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Precision Approaches

Objective​

Timing​

Format​

Overview​

  • Precision Approach Types
  • Localizer Principles of Operation
  • Glideslope Principles of Operation
  • Marker Beacons
  • ILS Receiving Equipment
  • ILS Categories
  • ILS Errors and Irregularities
  • Localizer and Glideslope Critical Areas
  • Precision Instrument Approaches
  • Back Course Approaches
  • APV Instrument Approaches and GPS Approaches

Elements​

Precision Approach Types​

Precision approaches are characterized by vertical and horizontal guidance that positions the aircraft close to the runway.

  • PAR - Precision approach radar
  • GLS - GBAS landing system
  • ILS - Instrument landing system

Localizer Principles of Operation​

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  • Provides horizontal guidance along the extended runway centerline
  • Transmitter located at the far end of the runway
  • Operates on 40 channels within the 108.10 MHz to 111.95 MHz frequency range (odd tenths)
  • Adjusted for a course width of 700 feet at the runway threshold
  • Normal service volume extends to 18 NM within 10° of centerline and 10 NM within 35°
  • Identification consists of a three-letter Morse code designator preceded by the letter "I"
    • The morse code identifier is printed on approach and enroute charts

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Glideslope Principles of Operation​

  • Provides vertical guidance toward the runway touchdown point
  • Frequency is automatically paired with the localizer frequency
  • Operates in the UHF frequency range between 329.15 MHz and 335.00 MHz
  • Transmitter located 750 to 1,250 feet from the approach end and offset 250 to 600 feet from centerline
  • Normally adjusted to a projection angle of 3° above horizontal
  • Provides a usable signal to a distance of 10 NM

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Marker Beacons​

  • VHF transmitters operating on 75 MHz to provide range information along the approach path
  • Antenna array produces an elliptical pattern above the station
  • Types of markers
    • Outer Marker (OM): Located 4 to 7 miles out; indicates glidepath intercept point; blue light and low-pitch dashes
    • Middle Marker (MM): Located ~3,500 feet from threshold; indicates ~200' altitude above threshold; amber light and alternate dots/dashes
    • Inner Marker (IM): Indicates Decision Height for CAT II approaches; white light and high-pitch dots
    • Back Course Marker: Indicates the final approach fix for a back course approach; white light and pairs of dots
  • Markers beacons have mostly been decommissioned, except for inner markers required for CAT-III approaches at large airports. Per AIM 1-1-9, the following may be substituted for an outer marker:
    • DME fix
    • VOR or VOR intersection
    • Suitable RNAV/GPS system
    • NDB in certain scenarios

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ILS Receiving Equipment​

  • Combined VOR/Localizer receiver with common tuning
    • Navigation indicator includes localizer (vertical) and glideslope (horizontal) needles
    • Warning flags (NAV, GS, or OFF) appear if signal strength is insufficient or equipment fails
  • Audio switch panel used to monitor Morse code identifiers and marker beacon tones
  • Selective sensitivity on some marker beacon receivers (usually on the audio panel)

ILS Categories​

  • Category I: DH 200 feet and RVR 2,400 feet (1,800 feet with TDZ/CL lighting or HUD/Autopilot)
    • Special Authorization CAT I: DH 150 feet and RVR 1,400 feet using HUD to DH
  • Category II: DH 100 feet and RVR 1,200 feet (1,000 feet with special authorization)
    • Special Authorization CAT II: DH 100 feet and RVR 1,200 feet with reduced lighting
  • Category IIIa: No DH or DH below 100 feet; RVR not less than 700 feet
  • Category IIIb: No DH or DH below 50 feet; RVR between 150 and 700 feet
  • Category IIIc: No DH and no RVR limitation

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ILS Errors and Irregularities​

  • Reflection: Surface vehicles or aircraft below 5,000 feet AGL may cause signal disturbances
  • False glideslope:
    • Inherent GS signals at higher vertical angles (typically 9°– 12°)
  • Backcourse glideslope: A glideslope signal may also be received when flying the BC of an ILS; it should be ignored
  • Needle "chasing" often results from poor planning or over-controlling
  • Guidance anomalies may occur below 100' above the DA
    • In this case it's best to fly a known good heading and maintain the established descent rate

Localizer and Glideslope Critical Areas​

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  • Areas near antennas established to prevent signal interference by vehicles or aircraft
  • ATC provides protection when ceiling is < 800 feet or visibility is < 2 miles
  • Arriving aircraft inside the OM (or fix in lieu) triggers protection for both LOC and GS areas

Precision Instrument Approaches​

  • Requires pilot adherence to depicted altitudes, paths, and weather minimums
  • GS should be captured from below the intercept altitude to avoid false courses.
  • The Precise Final Approach Fix (PFAF) is the point of glideslope intercept at the published altitude
  • Stabilized approach concept (constant rate and configuration) is critical below 1,000 feet AGL
  • Descent rates exceeding 1,000 FPM are generally unacceptable on final

Back Course Approaches​

  • Approach using the localizer signal in the opposite direction of the front course
  • Identified as "LOC BC" on approach charts
  • Reverse sensing occurs on standard VOR indicators (fly away from the needle)
  • HSI eliminates reverse sensing if set to the front course heading
    • The HSI OBS should be set to the front course of the runway (180° off of aircraft heading)
  • Glideslope is not provided; vertical guidance must be ignored if received
  • For autopilot-coupled approaches, ensure it is configured to track a backcourse signal

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APV Instrument Approaches and GPS Approaches​

RNAV = Random Area Navigation

  • RNAV Approaches with Vertical Guidance
    • LPV - Localizer Performance with Vertical guidance
      • ILS-like minimums (200' minima) and is flown similar to an ILS
      • Gives L/R angular guidance, similar to an ILS
      • Not technically a "precision approach" (for the purposes of alternate planning)
    • LNAV/VNAV - Lateral/vertical navigation (Garmin L/VNAV)
      • Final approach has fixed 0.3NM sensitivity (not angular like an ILS or LPV)
      • Doesn't require a WAAS-based navigator
      • These were initially design for baro-aided FMS systems
  • RNAV Non-precision approaches
    • LP - Akin to a LOC approach
    • LNAV - Lateral guidance
    • GPS overlay approaches
  • GPS Sensitivity
    • ENR, TERM, LNAV, LP
  • Advisory Vertical Guidance (LP +V, LNAV+V)
    • GPS units can provide advisory vertical guidance which meets the steps downs
    • This approach still has an MDA however

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References​

  • Instrument Flying Handbook: 9-35 to 9-43
  • Aeronautical Information Manual: Chapter 1, Section 1; Chapter 5, Section 4; Chapter 5, Section 5
  • TERPS Manual (FAA Order 8260.3): Chapters 1, 3, 10, and 12
  • Pilot-Controller Glossary: 1952, 1954, 1975, 1984, 1986, 2041