What is the difference between RNAV and RNP navigation specifications?

RNAV NavSpecs require the aircraft to meet a specified lateral accuracy standard (total system error) but do not mandate that the avionics continuously monitor and alert the crew if that standard is not being met. RNP NavSpecs add the requirement for on-board performance monitoring and alerting (OPMA), meaning the system must detect and warn the crew any time it cannot guarantee the required accuracy. This self-monitoring capability is what allows RNP procedures to be designed with tighter obstacle clearance areas.

Do I need special authorization to fly RNP AR approaches?

Yes. RNP Authorization Required (RNP AR APCH) always requires a specific Letter of Authorization (LOA) for Part 91 operators or the appropriate Operations Specifications paragraph (typically C384) for Part 121 and Part 135 operators, in addition to a compliant AFM supplement and crew training. Standard RNAV (GPS) approaches using the RNP APCH NavSpec do not require a separate LOA for most operators, as long as the avionics meet TSO standards outlined in AC 90-105A.

What does it mean when the FMS shows ANP exceeding RNP during an approach?

ANP (Actual Navigation Performance, also called EPU) is the FMS's real-time estimate of its own position uncertainty. RNP is the accuracy standard required for the procedure being flown. When ANP exceeds the RNP value, the system cannot guarantee it is meeting the required accuracy, and on an RNP approach this triggers an alert that requires the crew to execute an immediate missed approach. Continuing the approach when ANP exceeds RNP is not permitted, because the obstacle clearance guarantees built into the procedure design are no longer valid.

Performance-Based Navigation Specifications and Nav Spec Selection

Modern aviation has largely moved away from sensor-specific navigation mandates — rules that say "you must use a VOR" or "you must have DME" — toward a more flexible, capability-based framework called Performance-Based Navigation (PBN). Under PBN, the FAA and ICAO specify what a navigation system must do, rather than dictating which hardware achieves it. The result is a family of Navigation Specifications (NavSpecs), each defining the accuracy, integrity, continuity, and availability that an aircraft's navigation system must deliver in a particular phase of flight or on a specific procedure.

For Airline Transport Pilot candidates and working flight crew, understanding how to read a NavSpec, select the correct one in the FMS, and verify that the aircraft is actually authorized to use it is not merely academic. Errors in NavSpec selection have led to crews flying procedures their avionics were not approved to fly, with potentially serious consequences for obstacle clearance. 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, is the primary FAA source governing this framework in the United States.

The PBN Framework: From RNAV to RNP

PBN is an umbrella concept that contains two broad families of navigation specifications:

Area Navigation (RNAV) – Specifies required total system error (TSE) accuracy but does not mandate on-board performance monitoring and alerting (OPMA). The aircraft must achieve the accuracy, but there is no requirement that the avionics continuously prove it is doing so.
Required Navigation Performance (RNP) – Adds the critical requirement for OPMA. The avionics must not only meet the accuracy standard but must also monitor their own performance and alert the crew if that standard cannot be met. This self-monitoring capability is what distinguishes RNP from basic RNAV and is what enables RNP procedures to have tighter obstacle clearance areas.

Each NavSpec is identified by a designator and a number. The number represents the total system error limit in nautical miles within which the aircraft must remain for at least 95% of flight time. For example, RNAV 1 requires lateral TSE no greater than ±1 NM for 95% of flight time, while RNP 0.3 (used on RNAV (GPS) approaches with RF legs) requires ±0.3 NM. The smaller the number, the tighter the accuracy and, in RNP specs, the more demanding the alerting requirement.

The Major NavSpecs and Where They Apply

AC 90-105A organizes the NavSpecs into groups tied to specific flight phases. The following are the most operationally significant:

RNAV 2 – En route operations on Q-routes and T-routes. Requires TSE ≤ 2 NM for 95% of time. No OPMA required. Achievable with a multi-sensor FMS or GPS/GNSS meeting TSO-C129A/C145/C146 standards.
RNAV 1 – Standard terminal arrivals (STARs) and departures (SIDs) that carry the RNAV 1 designation. TSE ≤ 1 NM for 95% of time. No OPMA required, but position error must be tightly managed.
RNP 2 – Oceanic and remote continental en route operations. Like RNAV 2 in accuracy but adds OPMA, which is essential when ATC radar surveillance is unavailable.
RNP 1 – Terminal and some en route operations requiring OPMA at ±1 NM. Found on procedures specifically designated RNP 1.
RNP APCH – The NavSpec for RNAV (GPS) approach procedures, including LNAV, LNAV/VNAV, and LPV minima lines. Required accuracy varies by segment (typically ±0.3 NM in the final approach segment). This is the most commonly used approach NavSpec in the U.S. NAS.
RNP AR APCH – Authorization Required. The most demanding approach NavSpec, enabling curved (RF leg) and very precise straight approaches with accuracy as tight as RNP 0.1. Requires special FAA operational approval, specific avionics (TSO-C145/C146 or equivalent), specific training, and is only available at designated airports. This NavSpec enables approaches with radius-to-fix (RF) legs that no other NavSpec permits.
A-RNP – Advanced RNP; an ICAO NavSpec that encompasses scalable RNP values and RF legs, used primarily in international operations.

