Most of us understand how paired LOC and GS signals provide a course and a glideslope to follow when flying an ILS to a decision altitude (DA). (If you’d like a detailed review, see, for example, How ILS Works on the FlightInsight YouTube channel).
But the glidepath you see when flying an RNAV (GPS) approach to LPV minimums (usually shown as a magenta diamond on the vertical deviation scale) is more mysterious.
To make an LPV mimic an ILS’s behavior, LPV relies on programmed coordinates and instructions contained in a Final Approach Segment (FAS) data block. The FAS data block contains instructions for the approach, including coordinates for the runway, threshold crossing height, elevation, glidepath angle, and horizontal and vertical alert limits. The GPS receiver computes both linear and angular deviation but, as previously mentioned, only angular is displayed. They can be thought of as instructions to provide a pseudo localizer and glideslope.
In other words, the IFR-approved navigation receiver in your panel receives GPS-WAAS signals from space, and then creates a glidepath. The GP that you see and your AP/FD follows doesn’t emanate from a transmitter on the ground. That’s one reason we have so many RNAV (GPS) approaches with LPV minimums, even at small, quiet airports. Except for runway lights and (optional) approach lights, an approach with LPV minimums that mimics an ILS does not require expensive transmitters on the ground.
You GPS-WAAS receiver uses the same principles to create the advisory glidepaths (e.g., LNAV+V and LP+V) that you can use to help you descend to the MDA when flying a 2D (non-precision) approach, or to fly the vertical guidance on the visual approaches available with the latest GPS navigators.
The IFR flight was in benign weather, but I did need to fly the RNAV (GPS) RWY 35 approach at KUAO. It was a good opportunity to exercise the VNAV capability of the Garmin GFC 600 autopilot. Enjoy the scenery en route and observe the autopilot in action as I explain and use the VNAV feature.
A warm front recently brought widespread IMC to the Seattle area, and because the freezing level had climbed to about 5000 ft., I had a good opportunity to fly a couple of approaches for practice.
Ride along as I hop from Boeing Field (KBFI) in Seattle to nearby Bremerton National (KPWT), just across Puget Sound, for an RNAV (GPS) approach with a ceiling that was just 100 ft above the published DA of 200 ft. I let the GFC 600 autopilot fly the approach–my standard operating procedure when the weather is near minimums–so that I could could make sure that the airplane and avionics were properly set up. The GFC 600 also can fly a coupled missed approach, as you’ll see.
To learn more about how I prepare for IFR flights, see:
As the video below shows, I spent most of the flight in the clouds, but the tops in the area were at about 3500 ft., and I was able to cloud surf in an interesting sky for a little while.
Note that I filed three-letter codes in the remarks section of my flight plan to help Seattle Approach Control anticipate the procedures that I wanted to fly. For an explanation of the codes you can use in the TRACON’s airspace, see this document (PDF).
Join me on an IFR flight from Boeing Field (KBFI) in Seattle to Hoquiam (KHQM) on the Washington coast.
I wanted to exercise the Bonanza after an oil change and other maintenance (I had already completed a couple of VFR test flights). The weather was marginal VFR around Seattle, but a typical layer of coastal low clouds and fog shrouded Hoquiam in low IFR conditions, making a good setup for practicing an RNAV approach–the RNAV (GPS) RWY 24–at the normally sleepy KHQM before diverting to Bremerton to top off the fuel tanks.
As you’ll see, however, this flight included a couple of twists, including an ad-hoc hold en route to allow a preceding aircraft to complete its approach. And when I arrived at the DA, about 200 ft above the ground, the reported variable ceiling wasn’t so variable. I had to fly a real missed approach.
If you just want to see the final approach segment, watch the 5 minute clip below. A longer version, showing the departure, en route ad-hoc hold, and the approach appears at the end of this post.
