ILS & Precision Approaches
The Instrument Landing System is the most accurate civilian approach. Three components — localizer for lateral, glideslope for vertical, marker beacons for fixes — combine to give you precision guidance from intermediate fix all the way to a 200 ft DA. Modern equivalents (LPV) approach the same accuracy via WAAS GPS. Most operational errors on ILS approaches involve chasing the needles or misjudging localizer interception.
Localizer (LOC)
- Function: Lateral guidance along the runway centerline.
- Antenna: Located past the opposite end of the runway.
- Frequency: 108.1-111.95 MHz on odd tenths (LOC) — even tenths are VOR.
- Range: 18 NM from antenna, up to 4,500 ft above antenna.
- Course width: 5° total (full-scale deflection at 2.5° from centerline).
- Identification: Morse "I" + 3 letters (e.g., I-PDX).
- Receivers automatically switch from VOR to LOC mode when tuned to an odd-tenth frequency between 108-112 MHz.
Localizer is sharper than VOR — full-scale deflection happens at much smaller course deviation. New ILS pilots often over-correct; smooth small inputs are the rule.
Glideslope (GS)
- Function: Vertical guidance.
- Antenna: 750-1,250 ft from approach end of runway, 400-600 ft offset from centerline.
- Glideslope angle: 2.5°-3.5° (typically 3°).
- Beam thickness: 1.4° total (very narrow — high needle sensitivity).
- Outer Marker (OM) crossed at approximately 1,400 ft AGL.
- Middle Marker (MM) crossed at approximately 200 ft AGL.
- GS signal radiates on front course only — never trust GS on a back course.
- False courses exist at higher angles (9°, 12°). Avoid by flying chart-published altitudes to glideslope intercept.
The false-course problem: if you intercept the glideslope from above (for example, descending into the approach), you may capture a 9° or 12° false course. Always intercept from below at the published altitude.
Marker beacons
- Outer Marker (OM): 4-7 NM from threshold. Low-pitched continuous dashes (2/sec). Purple/blue light.
- Middle Marker (MM): ~3,500 ft from threshold. Alternating dots and dashes (95/min). Amber light.
- Inner Marker (IM): Used on Cat II/III ILS only. High-pitched dots (6/sec). White light.
- Marker receiver "TEST" position usually only tests the bulb, not the receiver.
- Many modern ILS approaches use DME or GPS in lieu of markers.
Approach Light Systems (ALS)
Visual aids extending the runway approach to provide directional, distance, and glide path information.
- ALSF — Approach Light system with Sequenced Flashing lights (high-intensity precision)
- SSALR — Simplified Short ALS with Runway alignment indicators
- MALSR — Medium intensity ALS with Runway alignment indicators
- MALSF — MALS with Sequenced Flashing
- ODALS — Omnidirectional ALS
- REIL — Runway End Identification Lights (synchronized flashers either side of threshold)
- "The Rabbit" is the high-intensity sequenced flasher running toward the runway at 2 flashes per second
- VASI / PAPI — Visual Approach Slope Indicator / Precision Approach Path Indicator: visual glideslope reference
Common ILS errors
- Chasing needles instead of flying the airplane — fixate on attitude and trim first, corrections second.
- Failing to recognize that LOC sensing is sharper than VOR — small inputs.
- Incorrect localizer interception angles — turn at first needle movement, don't wait.
- Trusting glideslope on a back course — never. Disregard GS on BC approaches.
- Capturing a false glideslope — always intercept from below at chart altitude.
- Failing to brief the missed before the FAF — missed approaches go badly when the brief is mid-execution.