Stein Mjåtveit
12.08.19

What is an Instrument Approach Procedure?

WRITTEN BY
Stein Mjåtveit
PUBLISHED
12.08.2019

Instrument Landing System (simplified)

During pilot training extensive time and effort is spent on mastering the different types of Instrument Approach Procedures. In this article I am going to explain what the different types of IAP's are and what makes them different from one another. Are you ready for the approach? Let's get you a clearance.

When flying under Instrument Flight Rules (IFR) pilots use the instruments on-board their aircraft to determine their position and navigate between points. When departing from an airport we usually follow a Standard Instrument Departure (SID) or an Obstacle Departure Procedure (ODP). Thereafter we transition to the en route segment where we fly along airways and/or between points until we get close to our destination. Once we get close enough, we latch onto a Standard Terminal Arrival Route (STAR) and/or receive radar vectors from Air Traffic Control (ATC), until we connect with the Instrument Approach Procedure we are cleared to fly.

Instrument Approach Procedures (IAP's) allow pilots to align their aircraft with the runway and fly towards it without being able to see anything outside their windows. This enables us to fly into airports even if they are obscured by clouds or fog, as long as we can make visual contact with the runway environment before reaching what is known as our Decision Altitude/Height (DA/DH) or Minimum Descent Altitude/Height (MDA/MDH). If visual contact has not been acquired at that point, we must initiate a Missed Approach Procedure (MAP) and either come back for another attempt, enter a holding pattern while waiting for the weather to improve or divert to our alternate airport. Pilots are constantly faced with these kind of decisions-making and problem solving scenarios, both during their training and when operating on the line.

There are two main types of IAP's available at most airports, 3D and 2D approaches. 3D approaches provide both lateral and vertical guidance so the pilots can compare their flight path to a Course Deviation Indicator (CDI) and a Glide Path (GP) indicator. When pilots fly a 2D approach we will only receive lateral guidance and must therefore calculate a suitable Rate of Descent (ROD) to achieve a Continuous Descent Final Approach (CDFA).

INSTRUMENT LANDING SYSTEM (ILS)

Type of approach: 3D

The ILS is a widely used Instrument Approach Procedure and is available on all major and many medium sized airports. When flying an ILS approach the pilots are presented with localizer and glide path indications on the instruments in the cockpit. This allows the pilots to make a smooth and stabilized approach towards the runway, while making precise corrections for any deviations along the way.

 

ILS Overview FAA Instrument Flying Handbook

Image source: FAA Instrument Flying Handbook

ILS approaches also have different sub-categories, such as CAT I, II, IIIA and IIIB. A CAT I ILS will usually allow the pilots to fly the aircraft down to 200 feet above the runway threshold without any outside visual references. During a CAT I approach the Runway Visual Range (RVR) must also be at least 550 meters, it might even be required to be higher if parts of the Approach Lighting Systems (ALS) are not working.

In the table below you will find an overview of the different criteria used for categorizing ILS approaches:

Category of Operation Decision Height (DH) Runway Visual Range (RVR) Visibility not less than
CAT I Not lower than 200 ft (60 m) Not less than 550 m 800 m
CAT II Lower than 200 ft (60 m), but not lower than 100 ft (30 m) Not less than 350 m*  
CAT IIIA Lower than 100 ft (30 m) or no DH Not less than 200 m  
CAT IIIB Lower than 50 ft (15 m) or no DH Less than 200 m, but not less than 50 m  
CAT IIIC No DH No RVR limitation  

*Appendix 1 to JAR-OPS 1.430, Table 6, permits the use of an RVR of 300m for Category D aircraft conducting an autoland. 

LOCALIZER (LOC)

Type of approach: 2D

A localizer approach is basically an ILS without the vertical guidance. Since there is no glide path on a localizer approach a Minimum Descent Altitude (MDA) is used instead of a Decision Altitude (DA) and pilots are not able to descend as low as we can on 3D approaches. Since there is no vertical guidance, pilots must calculate the appropriate rate of descent for the approach based on their velocity across the ground (ground speed).

LOCALIZER PERFORMANCE WITH VERTICAL GUIDANCE (LPV)

Type of approach: 3D

The design of an LPV approach incorporates angular guidance with increasing sensitivity as an aircraft gets closer to the runway. You can think of LPV approaches as the "GPS version of an ILS", which means that it uses GPS signals to provide both lateral and vertical guidance to the pilots.

In the USA there is a network of ground stations that do the job of augmenting the GPS signals for integrity assurance, increased reliability and improved precision called the Wide Area Augmentation System (WAAS). It consists of Wide Area Reference Stations spread all over the country which sends data to a Wide Area Master Station for correction. The Wide Area Master Station then uplinks the corrected signal to GEO Synchronous Satellite(s), a satellite that matches the earths rotational speed, thus keeping the satellite over the same geographical location as the earth rotates around its axis.

