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A precision approach is an instrument approach and landing using precision lateral and vertical guidance with minima as determined by the category of operation. In contrast, a nonprecision standard instrument approach procedure only provides horizontal guidance.
Note. Lateral and vertical guidance refers to the guidance provided either by:
a) a ground-based navigation aid; or
b) computer generated navigation data displayed to the pilot of an aircraft.
c) a controller interpreting the display on a radar screen (Precision Approach Radar (PAR)).
The controller uses the PAR display to guide the pilot or flight crew through the final stages of landing, providing horizontal and vertical guidance. In real time, ATC directs the pilot/flight crew to change heading or adjust the descent rate to keep the aircraft on a path that allows it to touch down at the correct spot on the runway.
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 Operation||Decision Height (DH) (2)||RVR||Visibility not less than|
|CAT I||not lower than 60 m (200 ft)||not less than 550 m||800m|
|CAT II||lower than 60 m (200 ft), but not lower than 30 m (100 ft)||not less than 350 m (1)|
|CAT IIIA||lower than 30 m (100 ft) or no DH||not less than 200 m|
|CAT IIIB||lower than 15 m (50 ft) or no DH||less than 200 m but not less than 50 m|
|CAT IIIC||no DH||no RVR limitation|
(1) Appendix 1 to JAR-OPS 1.430, Table 6, permits the use of an RVR of 300m for Category D aircraft conducting an autoland.
(2) Vertical minima:
- CAT I Because the aircraft is unlikely to be flying over level ground at the same elevation as the touch-down zone when passing the Missed Approach Point, the vertical minima used in a CAT I approach is measured by reference to a barometric altimeter. In practice, this means that when flying a CAT I approach either a Decision Altitude/Height or Decision Altitude/Height may be used.
- CAT II/III Because greater precision is required when flying a CAT II or CAT III approach, special attention is given to the terrain in the runway undershoot area to enable a radio altimeter to be used. CAT II and CAT III approaches are therefore always flown to a DH with reference to a radio altimeter.
CAT II and CAT III instrument approach and landing operations are not permitted unless RVR information is provided.
DA is the specified altitude in the precision approach, charted in feet mean sea level (MSL), at which a missed approach must be initiated if the pilot has not established the required visual reference to continue the approach.
DH is the specified altitude in the precision approach, charted in height above runway threshold elevation, at which the pilot must decide either to continue the approach or to execute a missed approach.
- ^ ICAO Annex 6
- JAR-OPS 1 Subpart E (All Weather Operations).