The instrument landing system has guided aircraft onto runways in poor weather for the best part of a century, and it is still the benchmark precision approach. Under the jargon it is a simple idea: two radio beams, one for left and right, one for up and down, crossed in space to mark a single path down to the runway. Once you see it that way, the rest is detail.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
Two beams that make a path
An ILS is a precision approach because it gives guidance in both the lateral and the vertical plane, and it does so with two separate transmitters described in ICAO Annex 10.
The localiser provides lateral guidance. It is a VHF transmitter at the far end of the runway, radiating a beam aligned with the runway centreline, and your instrument shows whether you are left or right of that extended centreline. Fly to centre the needle and you track straight at the runway.
The glideslope (or glide path) provides vertical guidance. It is a UHF transmitter beside the touchdown zone, radiating a beam angled up from the ground, typically at about 3 degrees, and your instrument shows whether you are above or below that descent path. Fly to centre this needle and you descend at the correct angle.
Cross the two and you have a single line in space, the right side-to-side position and the right height at every point, that leads down to the runway. The instrument that displays both, needles crossing at the centre when you are on the path, is the heart of flying an ILS.
The markers and the distance cues
Older ILS installations added marker beacons, low-power transmitters on the approach that flashed a light and sounded a tone as you passed over them, to tell you how far along the approach you were:
- the outer marker, several miles out, near where the glideslope is intercepted and the final descent begins;
- the middle marker, close to the CAT I decision height;
- and, for lower categories, an inner marker nearer the threshold.
Many installations have replaced the markers with DME distance or named fixes, which give a continuous distance rather than a single beep, but the idea is the same: a way to cross-check your progress and your height against distance as you come down the beam. That cross-check is also your defence against following a false glideslope signal, by confirming you are at the right altitude for the distance.
The categories: how low you can go
Not every ILS, and not every aircraft or crew, can be used down to the same minima. The categories describe how low the decision height and how poor the runway visual range (RVR) may be, and they sit in ICAO Doc 8168 and the operating rules:
- Category I (CAT I): a decision height no lower than 200 ft, and an RVR no less than about 550 m (or a prescribed visibility). This is the standard ILS most pilots fly.
- Category II (CAT II): a decision height below 200 ft but not below 100 ft, and an RVR no less than 300 m. It needs more capable ground equipment, a radio altimeter and specific crew and aircraft approval.
- Category III (CAT III): lower again, subdivided by decision height and RVR. CAT IIIA reaches a decision height below 100 ft with RVR not less than 200 m (the long-standing operational value; current ICAO gives 175 m); CAT IIIB goes to below 50 ft or no decision height with RVR down to 50 m; CAT IIIC has no decision height and no RVR limit. These demand the most capable systems, including autoland, and full approvals.
The headline is that lower categories are not just about the weather; they require progressively more capable equipment on the ground, in the aircraft, and in the crew's training. The category is the combination, not the cloud base alone.
When part of it fails
The ILS degrades gracefully, which is part of why it has lasted. If the glideslope fails or is unavailable, you keep the localiser, so the approach becomes a localiser-only approach, a non-precision approach flown to a minimum descent altitude rather than a decision height. You stay aligned with the runway by the localiser and descend by reference to altitudes and distances instead of riding the beam down. If the localiser too is lost, the ILS is no longer usable and another procedure is needed. Knowing which guidance you still have, and therefore which minima apply, is a basic part of flying the approach.
Intercepting and monitoring the beam
How you join the ILS matters as much as the beams themselves. The standard sequence is to intercept the localiser first, in level flight, and then capture the glideslope from below as it descends to meet you. Intercepting the glideslope from above is discouraged, because the transmitter produces weaker false glideslope signals at steeper angles, and capturing one of those would send you down far too fast. Joining from below, at the published altitude, avoids the trap.
Two habits guard the approach. The first is to identify the facility by its Morse-code ident before relying on it, confirming you have tuned the right ILS and that it is serviceable. The second is a gross-error check: as you pass the final approach fix or outer marker, your altimeter should read the published glideslope-crossing height, and if it does not, the glideslope picture is suspect and you should not follow it down. Even with the autopilot flying the approach, you keep monitoring the raw localiser and glideslope needles, because the displays are the truth and the automation is only as good as the signal it is tracking.
A modern relative
The ILS is no longer the only way to fly a guided, ILS-like descent. Satellite-based approaches, the LPV approaches flown with augmented GNSS, and GLS approaches flown with a ground-based augmentation system, present the pilot with the same picture, a lateral track and a glidepath to follow down to a decision altitude, without any localiser or glideslope antenna on the airfield. They are not an ILS, and their minima and equipment requirements are their own, but they are worth knowing as the family the ILS now belongs to: the precise, guided approach is increasingly delivered by satellites as well as by the two classic beams.
A worked example
You are vectored toward the final approach for an ILS. First you intercept the localiser: the lateral needle comes alive and you turn to centre it, now tracking the extended runway centreline. You hold your altitude until the glideslope needle, which has been pinned at the top, begins to move down toward the centre. As it reaches the centre you start your descent to keep both needles crossed, settling onto the roughly 3-degree path, and you cross-check your height against the DME or the marker as you pass it: at the right distance you should be at the right altitude.
You continue down the beam, both needles centred, to the CAT I decision height of 200 ft. There you make the decision the whole approach has been building to, the subject of our guide to decision altitude and the MDA: if you have the required visual reference and the landing is assured, you land; if not, you fly the missed approach.
Now replay the approach with a glideslope failure as you intercept. The vertical needle flags as unreliable, so you switch to the localiser-only minima: you keep the localiser centred for alignment, but you now descend by steps to a minimum descent altitude, higher than the 200 ft decision height, and look for the runway from there. The lateral picture is unchanged; the vertical picture, and the minimum, are not.
Common pitfalls
- Treating the ILS as one thing. It is two independent beams; you can have one without the other, and the minima change accordingly.
- Forgetting the distance cross-check. Confirming altitude against distance guards against following a false glideslope.
- Thinking the category is only about weather. Lower categories require more capable ground equipment, aircraft and crew approval, not just a lower cloud base.
- Confusing a localiser-only approach with a full ILS. Without the glideslope it is a non-precision approach to an MDA, not a decision height.
- Ignoring which minima apply. The decision height or MDA depends on the category and on what guidance you actually have.
In Pilot EFB
Pilot EFB is a study and planning companion for instrument procedures, helping you learn how an ILS is built and where its minima come from alongside the approach chart and the rest of your offline-first briefing. It does not fly the approach, display live guidance, or set your minima, and the procedure you fly comes from the current chart and your approved minima. Pilot EFB is not a certified Electronic Flight Bag, so treat it as a study and planning aid and fly the approach from your official source of record.