KLN 94 Gotcha Flying RNAV GPS Y and Z Instrument Approaches

Imagine calling the police and hearing “I was just thinking about calling you.” Those were the words I heard a few days ago when I phoned the California Highway Patrol (CHP). Fortunately, I wasn’t being investigated for a crime! Instead, a serious issue with the Bendix/King KLN 94 GPS, found in almost every Cessna built between 1997 and 2004, had been uncovered.

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CHP’s chief pilot called me last year when he was purchasing some of my G1000 books and CDs, since one of their Cessna 206s is equipped with the G1000. The rest of their 206s are round gauge airplanes with the KLN 94. In these latter aircraft, there’s a potentially deadly issue with flying RNAV (GPS) Y and Z approaches, so he’d been thinking of calling me to discuss it. 

To quote from my Max Trescott’s GPS and WAAS Instrument Flying Handbook, “Whenever a runway or heliport has two or more RNAV approaches, a single alpha character differentiates them. The first GPS approach charted to a runway is labeled “Z,” and subsequent approaches are assigned other letters, moving backward through the alphabet. Thus, the second RNAV approach to the same runway is labeled “Y.” Where you have Y and Z approaches to the same runway, usually the Z approach has precision LPV minimums and the Y approach has higher LNAV minimums. In rare cases, for example at the Nut Tree Airport in Vacaville, Calif., the Y approach is for LPV minimums and the Z approach is for LNAV minimums.”

Max Trescott’s GPS and WAAS Instrument Flying Handbook has a full chapter devoted to flying GPS approaches with the Bendix/King KLN 94. Quoting from that chapter: “It is certified under TSO-C129A, so it cannot be used for WAAS-based approach minimums, such as LPV, LNAV/VNAV, or LNAV+V. However, it can be used to fly to non-precision LNAV minimums. It’s important to note there are several versions of this GPS, only one of which can be used to fly GPS approaches. Another version can be used for en route IFR operations, including flying Arrival and Departure Procedures, but not for instrument approaches. Yet another version can be used only for VFR GPS operations.”

When using the KLN 94 to fly RNAV (GPS) Y and Z approaches, there’s a huge gotcha that could kill an unsuspecting pilot. Specifically, the problem is that when you select a GPS approach on it, the letters Y and Z don’t appear in the approach titles, so it’s difficult to know which of these approaches you are selecting. Let’s look at the GPS (RNAV) Y 36L and GPS (RNAV) Z 36L approaches into the Napa County Airport, KAPC.

Both approaches have identical IAFs and fixes, except for the two fixes in the missed approach. The “Z” approach has LPV minimums of 300 feet and LNAV minimums of 680 feet; the latter would be used with the KLN 94, since it’s not WAAS-capable. The “Y” approach has LPV minimums of 1202 feet and LNAV minimums of 1620 feet.

Only one of these approaches is in the KLN 94 database. Since the Zulu approach is the one with the lowest WAAS minimums, you might guess the non-WAAS KLN 94 contains just the Yankee approach—but you’d be wrong! Even if you correctly guessed that it’s the Zulu approach in the GPS, there’s still a trap: that approach requires a higher, non-standard climb gradient to fly the missed approach! If you missed the note about the 250 ft/nautical mile climb gradient, you might find yourself unable to clear the mountains immediately north of the airport!

There’s one way to determine if a GPS approach in the KLN 94 is a Y or Z. You must start to load the approach, inspect each of the waypoints displayed before the final button push to load the approach, and compare them to the waypoints in the instrument approach chart.  Let’s look at loading the GPS (RNAV) Z 36L approach into Napa County Airport. 

After, you push the PROC key and select the airport, you’re presented with a list of approaches (see screenshot at top of this article). Notice there’s just one RNAV (GPS) 36L approach, but it doesn’t say whether it’s the Y or Z approach. 

Then you’re presented with a list of IAFs. I scrolled down and chose MICRA, though it could have been any one of the IAFs or Vectors.

Finally, you’re asked if you’d like to add the approach to Flight Plan 0, the active flight plan. You’ll note that the first two waypoints, the IAF SABLO and the intermediate fix FESAV, are identical for both the Y and Z approaches. You can scroll to see the missed approach waypoints, including the hold at POPES. If you inspect the Y and Z approach plates, you’ll discover that this is the hold for the Zulu approach (the missed approach hold for the Y approach is HABAP). Now you’ve determined that the only GPS approach in this GPS for KAPC is the Zulu approach.  

The chief pilot contacted Bendix/King and they may send out an update to owners in the future. But that will leave a lot of renter pilots totally in the dark! But at least now YOU know. To learn more of the ins and outs of flying GPS and WAAS approaches with the Garmin 430, 530, G1000, Perspective, KLN 94 or GNS 480, order a copy of Max Trescott’s GPS and WAAS Instrument Flying Handbook and/or Computer CD course online or call 800-247-6553.

Comments

[Roy]

Did Honeywell actually acknowledge this problem or is the phrase "they may send out an update to owners in the future" just speculation? I ask because the company failed to respond at all to my repeated inquiries on the disappearance of the depiction of the DC SFRA boundary from the KLN-94

[Max Trescott]

I heard there was successful contact with both Honeywell and Jepp. I recall that one said they would send out an update, but I didn't recall which one (I think it was Honeywell). Since I wasn't 100% positive about that point, I worded it the way I did.

[Roy]

Cool, thanks.

[Jim McCord]

Max - this is really important information to get out! Thanks for sharing it. We've got a KLN94 Cessna on the line in Santa Rosa and I've done training in it to APC (although not since APC got WAAS, so it's been a while). I think this is so important, have you thought about writing a SPANS notice for pilots? Or an article for FAA Safety Briefing?

[John White]

Max:

Thanks for the heads-up on this. I have been flying an aircraft with the KLN94 and did not realize this.

Great Post!

John