Friday, May 18, 2012

The Case of the Missing Waypoint

If you never use a KLN94, this post might not be of much interest to you.

Situated south of Sacramento, California are four television broadcast towers, locally known as the Walnut Grove Towers. Over 2000 feet tall, these four masts are among the tallest in the US, let alone the world.

The Sacramento Valley has few natural obstructions that would affect the design of an instrument approach, but these tower have done just that. When it comes to TERPs (the design standards for instrument terminal procedures) I'm no expert, but these towers undoubtedly affected the choice of altitudes at WAGER and WIMUP.

If human-made obstructions weren't enough, avionics designers have created insidiously convoluted interfaces. But that's only part of the complexity equation. In addition to TERPs, you may need to factor in the way instrument approaches are coded in the GPS database. The shortcomings and complexities in these designs have a combinational effect, resulting in a mess that pilots and flight instructors must live with on a daily basis. In some cases, we bet our lives on this stuff.

Flying this approach with an instrument student, imagine our surprise when we noticed that WIMUP was not displayed by the KLN94. After passing WAGER, the current waypoint was GIFME and it required some fast thinking to figure out how to determine when we'd passed WIMUP. Assuming this was an error, I decided to ask the folks at Honeywell and their response was enlightening.

Turns out that some GPS receivers (the KLN94 being one) can't handle intermediate waypoints between the Final Approach Course Fix (FACF), the Final Approach Fix (FAF) and the Missed Approach Point (MAP). For the KLN94, Jeppesen coded the database with WAGER as the FAFC, GIFME as the FAF, and RW25 as the MAP. Since WIMUP is located between the FACF and the FAF, it's not in the database. If you're a dive-and-drive kind of pilot, this bit of information may give you pause. You'd need to descend pretty rapidly and be waay off course to get into trouble, but running into the guy wires that hold up those tall antennae would not be pretty.

Staying up-to-date on these bits of minutia is a challenge and it's unclear how many other approaches out there are affected by this issue. So always cross-reference what your GPS says with the approach chart and remember that when it comes to RNAV, it's a jungle out there.


Ryan said...

Wow, that is enlightening. It's been many years since I used a KLN94, but after selecting the approach, is it possible to add missing the waypoint(s) into the approach, or are they simply not recognized at all in the Honeywell database? This can potentially be a huge safety risk if the approach is not reviewed carefully.


John Ewing said...


No, you can't add waypoint within an approach on the KLN94. I don't know if WIMUP is even in the database (I suspect it is), it's just that the KLN94 design precludes that waypoint from being displayed because of the way Jeppesen coded the approach.

I agree that carefully comparing the approach chart with the GPS flight plan is a good SOP.

CG said...

The software engineer in me cringes...

Dave Starr said...

Very interesting John. An approach ignoring a stepdown fix by design just floors me. I didn't think intentional omissions like this were legal ... certainly not advisable.

An eerily similar setup I noted here:

The pilots in the accident at MKY were responsible for cooking their own goose, so to speak, but the proximate factor was indeed ignoring a stepdown fix that was in the approach for a very definitive, solid steel reason.

Obviously, "Diving and driving" can be as deadly as "Drinking and driving".