When I began working on this article, my intention was to discuss the different types of instrument approaches, their minima and how they will look and perform while being flown. Diving in, I inevitably encountered the RNAV/RNP/PBN naming convention, which is one of the drivers of confusion in GPS approaches. So, let’s start with a top-down review, a brief overview of each type of approach, and throw in some pitfalls and common mistakes that can occur while flying GPS, err RNAV, I mean RNP approaches.
RNAV: Area Navigation
In modern aviation, RNAV has become synonymous with GPS, but this was not always the case. From the FAA: “RNAV is a method of navigation that permits aircraft operation on any desired flight path within the coverage of ground- or space-based navigation aids, or within the limits of the capability of self-contained aids, or a combination of these.”
Area navigation originally referred to systems that could detect an aircraft’s position outside the normal hub-and-spoke VOR system. Aircraft could use internal devices, like an IRU or IRS (internal reference unit/system) which calibrated/aligned the aircraft’s position on the ground and used accelerometers and gyroscopes to detect aircraft movement and estimate its relative position. The key word is estimate, because the quality of the navigation would degrade, especially on longer flights.
Other RNAV systems were VOR/DME systems where multiple DMEs could be used to triangulate positions, or Loran (LOng RAnge Navigation). How these technologies work is neat, especially considering some were developed and implemented during WWII, but it’s not really relevant to the modern pilot. Obviously, using GPS technology has outpaced the capabilities for these older systems and is far more accessible for GA aircraft.
RNP: Required Navigation Performance
According to the FAA, “Required Navigation Performance (RNP) is similar to Area Navigation (RNAV); but, RNP requires on-board navigation performance monitoring and alerting capability to ensure that the aircraft stays within a specific containment area.”
A GPS unit with RAIM and/or WAAS (wide area augmentation system) is considered RNP because it can detect and alert the pilot about loss of integrity. Here is a huge source of confusion for pilots, because frankly the naming convention is confusing. An RNAV (RNP) approach requires aircrew and aircraft certification, which is possible under Part 91 but I have only seen corporate operations with special needs seek it out. There are more intricacies involved with RNAV and RNP, such as accuracy requirements, different modes depending on the phase of flight and so on.
The most important takeaway here is RNAV (RNP) approaches are labeled as AR (authorization required) and if you see minimums predicated on RNP .1 or .3, the approach cannot be flown to those minimums without said authorization.
PBN: Performance Based Navigation
The FAA definition: “Within PBN there are two main categories of navigation methods or specifications: area navigation (RNAV) and required navigation performance (RNP). In this context, the term RNAV x means a specific navigation specification with a specified lateral accuracy value. For an aircraft to meet the requirements of PBN, a specified RNAV or RNP accuracy must be met 95 percent of the flight time.”
Performance based navigation encompasses RNAV GPS navigation as well as the more stringent requirements of RNP.
Types Of RNAV Approaches
A quick note…different minima do not technically mean different approaches. You can look at an RNAV (GPS) XX procedure and see all these minimums, or a variation of them. It is just easier to communicate about the style of approach this way, although I realize some may be bothered by the semantics. Some of the options you’ll see include:
LPV: Localizer performance with vertical guidance approaches utilize WAAS-capable avionics to allow the lowest Category 1 minimums an airport is capable of. For example, if an airport has a CAT 1 ILS with minima of 1800 RVR, often an RNAV approach will be available with the same LPV minimums. Although it is flown to a DA, it is considered an approach with vertical guidance, not a precision approach.
For you instrument students and CFIIs out there, here is a pertinent passage from the instrument Airman Certification Standards, ACS:
“Localizer performance with vertical guidance (LPV) minimums with a decision altitude (DA) greater than 300 feet height above touchdown (HAT) may be used as a nonprecision approach; however, due to the precision of its glidepath and localizer-like lateral navigation characteristics, an LPV minimums approach can be used to demonstrate precision approach proficiency if the DA is equal to or less than 300 feet HAT.”
LNAV/VNAV: Lateral Navigation/Vertical Navigation approaches started life utilizing baro-aided GPS units to provide a glidepath. Since then, WAAS has largely replaced baro-aided GPS in general aviation aircraft, but if you do happen to be flying to LNAV/VNAV minimums with a baro-aided GPS there are often temperature limitations to consider. LNAV/VAV minimums are also flown to a DA, albeit almost always higher than LPV minimums.
LP: Localizer performance approaches are designed with WAAS lateral capabilities in airports when obstacles prevent the addition of a glideslope. Flying an LP approach will feel similar to a localizer approach, including the increasing sensitivity as you approach the runway. Because of the increased accuracy, LP approaches have a narrower Obstacle Clearance Surface (OCS) and therefore lower minimums than LNAV only approaches. Interestingly enough, some GPS or FMS systems with WAAS were created before LP approaches and cannot fly them. If there are no published LPV minimums and your GPS/FMS denotes LNAV/VNAV or LNAV minimums, do not descend below those minimums.
