ILS glidepath, false lobes and signal protection

What height belongs to a three-degree path?

Separate angle from a fixed feet-per-mile rule.

Height as a function of distance

For a straight geometric path, height above the chosen reference is horizontal distance × tan(path angle), plus the reference crossing height if supplied. At 3°, one nautical mile corresponds to about 318 ft of rise. At 5 NM, with an assumed 50 ft threshold crossing height, the ideal threshold-referenced height is about 1,642 ft.

The real glidepath is generated by a ground antenna, and a DME may be offset from the runway threshold. A charted distance therefore cannot automatically be inserted as threshold distance. The laboratory explicitly labels its distance reference and simplified straight-line geometry.

Rate of descent follows groundspeed

Required vertical speed is groundspeed × tan(angle), converted to ft/min. At 120 kt on a 3° path it is about 637 ft/min. A faster groundspeed needs a greater descent rate for the same angle. This links energy management and wind to the vertical path without changing the beam itself.

Resolve a reference mismatch

The synthetic aircraft is 5 NM from the threshold, and the approach DME antenna is assumed to lie 0.8 NM beyond that threshold on the extended centreline. Ignoring slant range for the moment, the DME would read about 5.8 NM. Inserting 5.8 as threshold distance into a 3° calculation would add about 255 ft of fictitious path height.

The correct method begins with the charted distance reference and the actual altitude comparison, rather than treating every distance as interchangeable. A separate slant-range adjustment may also matter, depending on geometry and the task. This exercise is intentionally simple: operational altitude/distance checks come from the published procedure and approved aircraft information. The calculation teaches why reference labels belong beside a number, not in an easily missed footnote.

Why is capture from the correct position important?

A centred vertical pointer is meaningful only within valid approach geometry.

False glidepath indications

The radiation pattern can contain additional lobes above the intended glidepath. Depending on the signal geometry and receiver response, these can create misleading guidance. Intercept from the published position and altitude, normally from below the intended path, using the approved procedure. Do not search upward for any centred indication.

The localizer and glidepath are distinct components. One can be valid while the other is flagged or unavailable. A localizer-only procedure, if available and authorised, has its own minima and profile; loss of vertical guidance does not authorise continuing on ILS assumptions.

Compare independent cues

Check the indicated path against charted altitude/distance, the identified approach and the aircraft’s position. Protect critical areas according to the aerodrome procedures; nearby objects can distort guidance. The illustration offers a false-lobe explanation only and is not a depiction of any facility’s actual lobe pattern.

Challenge a plausible but steep path

At 5 NM from the threshold, a synthetic 3° path with a 50 ft crossing height is about 1,642 ft above threshold elevation. A 6° straight line would instead be about 3,243 ft. A pointer that seems centred at the much higher value deserves immediate challenge against the procedure. The lesson does not claim that every false lobe lies at 6° or has a usable capture behaviour.

The independent altitude comparison is useful because it tests the guidance with another measurement and a published reference. It must still use the correct altitude datum, pressure setting and distance source. A wrong QNH or an offset DME could undermine that check. Build the check from defined references, and follow the approved response if the approach cannot be supported by valid, consistent guidance.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.