How NavSpec Authorization Works

Possession of capable avionics is necessary but not sufficient. A carrier or operator must hold specific operational authorization before exercising a NavSpec, and the authorization pathway differs by operation type:

For Part 91 operators, many RNAV and RNP APCH operations are authorized by regulation and the relevant Airplane Flight Manual (AFM) supplement without a separate letter of authorization (LOA), provided the avionics meet the TSO standards cited in AC 90-105A. However, RNP AR APCH always requires an LOA regardless of operator category.

For Part 121 and Part 135 operators, NavSpec authority flows through the Operations Specifications (OpSpecs) issued by the operator's Principal Operations Inspector (POI). The relevant OpSpec paragraphs (such as C384 for RNP AR APCH; B036 covers RNP 10 and related oceanic/remote RNP operations) authorize both the equipment standard and the operational procedures that must be followed. No flight crew may fly an RNP AR approach at an airline unless that airline holds the appropriate OpSpec.

The aircraft itself must also have an AFM or AFM supplement that explicitly approves the NavSpec being used. AC 90-105A provides detailed tables of required avionics standards: for example, GNSS receivers used for RNP operations must typically meet TSO-C145 or TSO-C146 (WAAS) or, for certain oceanic RNP 4 operations, TSO-C129A class A1/B1/C1 with specific RAIM prediction requirements.

Selecting the Correct NavSpec in the FMS

At the practical level, flight crew interact with NavSpecs primarily through the FMS. When loading an arrival, departure, or approach procedure, the FMS database encodes the NavSpec for which that procedure was designed. The crew's responsibility is to verify that the active NavSpec displayed by the FMS matches the procedure's requirement and that the estimated position uncertainty (EPU), also called Actual Navigation Performance (ANP), remains below the Required Navigation Performance (RNP) value shown on the display.

A critical discipline is the ANP/RNP relationship: the FMS continuously computes ANP (its current best estimate of position uncertainty) and compares it to the active RNP value. If ANP exceeds RNP, the FMS must generate an alert. This is the OPMA requirement in action. On an RNP AR approach, for example, if ANP rises above the active RNP value (which may have been set to 0.10 NM on the final segment), the crew must immediately execute the published missed approach — continuing is not an option.

Before commencing any RNP procedure, crews should verify: (1) the correct NavSpec is active, (2) RAIM or WAAS availability has been confirmed for the route and timeframe, (3) the FMS database is current, (4) the aircraft holds the required authorization, and (5) all required sensors are operational and selected.

Why NavSpec Selection Matters: Safety Implications

PBN procedures are designed with obstacle clearance areas sized to match the NavSpec's accuracy requirement. An RNP AR approach corridor may be drawn with only the obstacle clearance required for RNP 0.1 accuracy. If a crew attempts that approach with avionics that can only meet RNP 0.3 accuracy — or with avionics that lack OPMA entirely — the actual flight path could intrude into terrain or obstruction clearance areas that the procedure designer assumed were protected. This is not a theoretical risk; mountainous terrain approaches like those in Alaska, the Andes, and the Himalayas rely on RNP AR specifically because the precision and monitoring capability of the specification allow designers to carve safe corridors through terrain that would otherwise be inaccessible in low visibility.

Key Numbers and Rules

RNAV 2: TSE ≤ 2 NM (95%), no OPMA — used on en route Q/T routes.
RNAV 1: TSE ≤ 1 NM (95%), no OPMA — used on many SIDs and STARs.
RNP APCH: OPMA required; final approach segment accuracy typically ±0.3 NM.
RNP AR APCH: OPMA required; accuracy scalable to 0.1 NM; RF legs permitted; always requires LOA/OpSpec.
RNP 4: TSE ≤ 4 NM (95%) with OPMA — used in oceanic/remote operations such as PACOTS tracks.
ANP must be less than RNP at all times during an RNP operation; alert triggers mandatory missed approach or route deviation contingency.
Avionics must meet applicable TSO standards per AC 90-105A tables — WAAS receivers (TSO-C145/C146) satisfy most domestic RNP APCH requirements.

Common Test Traps

Confusing RNAV with RNP: The single most-tested distinction is that RNP includes OPMA and RNAV does not. Both may use the same sensors; the difference is the monitoring and alerting requirement built into the avionics and approved in the AFM.
Assuming capable avionics = authorization: An aircraft can have WAAS avionics capable of RNP AR and still be prohibited from flying an RNP AR approach if the operator lacks the required LOA or OpSpec paragraph.
Misreading the NavSpec number: A smaller number means tighter accuracy (RNP 0.1 is more demanding than RNP 1.0), which is counter-intuitive to students who initially think a higher number means higher performance.
Ignoring the ANP/RNP comparison: Examiners test whether candidates know that exceeding the RNP value (ANP > RNP) requires an immediate missed approach on an approach procedure, not a crew decision to continue if they feel confident.
Thinking RNP APCH and RNP AR APCH are the same: RNP APCH is the standard NavSpec for RNAV (GPS) approaches and requires no special authorization beyond equipment compliance. RNP AR APCH is a separate, more demanding NavSpec that always requires specific approval and training.