Not all RNAVs are created equal

A few years ago, I hadn’t flown in a few years, so I went to my local FBO to for biennial flight review/instrument proficiency check combo platter.

Wrapping up air work, my instructor launched right into approaches. “You’ve done LPVs before, right?” he asked me. “Sure,” I replied, thinking back to a previous life where he and I’d both flown turboprops at the same commuter airline. The FMS systems onboard those had LNAV/VNAV/RNAV databases loaded in, so in my mind, I conflated LPV with the rest of the abbreviations.

Once he loaded up an LPV approach and started briefing the approach plate with reference to the electronic glide path indicator, I realized this was something I’d never seen before. Before I had a chance to admit my error, he shut down an engine on me.

I went into autopilot mode and once “dead foot, dead engine” kicked in, muscle memory had the power lever pulled, the fuel shut off, and the propeller feathered. All secured, I declared an emergency and continued on the approach. The GPS was tracking, so I followed the glide path to the DA and then went missed. After the lesson, I admitted that had been my first ever LPV approach.

LPVs, LNAVs, and VNAVs are all RNAVs, but not all RNAVs are created equal. Sorting out the various RNAV approaches can be difficult.  An LPV is a Localizer with Vertical Guidance approach. It takes advantage of the refined accuracy of the wide area augmentation system (WAAS) lateral and vertical guidance to provide approaches approximating the accuracy of a Category I ILS. This is not be confused with an LP, a Localize Performance without Vertical Guidance or Lateral Navigation.

With a precision ILS approach, if the glideslope indication fails but the localizer frequency is still good, the approach can degrade to a non-precision one as long as the associated minimums can be maintained. An LP is not a degraded version of an LPV. They are independent approaches, partially because they’re both non-precision approaches. Both provide increasing lateral sensitivity (similar to a localizer) the closer the aircraft gets to the runway. LPs are generally created where terrain or other obstructions prohibit consistent vertical guidance.

Confused yet? You’re not alone.

A Part 135 flight crew flying the RNAV (GPS) 22L approach into Chicago Midway Airport (KMDW) thought they were flying an LPV approach. For a variety of reasons, it wouldn’t load from their flight management system (FMS) although they could see it in the database. The crew ended up flying a visual approach as they were in the clear in VFR conditions.

After the event, the reporting pilot wrote in his report to the Aviation Safety Reporting System, “I thought, through some research on LP approaches, that they were supposed to have LP in their title, but they do not.” He further lamented that a pilot cannot revert to an LNAV approach if he encounters a problem while flying the LP approach. A missed approach is the procedure with an attempt at a new approach.

An LP is an RNAV/GPS approach, but without the benefit of its own plate. LP information is found in the DA/MDA section of the approach plate. If a line of minima is present on the approach plate for an LP, it will be listed as a minimum descent altitude (MDA), not as a decision altitude (DA) the way LPVs and LNAV/VNAV minima are depicted.

But wait, aren’t all RNAV approaches non-precision? Yes. Yes, they are.

And yet LPVs and LNAV/VNAVs have DAs just like a precision ILS approach? Also true.

Flying an LPV or an LNAV/VNAV approach requires a WAAS-enabled GPS unit onboard. The benefits of WAAS include it being unaffected by snow reflections that can impact ILS operations; use of RNAV approaches to be used for alternate airport flight planning; and the use of LNAV/VNAV minima without temperature restrictions.

Unless…

A pilot filed a NASA report after failing to successfully interpret two RNAV approach plates for Boca Raton Airport (KBCT).

“The charts for the GPS to Runways 5 and 23 at BCT both contain a note that Baro-VNAV is not authorized when using the Palm Beach International (KPBI) altimeter,” he wrote.

On the same approach plates, though, there are notes authorizing LNAV/VNAV approaches with adjusted minimums when using the KPBI altimeter setting.

The pilot’s concern stemmed from the presidential activity in that area.

“With the increase in activity at BCT when presidential TFRs are in place, operations using PBI’s altimeter are very possible,” he continued in his report. Nobody wants to bust a TFR.

