ILS and Approach Guidance
Core concepts
ILS comprises a localiser for azimuth, a glide path for vertical guidance, and marker beacons or DME for range. The localiser transmits on VHF (108–112 MHz) with a 90 Hz/150 Hz modulation difference that drives the deviation indicator; full-scale deflection is typically 2.5° each side, narrowing to about 0.45° at the threshold. The glide path transmits on UHF and is normally set at 3°, giving full-scale deflection of about 0.7°. Key concepts are the decision altitude or height, the approach category, the precision approach classification (Cat I, II, III), and the role of the outer, middle and inner markers. Critical and sensitive areas protect the signals from reflections.
Traps in questions
Common traps: confusing decision height with decision altitude; forgetting that the localiser is the more sensitive guidance close to the runway; misunderstanding false glide paths above the true one; and the effect of large metallic objects or aircraft on the ground causing bends in the localiser. Also expect questions on the back course (no glide path), on the fact that ILS is a precision aid only in the approach direction, and on how marker beacons give range rather than guidance.
Worked example
A Category I ILS has a decision height of 200 ft. On a 3° glide path the aircraft is about 1 NM from the threshold at 300 ft; the rule of thumb is height in feet ≈ 300 × distance in NM on a 3° path, so 200 ft corresponds to about 0.7 NM.
Rule of thumb
On a 3° glide path, height above threshold in feet ≈ 300 × distance in NM. Localiser sensitivity increases as you get closer, so small corrections near the threshold.