ILS, RNAV, RNP — Modern Approach Procedures Explained in Detail

ILS, RNAV, RNP — Modern Instrument Approach Procedures in Detail

The instrument approach is the most demanding phase of an IFR flight. The pilot must guide the aircraft precisely along a published procedure to the runway — often without any outside visual reference until shortly before landing. Available approach procedures have evolved dramatically over recent decades: from classic ground-station-based systems (ILS, VOR/DME, NDB) to satellite-based systems (RNAV, RNP, LPV) that offer greater precision with less ground infrastructure.

ILS — The Gold Standard of Precision Approaches

The Instrument Landing System (ILS) has been the world's most widely used precision approach system since the 1940s and, despite all technological advances, will likely remain so for decades to come. It is the only ground-based system capable of supporting approaches down to Category III (near-zero visibility).

Components of the ILS:

• Localizer (LOC): The localizer transmits on frequencies between 108.10 and 111.95 MHz (odd tenths). Installed at the far end of the runway, it generates a directional beam providing lateral guidance (left/right of centerline). The total localizer beam width is typically 3-6 degrees, with the usable range (full-scale deflection) approximately 2.5 degrees either side of centerline. At the threshold, full-scale deflection corresponds to roughly 330 ft (100 m) of lateral deviation.
• Glide Slope (GS): The glide slope transmitter is installed beside the runway, typically 1,000 ft (300 m) beyond the threshold. It transmits on UHF frequencies (329.15-335.00 MHz), automatically paired with the localizer frequency. The glide path has a standard angle of 3 degrees (varying between 2.5 and 3.5 degrees depending on obstacle clearance). Vertical sensitivity is 0.7 degrees full-scale, corresponding to approximately 50 ft (15 m) of vertical deviation at the threshold.
• Marker Beacons (increasingly decommissioned): Outer Marker (OM, blue, 4-7 NM), Middle Marker (MM, amber, 0.5-0.8 NM), and Inner Marker (IM, white, at the threshold). These are increasingly being replaced by DME distance information or GPS positioning.
• Approach Lighting System (ALS): Not part of the ILS in the strict sense, but essential for the transition from instrument flight to visual landing. ALSF-2, MALSR, or SSALR depending on airport and category.

ILS Categories:

For private pilots in General Aviation, CAT I is the relevant standard. CAT II and above require special aircraft certification, ground infrastructure (high-performance ILS, RVR sensors, enhanced lighting), and pilot qualifications that are generally not available in the GA sector.

VOR/DME Approach — The Classic Non-Precision Approach

The VOR/DME approach (VHF Omnidirectional Range / Distance Measuring Equipment) is a Non-Precision Approach (NPA). This means it provides only lateral guidance (course) but no vertical guidance (glide path). The pilot must independently ensure correct altitude through stepwise descents between defined fixes (step-down fixes).

Characteristics of the VOR/DME approach:

• No glide path: Instead of a Decision Height (DH) as with the ILS, there is an MDA (Minimum Descent Altitude). The pilot descends to the MDA and flies it level until reaching the MAP (Missed Approach Point) or acquiring the runway visually.
• Step-down fixes: Between the Initial Approach Fix (IAF) and the Final Approach Fix (FAF), there are defined points at which the pilot may descend to the next altitude — only between fixes, never before.
• Higher minima: The MDA of a VOR/DME approach is typically 100-300 ft higher than the DH of an ILS on the same runway. Required visibility is also greater.
• VOR frequency: VOR transmits on 108.00-117.95 MHz (even tenths at lower frequencies, all tenths above 112.00 MHz).
• Dive and Drive vs. CDFA: Traditionally, the NPA was flown using the "dive and drive" technique: descend to the MDA, then fly level. ICAO has recommended the CDFA (Continuous Descent Final Approach) technique for years, where the pilot maintains a continuous descent like an ILS and uses the MDA only as a safety floor. CDFA is more stable, quieter, and safer.

