Instrument Ground School: Understanding RNAV approaches
Remember GPS Overlay approaches? The first instrument approaches to utilize GPS capability were approaches overlaid on existing VOR or NDB approaches, allowing us to fly the VOR or NDB non-precision approach using the GPS for lateral guidance. We have come a long way.
In 2013, we celebrated ten years of the Wide Area Augmentation System’s (WAAS) availability to the public. Pilots are now benefiting from the proliferation of Area Navigation (RNAV) Global Positioning System (GPS) approaches and lower minimums provided by WAAS-enabled systems. As of July 2011, there were twice as many WAAS approaches as Instrument Landing Systems (ILS) approaches. As of 2013 there were over 3000 Localizer Performance without Vertical Guidance (LP) and Localizer Performance with Vertical Guidance (LPV) procedures.
Consistent with the long term FAA schema to categorize approaches by their type or ultimately by the navigational precision required to fly them safely, rather than by the type navaid (VOR, NDB, Localizer LDA) employed, we now fly RNAV (area navigation) approaches. Most can be flown with any one of several area navigation devices and systems … but whom are we kidding? For general aviation pilots that device will be an approved and installed, usually WAAS-capable, GPS navigator.
Sorting out the various RNAV approaches can be difficult. At present, no RNAV approach is considered to be a precision approach, so they cannot be broken out into precision and non-precision. (That distinction has bearing only on the alternate weather requirements for IFR filing, and for RNAV approaches those requirements and standards are spelled out in the AIM.) The GBAS system may yet come to be considered a precision approach, but as of 2016 that system is in use at only a couple of airports [Houston and Newark] according to the FAA w[1] The GBAS system is the ICAO acronym for a Ground Based Augmentation System. The US system was originally called the LAAS Local Area Augmentation System. It is approved for CAT 1 approaches. See this FAA FAQ on GBAS.
RNAV approaches may more usefully be divided into approaches that provide approved vertical guidance and are therefore flown to a DA and those that do not provide approved vertical guidance and are flown to an MDA. Bear in mind that a single RNAV approach chart typically represents several different approach types, just as an ILS approach chart represents both a precision ILS approach and a non-precision Localizer approach. These various RNAV approaches, while separate only in the minima table, are truly different approaches, with different obstacle clearance structures, different piloting techniques, and different missed approach points. They are not the same approach flown to different minimums.
The GPS equipment will determine which version of the RNAV approach the signal supports — LNAV, LNAV/VNAV, LPV, etc. The minimums on the approach chart will provide the DA or MDA and the required visibility. The pilot will understand how each of these different approaches must be flown.
Here, in tabular form, are the several types of RNAV approaches and their characteristics. Explanations to follow.
| Approach | Vertical guidance | Requires WAAS | DA | MDA |
| LNAV | no | no | yes | |
| LP | no | yes | yes | |
| LPV | yes | yes | yes | |
| LNAV/VNAV | yes | Yes (or baro) | yes | |
| LNAV+V | Advisory only | yes | yes | |
If you have a WAAS receiver, you may fly an
- LNAV approach to an MDA
- LP approach to an MDA
- LNAV/VNAV approach to a DA
- LPV approach to a DA
If you have a non-WAAS receiver, you may fly an
- LNAV approach to an MDA
RNAV APPROACHES WITHOUT VERTICAL GUIDANCE
Localizer Performance without Vertical Guidance (LP) and
Lateral Navigation (LNAV)
LNAV approaches are non-precision approaches that provide lateral guidance only. They do not require WAAS equipment. The pilot must check RAIM (Receiver Autonomous Integrity Monitoring) prior to the approach when not using WAAS equipment. See AIM 1-1-19, 5-1-16, and AC 90-105. Both LP and LNAV lines of minima are Minimum Descent Altitudes (MDAs) rather than DAs (decision altitudes). It is possible to have LP and LNAV published on the same approach chart. An LP is published if it provides lower minima than the LNAV. See AIM 1-1-20.
