Everything You Need to Know about RNAV GPS Approaches

RNAV GPS (aRea NAVigation) stand-alone instrument approaches have become commonplace as GPS and the Wide Area Augmentation System (WAAS) hit the mainstream.

Virtually all GPS approaches require an RNP (Required Navigational Performance) of 0.3, which means an aircraft tracking the final approach course with a centered needle can be expected to be within 0.3 nm of the centerline 95 percent of the time. All IFR-certified GPS units meet 0.3 RNP.

Some RNAV units use DME cross referencing (DME/DME) to achieve RNP 0.3. Certain RNAV (GPS) approaches are not available to these type units as the airplane could be beyond the service volume of a necessary DME facility. Check the chart notes (e.g., DME/DME RNP-0.3 NA).

LNAV (Lateral NAVigation) (aka GPS NPA) — A nonprecision approach that uses GPS and/or WAAS for LNAV. Lateral sensitivity does not increase as the aircraft gets closer to the runway. Pilots may use a WAAS-enabled GPS for LNAV, but WAAS is not mandatory. Vertical guidance is not provided. When the aircraft reaches the final approach fix, the pilot descends to a minimum descent altitude (MDA) using the onboard barometric altimeter (aka "dive and drive").

LP (Localizer Performance) — Nonprecision WAAS-mandatory approach. Lateral-only WAAS guidance found at locations where terrain or obstructions prevent vertically guided LPV procedures. Typically use barometric altimeter data for descent to MDA. Lateral sensitivity increases as the aircraft gets closer to the runway (or point in space for helicopters). LP is not a fail-down mode for LPV — LP and LPV are independent. LNAV is not a fail-down mode for LP. LP will not be published with lines of minimums that contain approved vertical guidance (i.e., LNAV/VNAV or LPV). LP is the GPS equivalent of a localizer approach. Older WAAS receivers may not contain LP capability unless the receiver has been upgraded. LP minimums are published only if they provide lower minimums than LNAV.

APV (APproach with Vertical Guidance) — An instrument approach based on a navigation system that is not required to meet the precision approach standards of ICAO Annex 10 but provides course and glidepath deviation information. For example, baro-VNAV, LDA with glidepath, LNAV/VNAV and LPV are APV approaches. (OpSpec C052)

LNAV/VNAV, aka L/VNAV (Lateral NAVigation/Vertical NAVigation) — Horizontal and approved vertical guidance to the LNAV/VNAV line of minimums. Lateral sensitivity does not increase as the aircraft gets closer to the runway. Vertical guidance is provided either by WAAS or approach-certified baro-VNAV systems. LNAV/VNAV approaches are flown to a decision altitude rather than MDA. Decision altitude is the altitude at which you're supposed to look out the window and contemplate if you're going to land or go around — while you continue to descend — rather quickly! If your airplane depends on baro-VNAV (barometric Vertical NAVigation) instead of WAAS for VNAV, you may be restricted by temperature from using the (sometimes) lower VNAV minimums. Check the chart notes. Example: Baro-VNAV NA below negative 15 degrees Celsius (5 degrees Fahrenheit) or above 47 degrees Celsius (117 degrees Fahrenheit). Pilots must adhere to temperature limitations unless employing temperature compensation under an authorization from ATC. Some LNAV/VNAV minimums are higher than LNAV minimums.

LNAV+V — LNAV approach plus advisory Vertical guidance. If you see LNAV+V displayed on your WAAS unit's annunciator, you may fly the glideslope to the LNAV MDA. LNAV+V is not the same as LNAV/VNAV or LPV. Pilots must use the barometric altimeter to meet all altitude restrictions. "LNAV+V" is not listed on a chart. However, it may appear when you load the approach if the GPS is compatible. The advisory glideslope does not always ensure obstacle clearance.