VOR/DME approaches are increasingly being replaced by RNAV procedures but remain important as a fallback option during GPS outages and are still the only available approach aid at many smaller airports.

RNAV — GPS-Based Navigation of the Next Generation

RNAV stands for Area Navigation and refers to navigation procedures that are not dependent on ground-based navigation aids (VOR, NDB). In practice, RNAV today is almost exclusively based on GNSS (Global Navigation Satellite System) — GPS, GLONASS, and increasingly Galileo.

RNAV approaches (also called RNAV GNSS approaches) are divided into different service levels, differing in precision and requirements:

LNAV (Lateral Navigation):

• Lateral guidance only, no vertical component
• Equivalent to a non-precision approach
• MDA rather than DH
• Requires: Approved GPS receiver (e.g., Garmin GNS430W, GTN650/750)
• Accuracy: 0.3 NM (556 m / 1,823 ft) in the final approach segment
• Available at virtually all RNAV-capable airports

LNAV/VNAV (Lateral Navigation / Vertical Navigation):

• Both lateral and vertical guidance
• VNAV is calculated either barometrically (Baro-VNAV) or geometrically (SBAS)
• DA (Decision Altitude) rather than MDA, typically 250-350 ft
• Requires: GPS with VNAV capability or SBAS receiver
• Temperature limitations with Baro-VNAV (incorrect altitude indication at extreme temperatures)

LPV (Localizer Performance with Vertical Guidance):

• Highest precision of RNAV approaches
• Comparable to ILS CAT I in performance
• DA typically 200-250 ft, RVR 1,800 ft / 550 m
• Requires: SBAS-capable GPS receiver — WAAS (Wide Area Augmentation System) in the US, EGNOS (European Geostationary Navigation Overlay Service) in Europe
• Accuracy: 13 ft (4 m) vertical, 3 ft (1 m) horizontal in the final approach
• No ground infrastructure required at the airport (only survey)
• Increasingly replacing aging ILS installations at smaller airports

RNP-AR — Curved Approaches for Complex Environments

RNP-AR (Required Navigation Performance — Authorization Required) is the most demanding category of RNAV procedures. The "AR" stands for "Authorization Required" — airline, aircraft, and crew each need specific approval from the aviation authority.

What RNP-AR can do that other procedures cannot:

• Curved Approaches (RF Legs): RNP-AR permits curved flight paths with defined radii (Radius-to-Fix). The aircraft flies an exact arc with guaranteed navigation precision. This enables approaches through narrow valleys, around obstacles, or along noise-abatement routes over populated areas.
• Tighter RNP values: Standard RNAV requires RNP 0.3 NM in the final segment. RNP-AR can be specified at RNP 0.1 NM or even RNP 0.03 NM — meaning the aircraft remains within 0.1 NM (600 ft / 185 m) or 0.03 NM (180 ft / 56 m) of the desired position 99.999% of the time.
• Lower minima: The higher precision allows RNP-AR approaches to have lower DA and RVR values than standard RNAV, sometimes comparable to ILS CAT I.

Notable RNP-AR procedures:

• Innsbruck (LOWI): The RNP-AR approach to Runway 08 is one of the best-known examples in Europe. It guides aircraft through the Inn Valley with precise curves around mountain ridges — an approach that conventionally would only be possible in good visibility.
• Queenstown (NZQN): The famous RNP-AR approach in New Zealand through mountainous terrain.
• Juneau, Alaska (PAJN): One of the most well-known RNP-AR approaches in the United States, threading through a narrow channel surrounded by mountains.

RNP-AR is primarily a procedure for commercial aviation. General aviation aircraft typically lack the required avionics (FMS with RF-leg capability, found only in modern airliners and business jets).

NDB Approach — The Dinosaur Among Approaches

The NDB approach (Non-Directional Beacon) is based on the ADF receiver (Automatic Direction Finder) in the aircraft and a simple omnidirectional transmitter on the ground. It is the oldest instrument approach procedure still in use and is increasingly being decommissioned.

