A precision approach (PA) is an instrument approach that is based on a navigation system that provides course and glide path deviation information meeting the precision standards of ICAO and ICAO Annex 10. For example, PAR, ILS, and GLS are precision approaches. 

The basic components of a standard ILS (Instrument Landing System) include the following: 

1. Guidance Information—localizer and glide slope. Localizers operate on odd tenths within the 108.10 to 111.95 MHz band. The localizer antenna is located at the far end of the approach runway. The localizer signal provides course guidance throughout the descent path to the runway threshold from a distance of 18 NM from the antenna site. The localizer signal is adjusted to provide an angular width of between 3° to 6°, as necessary to provide a linear width of 700 feet at the runway approach threshold. Full left or full right deflection occurs at approximately 2.5° from the centerline of a localizer course, which is four times greater than when tuned to a VOR, where full-scale deflection equals 10° from the centerline. The glide slope transmitter is located between 750 feet and 1250 feet from the approach end of the runway and offset 250 feet to 650 feet from it. The glide slope is normally usable to a distance of 10 NM. The glide path projection angle is normally 3° above horizontal so that it intersects the middle marker (MM) at about 200 feet and the outer marker (OM) at about 1400 feet above the runway elevation.

2. Range Information—marker beacons and DME. Ordinarily there are two marker beacons associated with an ILS, the outer marker (OM) and middle marker (MM). Locations with a Category II ILS also have an inner marker (IM). The following are legal substitutions for an inoperative outer marker: Compass Locator; Precision Approach Radar (PARL) or Airport Surveillance Radar (ASR); DME, VOR, or NDB fixes authorized in the standard instrument approach procedure; or a suitable RNAV system in conjunction with a fix identified in the standard instrument approach procedure. 

Visual and aural indications that a pilot would receive when crossing the outer, middle, and inner markers of a standard ILS: 

a. Outer Marker: blue light, dull tone, slow speed, all dashes (– – – – –) 

b. Middle Marker: amber light, medium tone, medium speed, dash dot ( –. –. –. –. –.)

c. Inner Marker: white light, high tone, high-speed, all dots (. . . . .)

Distances from the landing threshold of the outer, middle, and inner markers: 

a. Outer Marker—4 to 7 miles from threshold 

b. Middle Marker—3500 feet from threshold 

c. Inner marker—between middle marker and threshold

3. Visual Information—approach lights, touchdown and centerline lights, runway lights. A VASI system provides visual descent guidance during an approach to a runway; safe obstruction clearance is assured within ±10° of the extended runway centerline up to 4 NM from the runway threshold. Two-bar VASI installations normally provide a 3° visual glide path.

When on the last segment of an ILS final approach, change pitch to control glide path, and change power to control airspeed. If wind shear is encountered, expect the following:

1. Loss of Tailwind—when a tailwind shears to either a calm or a headwind component, the airspeed initially increases, the aircraft pitches up, and altitude increases. Pilots should respond by reducing power initially, then increase power once the glide slope is regained.

2. Loss of Headwind—when a headwind shears to either a calm or a tailwind component, the airspeed initially decreases, the aircraft pitches down, and altitude decrease. Pilots should respond by increasing power initially, then decreasing power once the glide slope is regained.

The Final Approach Fix (FAF) on an ILS is the Glide Slope Intercept; the Missed Approach Point (MAP) is arrival at the Decision Altitude/Decision Height (DA/DH) on the glide slope. Do not descend below the DA/DH unless you are in a position to make a normal landing and can see the runway, approach lights, etc.

Decision Altitude/Decision Height (DA/DH)

Precision approaches show the Decision Altitude (DA) in feet MSL, followed by the Decision Height (DH), in feet AGL referenced to the Height Above Threshold elevation (HAT). The DA/DH is the point at which a decision must be made during an ILS, MLS, or PAR instrument approach to either continue the approach or execute a missed approach.

According to Part 91 regulations:

No person may operate an aircraft below the prescribed MDA or continue the approach below the authorized DA/DH unless: 

a. 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.

b. The flight visibility is not less than the visibility prescribed in the standard instrument approach procedure being used. ATC will provide the pilot with the current visibility reports appropriate to the runway in use. This may be in the form of prevailing visibility, runway visual value (RVV), or runway visual range (RVR). However, only the pilot can determine if the flight visibility meets the landing requirements indicated on the approach chart. If the flight visibility meets the minimum prescribed for the approach, then the approach may be continued to a landing. If the flight visibility is less than that prescribed for the approach, then the pilot must execute a missed approach, regardless of the reported visibility. RVR values converted to meteorological visibility in statute miles are shown below:

RVR – Statute miles 

1600 – 1/4

2400 – 1/2

3200 – 5/8

4000 – 3/4

4500 – 7/8 

5000 – 1

6000 – 1 1/4

c. At least one of the following visual references for the intended runway is distinctly visible and identifiable to the pilot:

1. The approach light system (except that the pilot may not descend below 100 feet above the touchdown zone elevation using the ALS as a reference unless the red terminating  bars or the red side bars are also distinctly visible and identifiable)

2. The threshold 

3. The threshold markings 

4. The threshold lights 

5. REIL (Runway End Identifier Lights)

6. VASI (Visual Approach Slope Indicator)

7. The touchdown zone markings 

8. The touchdown zone lights 

9. The runway and runway markings 

10. The runway lights