Flying approaches at Sanderson demonstrates how GPS has transformed IFR for light GA aircraft. The airport once had only an NDB approach. Now it has RNAV (GPS) procedures to both runway ends, and the approach to RWY 23, which you’ll see in this video, offers LPV minimums as low as 250 ft AGL and ¾ mi visibility, almost as good as those provided by a category one ILS.
The minimums are a little higher for several reasons—most importantly, RWY 23 does not have an approach lighting system to help guide you in during LIFR conditions. During preflight planning, it’s important to review such details so that you’re not surprised by the view when you break out of the clouds, especially when you are arriving at a relatively quiet, rural airport like Sanderson Field.
Note that this approach has two lines of LPV minimums—one set with a DA of 523 ft and ¾ mi visibility, another with a higher DA of 667 ft and visibility requirement of 1-1/4 miles. Which applies when you fly the approach?
The answer is provided by the hashtag associated with the lower LPV minimums.
The hashtag links to a note at the top of the chart, if you use FAA charts. The note explains that you can use the lower LPV minimums only if, while flying the missed approach, your airplane can achieve a climb gradient of 244 ft/nm until reaching 1700 ft, when the standard terminal climb gradient of 200 ft/nm applies.
Confirming that you can achieve that higher climb gradient requires using the aircraft performance data in the aircraft flight manual or POH, and checking the results against a climb rate/climb gradient table, such as the one published in the Chart Supplement.
As I’ve explained in other videos, I made my own table to more accurately reflect the performance of the aircraft that I fly, and I have rounded up the climb rate numbers to the next highest 100 fpm, a value that I can see on the instruments in the panel.
Ride along as I explain some of the preflight planning process and describe how I use the Garmin G500 TXi, GTN 750Xi, and GFC 600 autopilot in the Beechcraft A36 Bonanza.
The second video shows the same approach flown about a month earlier in mostly visual conditions. As I have noted before, it’s a good idea to have instrument students fly their first approaches in VMC, hood or Foggles off, so that they can compare the movements of the course and glidepath indicators with the view of the runway. They also see the terrain and obstacles along the approach path, which helps them understand the importance of remaining on the published courses and never going below the minimum altitudes as they descend.
I recently took advantage of solid IMC in the Seattle area to fly several instrument approaches. Flying in actual instrument meteorological conditions is different from practicing with a view limiting device (e.g., a “hood”), especially when the visibility beneath the clouds is limited.
An important new Advisory Circular 90-119 Performance-Based Navigation Operations was released for comment in May 2021 and is now in the review and coordination phase at FAA. You can find the draft PDF in my Aviation Documents folder here. FAA has not announced when the final AC will be published, but if past review cycles are a guide, it may appear by the end of 2022 or early 2023.
You can read my detailed initial comments to FAA here (PDF).
This advisory circular (AC) replaces and consolidates several ACs…and provides guidance for operators using Performance-based Navigation (PBN) in the United States, in oceanic and remote continental airspace, and in foreign countries that adopt International Civil Aviation Organization (ICAO) PBN standards.
The draft AC includes updates to FAA policy in some key areas, and it will replace several existing guidance documents, including:
AC 90-100A CHG 2, U.S. Terminal and En Route Area Navigation (RNAV) Operations, dated April 14, 2015;
AC 90-105A, Approval Guidance for RNP Operations and Barometric Vertical Navigation in the U.S. National Airspace System and in Oceanic and Remote Continental Airspace, dated March 7, 2016;
AC 90-107, Guidance for Localizer Performance with Vertical Guidance and Localizer Performance without Vertical Guidance Approach Operations in the U.S. National Airspace System, dated February 11, 2011; and
AC 90-108 CHG 1, Use of Suitable Area Navigation (RNAV) Systems on Conventional Routes and Procedures, dated April 21, 2015.
The language in this AC also affects sections of the AIM; FAA handbooks such as the Instrument Flying Handbook and Instrument Procedures Handbook; and Airmen Certification Standards that include instrument tasks.
Here are some of the key updates to FAA guidance and policy in the new AC. Detailed discussion of each item follows.
