062 Radio Navigation topic guide
ILS: Coverage, Errors and Categories
The ILS gives two independent guidance signals, a localiser for lateral guidance and a glide path for vertical guidance, and each has its own published coverage volume beyond which the signal cannot be relied upon. Knowing those volumes is not a memory exercise for its own sake: it explains why a localiser can be intercepted well off the runway centreline while the glide path only becomes usable much closer in.
The exam also probes what happens when the ILS geometry itself creates a second, false indication, because the glide path aerial radiates more than one lobe. Recognising a false glide path, and knowing the one habit that protects against following it down, is one of the highest-value pieces of ILS knowledge in the whole subject.
Coverage volumes
The localiser signal is usable within 10 degrees either side of the front course line out to about 25 NM, and within a wider 35 degrees either side out to about 17 NM; outside those angle and distance combinations the signal may still be present but is not guaranteed to meet accuracy standards. The glide path, by contrast, is only guaranteed to about 10 NM from the threshold, which is why aircraft are vectored or fly a published procedure to intercept the localiser well before they can expect reliable glide path guidance.
This asymmetry between lateral and vertical coverage is deliberate. Lateral guidance needs to work across a wide capture geometry so that radar vectoring and procedural turns can join the approach from varied directions, while vertical guidance only has to be trustworthy for the relatively short, straight final segment flown once the aircraft is already established on the localiser.
False glide paths
The glide path aerial array radiates the true path together with additional interference lobes above it. The first false glide path commonly appears at about twice the published glide path angle, so a standard 3 degree glide path has a false lobe near 6 degrees. Capturing that false lobe from below feels identical to capturing the real one: the needle centres and the approach looks normal, except it is roughly twice as steep and will arrive low and fast, or simply will not fit a stabilised profile.
The safeguard is procedural rather than something the needle itself can reveal: intercept the glide path from below and check the altitude at that point against the published crossing altitude at the final approach fix or outer marker. If the glide path centres well above that checked altitude, it is a false path, not the true one, and the correct response is to continue level, or go around, rather than follow the needle down.
Back beam and category minima
The localiser transmits behind the antenna as well as ahead of it, producing a back beam that can be flown as a back-course approach where published, though without a matching glide path and with reversed needle sensing unless the equipment corrects for it. Categories I, II and III describe progressively lower decision heights and required visual references, commonly quoted as a CAT I decision height not below 200 ft with a runway visual range not below 550 m, CAT II down to 100 ft with an RVR not below 300 m, and CAT III operations below 100 ft, or with no decision height at all, supported by progressively lower RVR. Exact minima depend on aircraft and operator approval, so treat these as commonly examined reference figures rather than universal constants.
Worked example
Worked example: identifying the false glide path angle
A runway's ILS glide path is published as 3.0 degrees. At approximately what angle above the horizon would a pilot expect to encounter the first false glide path?
- A1.5 degrees
- B3.0 degrees
- C6.0 degrees
- D9.0 degrees
Show the answer and walkthrough
Correct answer: C
- A. This halves the true angle rather than doubling it. False lobes appear above the true glide path, not below it.
- B. This is the true glide path angle itself. A false glide path is a separate, steeper lobe, not a repeat of the correct one.
- C. Correct. The first false glide path commonly appears at about twice the true angle, so 2 times 3.0 degrees gives 6.0 degrees.
- D. This triples the true angle. Higher-order false lobes exist further up, but the first and most commonly examined one sits at about twice the angle, not three times.
Step by step
- Identify the true glide path angle from the stem: 3.0 degrees.
- Recall that false glide paths are additional lobes above the true path, appearing at roughly whole-number multiples of it.
- The first, lowest false lobe is examined at about twice the true angle: 2 x 3.0 = 6.0 degrees.
- Sanity check: 6.0 is clearly double 3.0, and distinct from both the half value and the treble value offered as distractors.
Common mistakes
Assuming a centred, steady glide path needle guarantees the true path
A false glide path centres the needle just as convincingly as the real one. Without an independent altitude check the cockpit display alone cannot distinguish them, and following a false path down leads to an unstable, high-and-fast profile or worse.
Not cross-checking altitude at the outer marker or final approach fix
This is the one procedural defence against a false glide path, and skipping it removes the only evidence that would reveal an early, false capture before the approach becomes unstabilised.
Treating published coverage angles and distances as interchangeable
Exam stems sometimes ask for the angle limit at a given range or the range limit at a given angle; reading the wrong pairing from memory produces a confident but wrong answer.
Related topic guides
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Last reviewed July 2026