INSTRUMENT APPROACH TYPES

Instrument Approach Types Lesson by wifiCFI


Approach Types

There are 2 main categories of Instrument Approach Types, they are:

Precision Approaches

Provide the pilot with:

Azimuth Guidance (lateral/left and right guidance)

Vertical Guidance (vertical/up and down guidance)

Approach Light System

Distance Information (waypoints, DME distances, marker beacons)

Non-Precision Approaches

Provide the pilot with:

Azimuth Guidance (lateral/left and right guidance)

May not include the other systems included in a precision approach

Precision Approaches

The Precision Approach Procedures we will discuss in this lesson are:

ILS (Instrument Landing System)

LPV (Lateral Performance with Vertical Guidance)*

LNAV/VNAV (Lateral Navigation and Vertical Navigation)*

*LPV and LNAV/VNAV approaches are not technically Precision Approaches although they work in almost the same way as Precision Approaches. However, due to the cost and amount of paperwork required to certify a Precision Approach, LPV and LNAV/VNAV Approaches are technically considered APV Approaches (Approaches with Vertical Guidance). APV Approaches are considered Non-Precision Approaches.

The ILS (Instrument Landing System)

The Instrument Landing System (ILS) is perhaps the most common type of Precision Approach in the Instrument Flying Environment.

Azimuth Guidance is provided via use of a Localizer.

Vertical Guidance is provided via use of glideslope.

The Localizer

How does the Localizer Signal Work?

A Localizer sends out 2 signals. For our purposes we will call them “Signal A” and “Signal B.”

To receive an on-course indication, the Localizer Antenna on the airplane must be receiving equal amounts of both Signal A and B.

If it receiving 1 signal more than the other, the pilot will receive an off-course indication on his/her HSI/OBS.

Let’s begin by first discussing the Localizer on the ILS Approach.

The Localizer is always located at the back end of the runway it serves.

The Localizer has a service area. In this area, a pilot can be assured that he/she is receiving correct course indications from the localizer on their HIS/OBS.

The Localizer service area is depicted below. (AIM 1-1-9)

While the Service Volumes of the Localizer are depicted on the previous slide, a pilot should also know a Localizer’s Course Width.

This is the area in which the HSI/OBS Needle will be “alive.”

The course width of a Localizer can vary between 3 – 6 degrees.

This course width variation occurs due to the length of the runway.

Because the Localizer Course Width is always 700’ at the runway threshold, longer runways will have smaller course widths while shorter runways have longer ones.

The Glideslope

The glideslope works the same way as the localizer.

The glideslope is just tilted up on it’s side to create a vertical approach course instead of a lateral approach course.

However, the glideslope is usually located to the side of the runway it serves and its service volume and course width vary from the localizer. 

Marker Beacons (AIM 1-1-10)

Some ILS Approach Systems utilize Marker Beacons to alert a pilot to his/her location along the final approach course.

There are 3 types of Marker Beacons used on ILS Approaches, they are:

OM (Outer Marker)

Light Color: Blue

Beacon Code: - - - - 

MM (Middle Marker)

Light Color: Amber

Beacon Code: - . - .

IM (Inner Marker)

Light Color: White

Beacon Code: . . . .

Marker Beacons can also be set to high or low sensitivity.

When set to “high” sensitivity, the marker beacon will be identified from a further distance.

For this reason, it is best to set the marker to “low” sensitivity when executing an ILS approach for accuracy.

Marker Beacons are a ground based system that create a vertical signal from the station.

They are placed in strategic locations along the ILS Final Approach Course to alert the pilot of key points.

OM (Outer Marker)

Established at the glideslope intercept altitude (FAF).

MM (Middle Marker)

Established along glideslope at the 200’ AGL point (DH).

IM (Inner Marker)

Used on ILS CAT II Approaches.

Established at the CAT II Decision Height (DH).

Approach Light Systems (AIM 2-1-1)

The last piece of the ILS Approach to discuss are the Approach Light Systems.

There are many different types of approach light systems in use.

They extend from the end of the runway and are used to help the pilot transition the airplane safely from IFR to VFR flying and landing.

Parallel ILS Approaches (AIM 5-4-13)

ILS Approaches can be made simultaneously to parallel runways.

There are different types of parallel ILS Approaches we will cover in this lesson.

Dependent Parallel ILS Approaches (AIM 5-4-13)

These parallel approaches may be established where the runway centerlines are between 2,500’ and 9,000’ apart.

Aircraft on these approaches must be staggered as seen in the depiction below.

The minimum separation differences between these aircraft will vary based on the distance between the runway centerlines.

Simultaneous Independent Parallel ILS Approaches (AIM 5-4-13)

These parallel approaches may be established where the runway centerlines are between 4,300’ and 9,000’ apart.

Aircraft on these approaches may be parallel but must not encroach on the “No Transgression Zone” NTZ.