Use of suitable RNAV systems on conventional routes and procedures, which clarifies how you can use an IFR-approved GPS on all types of IFR procedures, including those that use localizer courses, until you reach the final approach segment.
Expanded definition and clarification of the term RNP APCH, which many pilots confuse with RNP AR APCH. FAA adds RNP APCH notes to approaches with RNAV (GPS) in the title and does not plan to adopt current ICAO conventions for naming PBN procedures (for background, see this post here at BruceAir). More details on this topic appear below.
Updated definition of precision approach, which now explicitly includes all RNAV (GPS) approaches with LPV minimums. The AC also aligns FAA terminology with the ICAO definitions for procedures that include 2D (lateral navigation) and 3D (lateral and vertical navigation).
Use of Suitable RNAV Systems on Conventional Routes and Procedures
For general aviation pilots who use IFR-approved GPS and WAAS avionics, the draft AC clarifies a key issue, the use of RNAV (i.e., GPS and WAAS) while flying routes and procedures based on navaids. In particular, Chapter 11 describes specific situations in which pilots can use RNAV as an alternate means of navigation or as a substitute for navaids, and the document clarifies when you can use GPS to navigate along localizer courses outside the final approach segment.
(I recently gave a webinar for the American Bonanza Society on this topic. You can watch that presentation here and read more about the topic here.)
The draft AC provides the following updated definitions:
22.214.171.124 Alternate Means of Navigation. The pilot’s use of a suitable RNAV system to navigate on a conventional route or procedure without monitoring the operational NAVAID(s) defining the route or procedure. “Alternate means” applies to a situation where the pilot has options or a choice, and uses the suitable RNAV system primarily for convenience.
126.96.36.199 Substitute Means of Navigation. The pilot’s use of a suitable RNAV system to navigate on a conventional route or procedure in an aircraft with inoperative or not-installed conventional navigation equipment and/or when conventional NAVAIDs are out of service. “Substitution” applies to a situation where the NAVAID or the aircraft equipment are inoperative or unavailable (i.e., the pilot cannot use or monitor the conventional NAVAID).
188.8.131.52 Suitable RNAV System. Per § 1.1, an RNAV system is suitable when it (1) is installed for IFR operations, and (2) meets the navigation performance criteria required by ATC for the route or procedure to be flown. Put simply, suitable RNAV systems are those that meet the criteria identified in this AC for RNAV operations.
Section 11.4.2 Authorized Uses of Suitable RNAV Systems describes specific situations in which you can use GPS to complement or substitute for ground-based navaids.
The most significant update is item 6 (reinforced by paragraph 184.108.40.206), which clarifies that you can use GPS to navigate all legs of any conventional approach procedure, including procedures based on a localizer, until you reach final approach segment:
1. Determine aircraft position relative to, or distance from, a conventional NAVAID, DME fix, or named fix based on a conventional NAVAID.
2. Navigate to or from any conventional NAVAID or fix, via direct or a defined course.
3. Hold over any conventional NAVAID or DME fix.
4. Fly a published arc based upon DME.
5. Fly a route, comprised of charted airways, fixes, and/or NAVAIDs.
6. Navigate to the FAS of a conventional IAP.
7. Navigate the lateral course of the FAS on an IAP based on a VOR, Tactical Air Navigation System (TACAN), or Non-Directional Beacon (NDB) signal. The aircraft equipment and underlying NAVAID must be operational and monitored for FAS course alignment. The underlying NAVAID remains the primary source of navigation for the FAS course and should be used for course alignment if the RNAV system track differs from the underlying NAVAID course.
Items 1-2 also mean that you can use RNAV to fly departures and arrivals based on conventional navaids.
Item 6 is backed up by additional paragraphs:
220.127.116.11 On Unusable or Not Authorized (NA) Procedures. Pilots may not fly any portion of a conventional route or procedure identified (by chart annotation or NOTAM) as unusable or not authorized (“NA”).