LPs are non-precision approaches with WAAS lateral guidance. They are added in locations where terrain or obstructions do not allow publication of vertically guided LPV procedures. Lateral sensitivity increases as an aircraft gets closer to the runway (or PinS type approaches for helicopters). LP is not a fail-down mode for an LPV. LP and LPV are independent. LP minimums will not be published with lines of minima that contain approved vertical guidance (LNAV/VNAV or LPV). The FAA has specifically declined to allow vertical guidance associated with LP approaches so as not to encourage pilots to confuse an LP approach with an LPV approach.
What is the difference? LNAV approaches may be flown without WAAS equipment, therefore the RAIM checking requirement.
LP approaches require WAAS, but specifically do not include vertical guidance, even advisory vertical guidance, so as not to be confused with LPV approaches.
Note that both are MDA approaches, even those LNAV approaches with advisory guidance (LNAV+VNAV). Do not fly advisory vertical guidance below MDA without the required criteria to descend below MDA.
RNAV APPROACHES WITH VERTICAL GUIDANCE
Localizer Performance with Vertical Guidance (LPV) and
Lateral Navigation / Vertical Navigation (LNAV/VNAV)
Localizer Performance with Vertical Guidance (LPV)
LPV approaches take advantage of the refined accuracy of WAAS lateral and vertical guidance to provide an approach very similar to a Category I ILS. Like an ILS, an LPV has vertical guidance and is flown to a Decision Altitude (DA). The design of an LPV approach incorporates angular guidance with increasing sensitivity as an aircraft gets closer to the runway (or point in space (PinS) type approaches for helicopters). Sensitivities are nearly identical to those of the ILS at similar distances. This is intentional to aid pilots in transferring their ILS flying skills to LPV approaches. Any pilot proficient in the technique of flying an ILS should be comfortable flying an LPV approach.
Lateral Navigation/Vertical Navigation (LNAV/VNAV)
LNAV/VNAV approaches provide both horizontal and approved vertical approach guidance. Vertical Navigation (VNAV) utilizes an internally generated glideslope based on WAAS or baro-VNAV systems. Minimums are published as a DA. If baro-VNAV is used instead of WAAS to determine vertical guidance, the pilot may have approach restrictions as a result of temperature limitations (which will be listed in the approach chart notes) and must check predictive RAIM (Receiver Autonomous Integrity Monitoring). These were the first RNAV approaches with approved vertical guidance, flown originally with Baro-VNAV systems and more recently and more commonly, with WAAS providing the approved vertical navigation. See AIM 1-1-19, 5-1-16, and AC 90-105.
IMPORTANT CONCEPTS
Advisory Vertical Guidance
Depending on the manufacturer, a few WAAS-enabled GPS units provide advisory vertical guidance in association with LP or LNAV approaches. Typically, the manufacturer will use the notation of LNAV+V or LNAV+VNAV. The system generates an artificially created advisory glide path from the final approach fix to the touchdown point on the runway, which is intended to assist the pilot in flying a constant descent path to an MDA. LNAV+V is not the same as LNAV/VNAV or LPV. Pilots must use the barometric altimeter as the primary altitude reference to meet all altitude restrictions. Advisory vertical guidance is not required and is an optional capability. The availability of +VNAV advisory vertical guidance does not constitute a separate approach type or category and does not alter the minima of the LNAV approach for which it is provided. Nor does it allow the pilot to continue below MDA unless the necessary criteria of 91.175 (c) exist See AC 90-107.
Approved Vertical Guidance
Approved vertical guidance provides pilots with glide path information to meet altitude approach restrictions for LPV, LNAV/VNAV, and ILS lines of minima. An LPV approach can provide WAAS vertical guidance as low as 200 feet AGL. The first approaches to provide approved vertical guidance were LNAV/VNAV approaches flown with certified baro-VNAV information in conjunction with a flight management system (FMS). These preceded the advent of WAAS. The introduction of the WAAS system has meant that today LNAV/VNAV approaches are available to far more aircraft via WAAS. (Baro-VNAV is still a valid means of flying an LNAV/VNAV approach but may be subject to temperature and other limitations mentioned in the approach notes.) Approved Baro-VNAV is not to be confused with barometric altimeter information, which remains the primary altitude reference for complying with any altitude restrictions. See AC 90-107.