• Precision: Low. The ADF shows only the direction to the station, not course deviation. The pilot must calculate corrections from bearing and heading independently.
• Susceptibility: NDB signals are vulnerable to atmospheric interference (thunderstorms), coastal effects, terrain effects, and nighttime effects (skywave interference).
• High minima: The MDA of an NDB approach is typically the highest of all available procedures at an airport.
• Training value: Despite declining operational significance, the NDB approach continues to be taught in IR training (particularly under EASA) as it promotes fundamental navigation understanding and precise flying. The FAA has largely moved away from NDB approach testing.

Comparison Table: Approach Procedures at a Glance

Reading Approach Plates — The Art of the Approach Chart

Every instrument approach procedure is published on an approach plate (approach chart). Worldwide, these are published primarily by Jeppesen (commercial provider) and government sources — the FAA publishes Terminal Procedures (TPP) charts for US airports, while European states publish through their national AIPs (Aeronautical Information Publication). An approach plate contains all the information the pilot needs to safely execute the approach.

Layout of a typical approach plate:

• Header: Airport ICAO code, procedure designation (e.g., "ILS RWY 25L"), effective date, frequencies
• Communications block: All relevant ATC frequencies (Approach, Tower, Ground, ATIS)
• Plan view: Top-down view of the procedure showing IAF, IF, FAF, MAP, missed approach route, obstacles, altitude restrictions, courses, and distances
• Profile view: Side view of the procedure with altitude profile, glide slope indication, step-down fixes, DH/MDA
• Minimums table: Decision Height/Altitude or MDA and required visibility/RVR for different aircraft categories (A-E based on approach speed)
• Missed approach description: Textual and graphical depiction of the go-around procedure
• Airport information: Runway length, field elevation, lighting

Aircraft categories for approach minima:

The Future of Approach Procedures

The trend is clearly toward satellite-based procedures. The advantages are compelling:

• Cost savings: An ILS installation costs $1-5 million and requires regular calibration via flight inspection. An RNAV/LPV procedure requires only a one-time survey and no ground infrastructure.
• Flexibility: RNAV procedures can be implemented at any airport, even where there is no space or budget for an ILS.
• Environmental benefits: RNP-AR procedures with curved approaches enable noise-optimized approach routes.
• Capacity increase: Closely spaced parallel approaches become possible through the high precision of RNP.

For General Aviation, this development means a gradual democratization of instrument flight. Airports that previously had no IFR procedures can now be equipped with RNAV approaches. Modern GPS receivers in GA aircraft (Garmin GTN 750Xi, Avidyne IFD550) support LPV approaches and deliver precision that was reserved exclusively for airlines just 20 years ago. In the United States, WAAS-enabled LPV approaches are now available at thousands of airports, many of which never had any ground-based approach aids.

At the same time, the conventional navigation network (VOR, NDB) is being progressively thinned out worldwide. In Europe, several VOR stations have already been decommissioned with plans to reduce the network to a Minimum Operational Network by the end of the 2020s. The FAA has undertaken a similar VOR MON (Minimum Operational Network) initiative. Pilots relying exclusively on conventional navigation will find themselves increasingly restricted.

Conclusion: From ILS to LPV — A Quiet Revolution

The evolution of approach procedures from NDB through VOR/DME and ILS to RNAV, RNP, and LPV is one of the most significant technological developments in instrument flying. For the private pilot, it is essential to understand these procedures and equip their aircraft accordingly. A modern GPS receiver with SBAS capability (WAAS in the US, EGNOS in Europe) is today the single most important investment in an aircraft's IFR capability. It makes precision approaches available at hundreds — in the US, thousands — of airports that previously had only non-precision approaches or no instrument procedures at all, and at an accuracy that is every bit the equal of the proven ILS.