Note: Pilots should take particular care when loading routes by name or title, especially routes defined solely by conventional NAVAIDs. Conventional routes designated as “unusable” by chart notation or NOTAM are unusable by any user. Pilots should not file for, and ATC should not use, the unusable route title or name in the IFR clearance. This does not preclude the use of direct clearances to usable waypoints along or across a charted route designated as “unusable.” [For more background on this issue, see Unusable Airways, Routes, and Segments here at BruceAir.]
18.104.22.168 As Sole Means of Navigation on Conventional FAS. Pilots may not use RNAV as the sole means of navigation to fly the final approach course on a conventional instrument approach.
22.214.171.124 To Navigate a Localizer (LOC) Final Approach Course. Pilots may not use RNAV to fly a FAS defined by an LOC signal.
Together, these sections mean that you can use an IFR-approved GPS to fly departures; airways; arrivals; feeder routes; track a VOR radial or localizer course to a course reversal or HILPT; fly DME arcs; fly holds, including holds based on DME fixes; and to fly the legs of missed approach procedures based on navaids, including localizers.
Paragraph 126.96.36.199As Sole Means of Navigation on Conventional FAS emphasizes that you cannot use GPS for lateral guidance along the final approach segment of a VOR or NDB approach unless the navaid is operational and you can monitor the course on a bearing pointer or CDI. This paragraph aligns with previous guidance in the AIM and AFM supplements, as I’ve noted in several posts here at BruceAir.
Paragraph 12.3 RNP APCH Overview provides background on the use of the note RNP APCH, which now appears on many charts.
This note is one way that FAA attempts to align its naming conventions for PBN approaches with updated ICAO standards (for background, see FAA InFO16020). As the AC explains:
In the United States, RNP APCH applies to all approach applications based on Global Positioning System (GPS), normally titled “RNAV (GPS)” or with “or GPS” in the title. These procedures provide operators one or more lines of minima (i.e., LNAV, LNAV/VNAV, LPV, or LP)…The RNP APCH NavSpec is intended to encompass all segments of the terminal approach operation: initial, intermediate, final, and missed approach…Charts for RNP APCH procedures will prominently display a standardized PBN Notes Box containing the procedure’s requirements including NavSpec(s) and, if needed, any required sensors or additional functionality (e.g., RF capability), as well as any minimum RNP value required for the procedure, and applicable remarks.
In other words, in the U.S., GPS-based approaches are titled RNAV (GPS) RWY xx. These approaches also include a RNP APCH note to confirm that they require the equipment needed to fly to the RNP terminal and final approach standards (i.e., RNP 1 and RNP 0.3 for LNAV minimums to an MDA or LPV, LNAV/VNAV to a DA, or LP standards for lateral guidance to an MDA).
RNP APCH is not the same as the authorization required to comply with the RNP AR APCH standard. In the U.S. approaches that require the RNP AR APCH standard are titled RNAV (RNP) RWY xx.
As the draft AC notes:
This AC does not apply to Required Navigation Performance Authorization Required Approach (RNP AR APCH) approaches, titled “RNAV (RNP)” (ICAO: RNP RWY xx (AR)). Guidance for RNP AR operations is in AC 90-101( ), Approval Guidance for RNP Procedures with AR.
Definition of Precision Approach
The draft AC updates the definition of precision approach, which now explicitly includes all RNAV (GPS) approaches with LPV minimums. The AC also aligns FAA terminology with the ICAO definitions for procedures that include 2D (lateral navigation) and 3D (lateral and vertical navigation).
In the past, precision approach applied only to procedures based on ground facilities that provide a glideslope or other approved vertical guidance to a DA–viz., an ILS or PAR. That obsolete definition required the creation of a new term, APV (approaches with vertical guidance), for RNAV procedures that offer approved vertical guidance to LPV or LNAV/VNAV decision altitude minimums.
Today, ICAO has updated its definition of precision approach by describing procedures that include approved vertical guidance to a DA, so-called 3D navigation. ICAO also describes so-called 2D approaches to MDAs.
