ILS (Localizer, Glide Path, Markers)
ILS provides precision approach guidance in three dimensions. The localizer (108-112 MHz VHF) gives lateral guidance with a 90 Hz/150 Hz modulation difference; the glide path (329.15-335 MHz UHF) gives vertical guidance, typically 3 degrees. Candidates must know the fly-up/fly-down and left/right indications, the sensitivity of the localizer (wider at the threshold, narrower at the runway end), and the meaning of the course deviation indicator.
Markers (75 MHz) indicate distance to threshold: outer (about 4-7 nm), middle (about 3500 ft from threshold) and inner (about 1000-1500 ft). DME may replace markers. Understand the ILS critical area and sensitive area to protect signals from interference, and the categories (CAT I, II, III) with their decision heights and RVR minima.
Traps: confusing localizer and glide path frequencies; forgetting that the localizer is the most sensitive near the runway; misinterpreting marker tones (outer = 400 Hz dashes, middle = 1300 Hz alternating, inner = 3000 Hz dots). Another trap is the assumption that ILS is unaffected by terrain; it is.
Rule of thumb: localizer 90 Hz = left of centreline, 150 Hz = right; glide path 90 Hz = fly up, 150 Hz = fly down.