LPV (Localizer Performance with Vertical guidance) — Offers the lowest minimums of all GPS approaches but are technically still considered nonprecision approaches (i.e., APproach with Vertical guidance — APV). Requires a WAAS receiver in the airplane and can have minimums as low as 200 feet agl and half-mile visibility with proper approach and runway lighting. Lateral sensitivity increases as the aircraft gets closer to the runway. However, unlike an ILS, which gets increasingly sensitive and difficult to fly near and below DA, the LPV course transitions to linear scaling 700 feet wide at the threshold (same as ILS) but then doesn't get any tighter. They are the operational equivalent of a legacy ILS and are flown to a DA, but are far more economical because no navigation infrastructure is needed at the airport. Several thousand LPV approaches are in use today, many at airports that previously did not have an ILS. As stated in the Instrument Airplane ACS (FAA-S-ACS-8, Appendix 7), when taking a flight test or performing an IPC, an LPV with a DA greater than 300 feet HAT may be used as a nonprecision approach. LPV minimums can be used to demonstrate a precision approach if the DA is equal to or less than 300 feet HAT. Always ensure that the WAAS channel number and ID displayed on the GPS match the WAAS numbers listed at the top of the approach chart.

GBAS (Ground Based Augmentation System), aka GLS (GBAS Landing System), aka LAAS (Local Area Augmentation System). GBAS augments GPS and provides corrections to aircraft to improve GPS navigation for approaches. It is considered a precision approach. LAAS is synonymous with GBAS. LAAS was the term initially used by the FAA, which has since migrated to the ICAO term GBAS. GLS is the FAA's official term for a GBAS approach (e.g., GLS RWY 23). GBAS and WAAS standards are different, so GBAS datalinks must be supported by compatible avionics in the aircraft. Pilots select a five-digit GBAS channel number within the FMS menu (or manually). Currently in use by several airlines at Newark, Houston and many other locations around the world.

Stand-alone GPS — Older nonprecision approach format. Example: GPS RWY 23 (very few are left). It is being replaced with a newer format prefaced by the acronym RNAV. Example: RNAV (GPS) RWY 23.

RNP, aka (RNAV) RNP, aka RNP AR — Required Navigation Performance with Authorization Required (AC 90-101). Special authorization from the FAA is required for these approaches, aka RNP SAAAR (Special Aircraft and Aircrew Authorization Required).

WAAS units are designed to evaluate the lowest minimums possible based on meeting required horizontal and vertical limits. The approach mode annunciator on the unit will notify you of which minimums you may use. Check for WAAS (D) notams. WAAS is required for LP, LPV, and LNAV/VNAV (without baro-VNAV) approaches.

Approved vertical guidance is available on LNAV/VNAV minimums, and existed before the WAAS system was certified. At that time, only aircraft equipped with a flight management system and certified baro-VNAV systems could use the LNAV/VNAV minimums. Today, LNAV/VNAV minimums may be flown using approved GPS WAAS receiver equipment.

Barometric aiding, aka baro-aiding, is an integrity augmentation that allows a GPS system to use a nonsatellite input source (e.g., pitot-static system) to provide vertical reference.

Barometric vertical navigation, aka baro-VNAV — Uses approach-certified barometric altitude info from the pitot-static system and air data computer to compute vertical guidance (large aircraft). May be restricted by temperature.

RNAV approaches normally list several approach minimums to ensure as many aircraft as possible can fly the approach and provide operational flexibility if WAAS becomes unavailable. Aircraft with standard GPS receivers (or WAAS) can fly to the LNAV MDA. Aircraft with GPS and approach-certified Baro-VNAV can fly to LNAV/VNAV decision altitude (DA). WAAS-certified aircraft can fly to LP, LPV or LNAV/VNAV minimums. If for some reason WAAS becomes unavailable, all GPS or WAAS-equipped aircraft revert to the LNAV decision altitude.

Alternates — When using TSO-C129 and TSO-C196 (non-WAAS) GPS equipment at an alternate, authorized users may file based on a GPS-based IAP at either the destination or the alternate airport, but not at both locations. When using TSO-C145 and TSO-C146 (WAAS) equipment at an alternate airport, planning must be based on flying the LNAV or circling minimum line, or GPS procedure, or conventional procedure with "or GPS" in the title. Upon arrival at an alternate, LNAV/VNAV or LPV may be used to complete the approach. WAAS users with authorized baro-VNAV may plan for LNAV/VNAV DA, or RNP 0.3 DA at an alternate.