With that change in mind, the new AC explains precision approaches (PA) and nonprecision approaches (NPA) that use GPS:
188.8.131.52 NPA Operations. These two-dimensional (2D) operations use GPS-derived lateral guidance from the RNP system. The procedure provides obstruction clearance as long as the pilots strictly adhere to the published minimum altitudes along the approach course, primarily by reference to the barometric altimeter. These 2D procedures typically have LNAV lines of minima to a minimum descent altitude (MDA).
184.108.40.206 PA Operations. These 3D operations use either ground-based, GPS-derived, and/or integrated electronic vertical guidance from the RNP system to enable lateral and vertical navigation to decision altitude (DA)/decision heights (DH) at or below 250 feet above ground level (AGL) (depending on the presence of obstacles). Typically, these are shown as LPV or LNAV/VNAV (and ILS/GLS) DA/DHs.
This section of the AC also notes that:
Note 2: Some approach procedures that contain both precision and nonprecision features are internationally designated as “Approaches with Vertical Guidance (APV).” These 3D approach procedures use GPS or SBAS to generate integrated electronic vertical and lateral guidance from the RNP system. These procedures typically have LNAV/VNAV lines of minima to DA/DH as low as 251 feet AGL. In the United States, all procedures to a LPV DA (regardless of height above touchdown (HAT)) are considered 3D precision operations. [Emphasis added]
This change should help reduce confusion about when IFR students and applicants for instrument ratings can use RNAV (GPS) approaches with LPV minimums to accomplish requirements for practicing and demonstrating precision approaches on practical tests.
The AC includes updated guidance in several other areas, beyond technical requirements for approval and use of RNAV systems.
For example, paragraph 220.127.116.11 Extract Procedures by Name notes that:
Pilots should extract PBN procedures by name from the onboard navigation database and ensure the extracted procedures match the charted procedures.
Note 1: Pilots operating aircraft with some early-model legacy RNAV navigators may not be able to extract certain PBN departure and arrival procedures by name from the navigation database. In these aircraft, pilots may load the departure or arrival procedures by extracting the individual fixes defining the procedures from the navigation database and loading them into the flight plan for their aircraft’s RNAV system. When this is necessary, pilots should confirm the resulting flight plan content matches the charted PBN procedure.
Note 2: Pilots are also cautioned that some procedures, even if extracted by name from the database, may not contain every segment, turn point, or conditional waypoint, or may contain “computer navigation fixes (CNF)” not shown on the procedure. It is always the pilot’s responsibility to ensure the aircraft’s flightpath conforms to the ATC clearance.
Paragraph 18.104.22.168 Extract PBN Routes in Their Entirety cautions that:
Pilots should extract the PBN routes from the navigation database whenever possible rather than loading the route by stringing individual fixes defining the route in sequence.
Note 1: This does not preclude a pilot’s use of a legacy RNAV navigator that cannot auto-load a PBN route into the navigator’s flight plan. A pilot may load a PBN route by extracting the individual fixes defining the route from the onboard navigation database and loading into the flight plan, fix by fix. When this is necessary, pilots should confirm the resulting flight plan route entries match the charted routes.
Note 2: Caution is warranted when loading routes into the RNAV system for convenience, especially conventional routes defined by ground NAVAIDs. Routes designated as “unusable” by chart notation or NOTAM are unusable by any user. Pilots should not file for, and ATC should not use, the unusable route title or name in the IFR clearance. This does not preclude the use of “direct to” clearances to usable waypoints along a charted route designated as “unusable.”
Paragraph 22.214.171.124 Creating or Altering Waypoints also warns that:
For any published (i.e., charted) PBN routes or procedures, pilots may only use waypoints downloaded from the aircraft navigation database. Pilots may not create waypoints (e.g., by using latitude/longitude coordinates, place/bearing, or any other means) for use on published PBN routes or procedures. Pilots also may not change any parameters of waypoints downloaded from the navigation database (e.g., changing a flyover waypoint to a flyby waypoint).
