and lost control.

          • Robert Chapin
            Robert Chapin says:

            Excellent insights, thank you.

            I think the NTSB distinguishes fairly between the navigation issue and the loss of control issue. It is tricky to say if one mistake leads to the other without more details.

  12. Rob
    Rob says:

    I was taught TISC or TICS. Tune the frequency. Identify the VOR is operational. Set the course in the OBS. Check the GPS/NAV button is correct. KLN 94 C172.

  13. David Megginson
    David Megginson says:

    As a proud new GTN 650 owner (after 14 years of conventional IFR in my Warrior II) I’ve just come back to this article—useful and succinct for someone working to learn the RNAV lore. Do you have some way of organising links to past articles so that we can find related ones all together (like tags or topics)?

  14. wayne johnson
    wayne johnson says:

    I would like to sign up for any newsletters you have
    also would like to read past articles you have written

    This article was very helpful and cleared up problems I have had

  15. Mark Hathaway
    Mark Hathaway says:

    It appears the introduction of this new-ish technology (GPS) has introduced one additional confusion point for pilots. The ability of several pieces of equipment to work together (or not) adds another. Conflicting advice from the FAA adds another.

    Take all this complexity and you have a big problem.

    Perhaps some day GPS+WAAS will be standard and flying anywhere, including the approaches will be done one and only one way. But, for now there are deadly gotchas.

    Discussions like this are good. But, you’ve got to be the loudest squeaky wheel to get the attention of equipment manufacturers, the FAA, and pilots associations.

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