Barometric Aiding (Baro-Aiding)
Barometric aiding is an integrity augmentation that allows a GPS system to use a non-satellite input source (e.g. the aircraft static system) to provide vertical reference and reduces the number of required satellites from five to four. Baro-aiding requires four satellites and a barometric altimeter to detect an integrity anomaly. The current altimeter setting may need to be entered into the receiver as described in the operating manual. Baro-aiding satisfies the RAIM requirement in lieu of a fifth satellite. See AIM 1-1-19.
Barometric Vertical Navigation (Baro-VNAV)
Baro-VNAV is an RNAV system which uses barometric altitude information from the aircraft’s altimeter to compute vertical guidance for the pilot. The specified vertical path is typically computed between two waypoints or an angle from a single way point. When using baro-VNAV guidance, the pilots should check for any temperature limitations which may result in approach restrictions. See AIM 5-4-5.
Conclusions, Counsel, and things to remember ….
- You do not load an LP, LNAV, LNAV/VNAV, or LPV approach. You simply load the RNAV rwy xx approach. The onboard GPS will evaluate the quality of the signal and determine which of the various approaches it can support.
- You can fly whatever approach annunciates on your GPS display – but you must fly it to the minimums and in a manner that adheres. The database will attempt to provide – and declare — the approach type requiring the highest level of signal acuity and integrity and default to the most desirable approach supported by the available signal quality.
- There, at the moment, is no such thing as a precision RNAV approach. So we should distinguish them as MDA or DA approaches.
- If you do not have a WAAS receiver, with the necessary FMS approval (An airworthiness approval in accordance with TSO Technical Standard Order TSO 145-A or TSO-146A and installed in accordance with AC 20-130A or AC20-138A) you are limited to LNAV approaches with an MDA. LNAV approaches may be flown without WAAS equipment, therefore the RAIM checking requirement.
- Barometric altimeter information remains the primary altitude reference for complying with any altitude restrictions. Altitudes for each segment of the approach, for any stepdowns, and for the minimum altitude are dictated by the barometric altimeter. In other words, whether we fly an RNAV approach to a DA or an MDA, that altitude is determined by the altimeter, not by the GPS.
- LNAV approaches may offer advisory vertical guidance (+VNAV). This is a construct of the manufacturer and is an arithmetic calculation that has nothing to do with approach design. Do not mistake it for a glideslope or for approved vertical navigation. The advisory glideslope information is nothing more than an alternate means of descending from the intermediate segment altitude to the MDA. You may recall that a non-precision approach, be it a VOR, NDB, Localizer, or RNAV approach with an MDA may be flown either as a “dive and drive” rapid descent to the MDA and a level-off at the MDA and continuation to the MAP or it may be flown as a steady-state descent to the MDA by calculation or with the aid of a flight management system. Advisory guidance on an RNAV MDA approach is nothing more than assistance with that arithmetic. It is simply another way to reach the MDA and changes nothing about the architecture of the approach. Do not follow advisory guidance to a DA. Observe the MDA and continue below only if the requirements of 91.175 (c) are met.
For more information please refer to the following:
Aeronautical Information Manual (AIM) Paragraphs: 1-1-19, 1-1-20, 5-1-16, and 5-4-5
Advisory Circulars:
AC 90-100A, FAA Advisory Circular RNAV en route and terminal operations AC_90-100A.pdf
[1] The GBAS system is the ICAO acronym for a Ground Based Augmentation System. The US system was originally called the LAAS Local Area Augmentation System. It is approved for CAT 1 approaches. Exists as of 2016 at only two airports. See this FAA FAQ on GBAS.