Paragraph 126.96.36.199 Cross-Check Flight Plan Against ATC Clearance recommends that:
Pilots should cross-check the navigation system’s flight plan against their ATC clearance and the charted routes and procedures. Both the flight plan’s textual display and the aircraft’s electronic moving map display (when available) can aid in this cross-check. When required by NOTAM or by the aircraft’s operating manual and SOP, pilots should confirm exclusion of specific ground-based navigation aids. If at any time during these cross-checks, a pilot doubts the validity of the route or procedure they extracted from the navigation database, they should not attempt to execute the route or procedure.
Note: Pilots may notice a slight difference between the navigation information portrayed on the chart and their primary navigation display. Differences of up to 5 degrees may result from avionics’ application of magnetic variation. These differences are acceptable.
Paragraph 3.6.10 and others in that section explain equipment required notes on PBN procedures. I won’t repeat all those details here. You can find background at Equipment Required Notes on IFR Procedure Charts here at BruceAir. I’ll update that post when I can distill the current FAA guidance as described in the new AC.
Finally, section 3.7 Training offers detailed guidance on the steps pilots should take to ensure that they understand how to use RNAV systems. A list appears in paragraph 3.7.3. Many of these items are also covered in existing industry guidance, such as the free Garmin GTN 750 Sample Training Syllabus (PDF).
During training for the instrument rating, we fly most approaches to published minimums. But in real-world IFR flying, the weather is usually well above the visibility (which actually controls) and ceiling required to complete an approach and land.
I recently flew a couple of approaches in VMC (visual meteorological conditions), albeit with light rain reducing visibility. I couldn’t log the approaches for currency, but they were still good opportunities to practice IFR procedures, use the avionics in the A36, and keep my head in the IFR game.
The following videos also show what the runway environment looks like as you approach the decision altitude (DA), first on an RNAV (GPS) approach with LPV minimums, then an ILS.
As of January 30, 2020, the FAA had published 4,048 RNAV (GPS) approaches with LPV (localizer performance with vertical guidance) minimums at 1,954 airports; 1,186 of those airports are not served by an ILS. The number of LPV-capable procedures is almost three times the 1,550 approaches with Category I ILS minimums, and 2,838 of the procedures with LPV minimums serve runways without an ILS,
This capability to fly ILS-like procedures to thousands of runways at small-town and rural airports is a boon to IFR pilots. But you must carefully prepare to fly approaches to runways that don’t have the ground infrastructure associated with an ILS–most importantly an approach lighting system and accessories such centerline and touchdown zone lighting. And keep in mind that a non-ILS runway may not be as a long as you’re accustomed to–the minimum length for a runway served by an approach with LPV minimums is just 3200 ft. (more details here).
For example, here’s video of the final approach segment of the RNAV (GPS) RWY 16 procedure at Chehalis, WA (KCLS) during a night approach.
As the video shows, the runway can be hard to spot. It has REILs and a PAPI, but it’s in a dark area near a river. And you must remember to activate the lights by clicking the transmit button on the CTAF as you approach the airport. (The camera makes the scene look a little darker than it really was to human eyes–but you get the idea.)
That approach has an LPV decision altitude of 476 ft MSL (300 ft. AGL). The visibility requirement is 1 sm. That’s not much greater than the 200 ft DA and 1/2 sm visibility for a typical Category 1 ILS. But the environment is vastly different than that presented by an ILS runway with its bright lights and other big-city features.
Here’s an overview of the approach lighting systems associated with ILS procedures.
Contrast the view at KCLS with the scene at Boeing Field (KBFI) in Seattle during the final stages of an ILS approach.
Here’s an approach to runway 20 at Bremerton, WA (KPWT). KPWT is a non-towered airport, but runway 20 is served by an ILS approach, which includes an ALS.
So, regardless of the type of approach you’re flying, make sure you review and prepare for the runway environment that you’ll encounter when you break out of the clouds and go visual. It’s especially important to know which of the visual cues described by 14 CFR §91.175 Takeoff and landing under IFR will be available.