LNAV: Lateral Navigation, like LP approaches, are flown to an MDA. WAAS is not required for LNAV approaches and therefore provides the least accuracy and highest minimums. LNAV minimums are almost always the highest on any given GPS approach, but if there is ever a major WAAS outage, we will be happy they stuck around!
RNP: A GPS (RNP) approach requires FAA authorization, aircrew training and aircraft certification. If you fly for an operation that has this, they will provide everything you need to know about RNP approaches. Otherwise, you are stuck flying the other four.
RAIM stands for Receiver Autonomous Integrity Monitor. It strictly interfaces between GPS satellites and the aircraft’s receiver to ensure lateral position accuracy. If sufficient satellite coverage exists, the system can isolate faulty data and continue to provide accurate data to the pilot. If coverage is inadequate, it will alert the crew about the loss of integrity. RAIM alone allows for LNAV minimums, or LNAV/VNAV if the unit is capable of baro-aiding.
WAAS, the Wide Area Augmentation System, provides greater accuracy by augmenting satellite-based navigation with ground-based stations. Because the positions of the ground stations are fixed and precisely surveyed, they can be used to “clean up” normal GPS positioning. With a functioning WAAS receiver, a RAIM check is not necessary. WAAS facilitates operations to LPV and LP minimums, as well as LNAV/VNAV and VNAV. Sixty seconds prior to the final approach fix, the WAAS GPS receiver will run an integrity check. If the test does not pass, you will see a message telling you to downgrade the minimums.
Pitfalls
There are a few pitfalls and common errors we want to be aware of when flying GPS approaches. First and foremost, always check your Notams. This should occur each and every flight, but I have noticed that more often a glideslope being out of service is caught by the pilot over a planned RAIM or WAAS outage. On this same note, just because your GPS receiver is indicating WAAS or RAIM on the ground does not invalidate the Notam and GPS approaches requiring integrity monitoring (LPV, LP, LNAV, VNAV) should not be conducted in IMC.
In most GA GPS/FMS systems, the GPS will notate the lowest approach minimums it has the integrity to fly. Just like it is possible to lose a glideslope while shooting an ILS, it is possible experience a loss of integrity during a GPS approach. First, know what this looks like if you have not experienced it before. Videos exist online of different failures and reversion modes for all types of avionic setups.
Second, have a plan for different scenarios. A full loss of GPS is rare, but possible. Of course, the first step will be to start climbing, but GPS approaches naturally utilize GPS for the missed. Some units have a DR (dead reckoning) mode that can help until ATC can be reached for vectors, but otherwise you just need to do whatever it takes. Often GPS missed approaches use GPS to navigate to radio nav aids, so this could also be an option. This is an emergency in IMC, so all available resources will be in play until ATC can be reached.
Outside of a full loss of GPS, you could have a downgrade in integrity. The minimums or signal quality should be kept in the scan during the approach. If the LPV changes to LNAV/VNAV, for example, the approach must not be continued below LNAV/VNAV minimums. A missed approach may be required and is often the safest bet. If the failure occurs above LNAV/VNAV minimums and said minimums are briefed, it is an option to continue. What is not a good bet is being startled by the LOI and going heads down instead of a safe instrument scan to find the higher minimums on the approach plate.
Never-Ending Acronyms
Aviation English involves a nauseating number of acronyms, and GPS usage is no exception. Having a general understanding of how these concepts interface ensures the correct usage, especially during abnormal operations.
On the flip side, once you activate an RNAV procedure, its annunciations should tell you which accuracy level and vertical navigation support are available. One trick to it all is to ensure you match the approach plate’s minima to the annunciated procedure.
There much, much more to RNAV-based terminal procedures, of course, and we’ll circle back to this topic in a future issue.
What About That +V?
Many GPS or FMS installations are capable of generating a glidepath where none is charted, strictly for situational awareness. If you load up an approach and the GPS annunciates “LP+V” or “LNAV+V” minimums, it’s generating a virtual glidepath. Care must still be taken to ensure any stepdown altitudes are honored. Glideslopes generated this way help stabilize approaches, and if you are frequently flying ILS or LPV approaches, it will more closely mirror that experience.
There is one threat to consider. From paragraph 5-4-5 of the AIM: “Obstacle clearance is provided to allow a momentary descent below DA while transitioning from the final approach to the missed approach.” Conversely, an MDA is a hard altitude. If you fly an approach with an MDA using a glideslope for situational awareness, consider setting minimums and executing a go-around 50 feet above published minimums to ensure the momentary dip made at the derived decision altitude does not bust the MDA.