His concern did not go unnoticed. As of this writing, at least the BCT RNAV GPS RWY 5 approach plate has been clarified. It now stipulates when the BCT altimeter setting is not available, it’s okay to use PBI’s, but the DA must be increased 43′ and all MDAs by 60′. All visibilities increase too. Baro-VNAV operations are still not authorized when using the PBI altimeter setting.

What differentiates Baro-VNAV/LNAV from plain old vanilla VNAV/LNAV?

Barometric Vertical Navigation is an RNAV system utilizing the aircraft’s barometric altimeter to compute vertical guidance. Normally, the vertical path is computed between two waypoints or an angular relationship from a single waypoint. Waypoints are satellite-based.

Baro-VNAV allows a manufacturer to offer a pilot Advisory Vertical Guidance. The guidance that appears on a pilot’s display is an artificially generated advisory glide path from the final approach fix (FAF) to a touchdown point on the runway. Use of it requires an operational barometric (as opposed to satellite-referenced) altimeter as the primary altitude reference.

Use of the system is also temperature limited. For example, Baro-VNAV is not authorized for the BCT RNAV GPS RWY 5 or RWY 23 approaches below 5° Fahrenheit or above 130°. Baro-VNAV is noted on the GPS unit with “LNAV + V.”

RNAV capability only began its present iteration in 2003, less than 20 years ago. So in human terms, it’s still in its first generation.

The FAA is aware of the limitations of RNAV. That’s why a capitalized white “W” on the black background in the notes section of an RNAV approach plate means that WAAS outages for vertical guidance may occur daily. It also means zero NOTAMS will be issued by the FAA for that airport in the case of an outage. That’s how unreliable the signal is. As a result, IFR-approved, WAAS-enabled GPS units are designed to signal to the pilot any degradation in capability. It’s up to the pilot to maintain situational awareness and act accordingly.

One single-engine pilot learned about that and lived to write a NASA report. He was en route to Sacramento Executive Airport (KSAC) to have an intermittently glitching autopilot repaired. He was using that same autopilot on the RNAV GPS RWY 2 approach from a Garmin 430W. The 430W will automatically select the LPV, lower minima approach option when available, when an RNAV approach is loaded. The pilot was on an LPV approach when his autopilot began to fail.

The 430W is programmed to perform GPS integrity checks 60 seconds prior to the final approach fix (FAF) to confirm all parameters are within limits. If the system detects anything less than optimal integrity, it downgrades the approach to LNAV and announces “Approach downgraded. Use LNAV minimums” on its screen.

“The autopilot malfunctioned as I started the approach,” he wrote. “While dealing with the problematic autopilot I failed to notice that the Garmin 430W indicated LNAV instead of LPV and there was no glide slope.”

He descended below the published segment altitudes, which triggered low altitude alerts from ATC. He recovered and landed successfully.

A Cessna 182 pilot reported a similar situation. After programming an RNAV (LPV) approach and commencing it, he noticed the glide path failed to activate. Unlike the previous pilot, he noticed the problem in time. He chose to revert to LNAV minimums about 100′ prior to reaching them.

“I believe I operated consistent with the requirements for the LNAV approach,” he wrote.

But he still got an altitude warning from ATC. He believes it was triggered by his rapid descent rate.

A Piper Aerostar pilot blames his ATC altitude warning during an RNAV RWY 34 approach to Hawkins Field Airport (KHKS) on the fact that “approaches are not simple to read and understand anymore.”

He had been vectored to final, where he intercepted the RNAV RWY 34 approach.

“After passing what was assumed to be the FAF,” he wrote, “the descent was started.”

He was using the higher MDA minimums for the approach as he was operating with a KLN 90B, non-WAAS GPS unit. He landed without incident. He concluded his report with, “The modern environment makes single pilot operations increasingly difficult.”

The FAA continues to roll out RNAV approaches at airports that do not have ground-based navigational aids. More than 900 non-ILS airports employ more than 1,500 LPV RNAV approaches. Their ubiquity and utility are fantastic, so be better than I was. Know the differences between all of them before you launch.