Simultaneous Close Independent Parallel ILS Approaches (AIM 5-4-13)

These parallel approaches may be established where the runway centerlines are between 3,000’ and 4,300’ apart.

Aircraft on these approaches may be parallel but must not encroach on the “No Transgression Zone” NTZ.

These approaches require PRM (Precision Runway Monitoring) by ATC.

LPV and LNAV/VNAV Approaches (AIM 1-1-18)

LPV and LNAV/VNAV work in much the same way as ILS Approaches except they provide Azimuth (Lateral) and Vertical Guidance via use of GPS Satellites.

These approaches require the use of an IFR Approved GPS with WAAS (Wide Area Augmentation System) capabilities.

Non-Precision Approaches

The Non-Precision Approach Procedures we will discuss in this lesson are:

Localizer Front Course Approach

Localizer Back Course Approach

Localizer Type Directional Aid Approach (LDA)

Simplified Directional Facility Approach (SDF)

VOR Approach

RNAV/GPS Approach

Localizer Front Course Approach

A localizer front course approach is essentially the same as an ILS Approach but without Vertical Guidance (glideslope).

This type of approach may also be flow if the pilot loses his/her glideslope indication when initially executing an ILS Approach.

In this situation, the approach would revert to a Non-Precision Localizer Only Approach.

The pilot will fly the intended approach courses but will use “step-down” altitudes between fixes instead of following the glideslope needle.

Localizer Back Course Approach

The localizer antenna (as seen in previous slides) creates 2 signals to establish on and off course indications to a pilot in flight.

The localizer antenna also sends these 2 signals from the reverse side of the antenna (going in the opposite direction).

This establishes a flyable, non-precision, Localizer Back Course Approach.

When flying a back course approach it is important for the pilot to still use the front course number published on the instrument approach plate (this is covered in more detail in the “Localizer Back Course Approach Maneuvers” lesson).

LDA Approach (AIM 1-1-9)

LDA stands for “Localizer Type Directional Aid.”

LDA’s work very similar to ILS Approaches, some even provide Vertical Guidance via the use of a glideslope.

The difference between an LDA and an ILS Approach is that an LDA is not aligned with the runway centerline.

If the LDA course lines up with the runway centerline within 30 degrees then straight-in minimums may be published.

If not, then only circling approach minimums will be published.

SDF Approach (AIM 1-1-10)

SDF stands for “Simplified Directional Facility.”

SDF approaches are also similar to Localizer and LDA Approaches.

The SDF approach differs from a Localizer Approach in the fact that it may not be aligned with the runway centerline.

The SDF approach differs from an LDA Approach in the fact that it is set to a 6 or 12 degree course width instead of the 3-6 degree width of a Localizer or LDA.

VOR Approaches

VOR Approaches are non-precision approaches that utilize VOR Radials to create approach courses to an airport or runway.

For information on how VOR’s work, see our VOR Lesson.

RNAV Approaches

RNAV Approaches are non-precision approaches that utilize GPS Satellites to create approach courses to an airport or runway.

For information on how the GPS System works, see our GPS Lesson.

Other Approaches

There are a few other approaches we will cover in this lesson, they are:

PAR Approach

ASR Approach

No-Gyro Approach

Visual Approach

Contact Approach

PAR Approach

PAR stands for “Precision Approach Radar.”

In a PAR Approach, an ATC Controller will give verbal instructions to the pilot making the approach to land.

These verbal instructions will include Lateral and Vertical Guidance.

ASR Approach

ASR stands for “Airport Surveillance Radar.”

In an ASR Approach, an ATC Controller will give verbal instructions to the pilot making the approach to land.

These verbal instructions are for Lateral Guidance only.

No Gyro Approach

A no-gyro approach can be requested by a pilot when he/she has gyroscopic instrument failure(s).

This approach will include verbal guidance to the approach to landing.

Turns made while executing a no-gyro approach should be made at half-standard rate.

Visual Approach

A visual approach can be assigned to a pilot by ATC when the pilot has either the airport in sight, or the preceding aircraft in sight.

During a visual approach, the pilot is responsible for his/her traffic clearances.

The pilot is able to make the necessary adjustments to the flightpath to approach and landing.

Contact Approach

A Contact Approach can not be assigned by ATC but may be requested by the pilot.

A Contact Approach is used when a pilot does not have the preceding aircraft or airport in sight but is familiar enough with the nearby environment that he/she can safely navigate to the airport using landmarks.

In order to request a Contact Approach, the following conditions must exist:

The destination airport has a published instrument approach procedure.

The reported ground visibility at the destination airport is at least 1 mile.

The pilot must be able to remain clear of clouds and have 1sm Flight Visibility during the approach.

FAA Sources Used for This Lesson

Federal Aviation Regulations Part 91

Instrument Flying Handbook

Aeronautical Information Manual


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