WAAS FAA Instrument Flying Handbook

Image source: FAA Instrument Flying Handbook

In addition to WAAS, which is a combination of Ground- and Satellite Based Augmentation Systems (GBAS/SBAS), Aircraft Based Augmentation Systems (ABAS) are also used to assure the integrity, reliability and precision of RNP approaches. There can also be equipment located at or close to the airport which in the American system is called Local Area Augmentation System (LAAS).  

WAAS in Airport environment FAA Instrument Procedures Handbook

Image source: FAA Instrument Flying Handbook

LOCALIZER PERFORMANCE WITHOUT VERTICAL GUIDANCE (LP)

Type of approach: 2D

An LP approach has the same accuracy requirements as an LPV approach, but no glide path or vertical guidance is provided. There could be several reasons for using LP instead of LPV approaches, but one of the more common reasons is that it might not be possible to design an LPV approach for a particular airport due to obstacles on the approach path.

LNAV/VNAV

Type of approach: 3D

LPV_Chart2

The main difference between LPV and LNAV/VNAV approaches is the Required Navigation Performance (RNP) required to fly the approach. You can read more about RNP in the "Related terms" section below. What this means is that an LPV approaches has stricter requirements for accuracy and aircraft equipment compared to LNAV/VNAV. This translates into LNAV/VNAV approaches having a higher Decision Altitude (DA) compared to an LPV approach. LNAV/VNAV utilizes an internally generated glide slope which is usually based on WAAS or a barometrically calculated glide slope.

LNAV

Type of approach: 2D

LNAV has the same accuracy requirements as LNAV/VNAV, but as you might have guessed there is no Vertical Navigation (VNAV) component on an LNAV approach. 

NON-DIRECTIONAL BEACON (NDB)

Type of approach: 2D

NDB's are not commonly found in the United States, however, they are still common in Europe. When flying an NDB approach the pilots use an Automatic Direction Finder (ADF) which gives them a relative bearing towards the NDB. The inbound course will be published on an approach plate, so pilots know which relative bearing to follow inbound to and outbound from the NDB.

VHF OMNIDIRECTIONAL RANGE (VOR)

Type of approach: 2D

VOR stations are navigational aids that act as connecting intersections between airways. Pilots often fly along airways and pass over VOR's during the en route segment of a flight, but they are also used for SID's, STAR's and IAP's. When navigating using a VOR station, the pilots can select any radial omitted from the VOR and intercept and track that radial. When used for approaches, the inbound radial to be used is specified on the approach plate. 

RELATED TERMS

Performance Based Naviation (PBN)

Performance Based Naviation (PBN) allow aircraft to fly three-dimensional paths through our airspace systems. For many, PBN is strongly associated with GPS/GNSS navigation, but the truth is that this PBN has been around for longer than GPS/GNSS. Pilots from the older generations are familiar with systems such as LORAN-C and Inertial Reference Systems (INS) which also allowed pilots to navigate point-to-point without having to fly directly over navigational aids on the ground. PBN has however, become a more widely used term as GPS/GNSS systems have matured and are being used on a wider scale.

Required Navigation Performance (RNP)

RNP is a system in place to monitor and alert the crew in case the accuracy deteriorates outside of approved limitations. One example of such a system is Receiver Autonomous Integrity Monitoring (RAIM), which ensures that GPS signals are within the prescribed RNP for any given segment of a flight. An aircraft must be within the specified accuracy at a minimum of 95% of the flight time in order to meet the PBN requirements.

RNP FAA Instrument Flying Handbook

Image source: FAA Instrument Procedures Handbook

In layman terms, the job of RNP is to ensure that the accuracy is within limits. If not, the crew should be alerted of the discrepancy.

Area Navigation (RNAV)

RNAV, or Area Navigation, is a broad term that encompasses all types of point-to-point navigation and acts as an umbrella-term for LPV, LP, LNAV/VNAV and LNAV approaches. Area navigation does not require pilots to fly directly over navigational aids which means that often a more efficient point-to-point route can be selected. Think of it this way, LPV, LP, LNAV/VNAV and LNAV approaches are all RNAV approaches, they just have different tolerances for precision. It is also worth noting that the more advanced the approach is, the more advanced equipment you will need on board the aircraft to fly the approach.

On PBN instrument approach chart titles, the term “RNP” is going to replace the term “RNAV”. RNAV has been used for decades to refer to instrument approach procedures enabled by Global Navigation Satellite Systems (GNSS). Read more about this change in this document from Eurocontrol (PDF). 

I hope you enjoyed this article on the different kinds of Instrument Approach Procedures, blue skies and safe landings my friends! 

 

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