Types of Precision Approaches: ILS, GLS, PAR systems


A precision approach (PA) is an instrument approach or instrument approach procedure (IAP) approach for landing an aircraft using both lateral and vertical guidance with a determined decision height and minimums regarding the category of the aircraft. This type of precision approach could be used as an IFR flight requirement.

Unlike the non-precision approaches which use only lateral guidance like VOR/DME vs RNAV.

Precision approach Concept

The precision approach is one in which the pilot uses aircraft instruments for lateral and vertical guidance to maintain a constant path towards the landing runway.

Precision Instrument Approaches

The concept of the Precision approach is a series of predetermined maneuvers for the orderly transfer of an aircraft operating under instrument flight rules from the beginning of the initial approach to a landing or to a point from which a landing may be made visually. These approaches are approved in the European Union by EASA and the respective country authorities and in the United States by the FAA or the United States Department of Defense for the military. The ICAO defines an instrument approach as a series of predetermined maneuvers by reference to flight instruments with specific protection from obstacles from the initial approach fix, or where applicable, from the beginning of a defined arrival route to a point from which a landing can be completed and thereafter if the landing is not completed to a position at which holding or en route obstacle clearance criteria apply. That’s why external lighting systems like, Nav lights, strobe lights, and landing lights help the pilot approach the runway safely.

Precision approach types

ILS glideslope and localizer
ILS illustration| Wikimedia Commons
  1. Ground Based Augmentation System (GBAS) Landing System (GLS) approaches provide exact alignment and descent guidance to aircraft. To the pilot, GBAS/LAAS approaches look similar to ILS approaches. LAAS is based on GPS signals that are then supplemented by ground equipment.
  2. Precision Approach Radar (PAR) approaches involve radar and two-way radio communications. The controller provides the pilot headings, altitudes showed by the Altimeter, provide necessary corrections to remain on course. The military mostly uses PAR approaches in the United States.
Ground approach radar equipment in operation.
Ground approach radar equipment | Picryl

Precision approach categories

Categories of precision approach and landing (including Instrument Landing System (ILS) and Autoland) operations are defined according to the applicable Decision Altitude/Height and Runway Visual Range (RVR) or visibility as shown in the following table.

Category of OperationDecision Height (DH) (2)RVRVisibility not less than
CAT Inot lower than 60 m (200 ft)not less than 550 m800m
CAT IIlower than 60 m (200 ft), but not lower than 30 m (100 ft)not less than 350 m (1)
CAT IIIAlower than 30 m (100 ft) or no DHnot less than 200 m
CAT IIIBlower than 15 m (50 ft) or no DHless than 200 m but not less than 50 m
CAT IIICno DHno RVR limitation
Categories of Precision approach

Precision approach minimums

  1. Published Approach Minimums:
    Approach minimums are published for different aircraft categories and consist of a minimum altitude (DA, DH, MDA) and required visibility. These minimums are determined by applying the appropriate TERPS criteria
  2. Straight-in Minimums:
    Obstacle clearance is provided within 30° either side of centerline (15 degrees for GPS IAPs) and a normal descent can be made from the IFR altitude shown on the IAP to the runway surface. When either the normal rate of descent or the runway alignment factor of 30 degrees (15 degrees for GPS IAPs) is exceeded, a straight-in minimum is not published and a circling minimum applies.
    The fact that a straight-in minimum is not published does not preclude pilots from landing straight-in if they have the active runway in sight and sufficient time to make a normal approach for landing
  3. Side-Step Maneuver Minimums:
    Landing minimums for a side-step maneuver to the adjacent runway will normally be higher than the minimums to the primary runway
  4. Circling Minimums:
    In some busy terminal areas, ATC may not allow circling and circling minimums will not be published. Published circling minimums provide obstacle clearance when pilots remain within the appropriate area of protection. Pilots should remain at or above the circling altitude until the aircraft is continuously in a position from which a descent to a landing on the intended runway can be made at a normal rate of descent using normal maneuvers. Circling may require maneuvers at low altitude, at low airspeed, and in marginal weather conditions. Pilots must use sound judgment, have an in-depth knowledge of their capabilities, and fully understand the aircraft performance to determine the exact circling maneuver since weather, unique airport design, and the aircraft position, altitude, and airspeed shwed by the static pitot system must all be considered.

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