Flying an ILS approach, down to the minimums

How an ILS works: localiser, glide path and distance

The localiser transmitter sits beyond the far end of the runway and marks out the extended centreline. The beam is angular, a few degrees either side, and set up so that full-scale deflection is roughly 350 ft either side of the centreline at the threshold. That makes it easy to find 15 NM out, where full scale is getting on for a mile each side, and very sensitive close in.

The glide path transmitter stands beside the runway near the touchdown zone. The path is normally 3°, crossing the threshold at about 50 ft. A 3° path loses about 318 ft for every nautical mile, which pilots round to 300 ft per NM: 3 NM out you should be about 1000 ft above the runway.

To know how far along the approach you are, most UK ILS procedures use a DME (distance measuring equipment) paired with the ILS, often zero-ranged to the threshold. Older procedures, and many in the US, use marker beacons instead: an outer marker roughly 4–7 NM out and a middle marker near the decision point, each flashing a light and sounding a tone overhead. The chart gives a table of altitude against distance so you can check you are on the right path.

Reading the needles: CDI, HSI and the glide path

On a round-dial panel the localiser shows on the CDI (course deviation indicator) or the HSI (horizontal situation indicator) as a vertical bar; on a glass cockpit it is the deviation bar on the PFD (primary flight display). The bar shows where the centreline is: bar left, the runway is to your left, so turn towards it. The glide path is a separate pointer: a horizontal needle or pointer on the HSI, or a diamond beside the altitude tape on a PFD. Pointer below centre means the path is below you, so you are high.

On an HSI or a PFD each scale has two dots either side, so one dot is half scale; a round-dial ILS indicator usually has five, so half scale there is two and a half dots. Full scale on the glide path is only about 0.7° either side of a 3° path, which near the runway is a few tens of feet. Because both beams narrow as you get closer, a correction that was right at 6 NM is far too big at 1 NM. See Moving from round dials to a glass cockpit for how the same information is laid out on each panel.

Try it below: put the aircraft anywhere on the approach, point it any way you like, and compare what each instrument shows.

Capturing the localiser, then the glide path

Radar will usually vector you onto the localiser with a 30° intercept, at an altitude that puts you below the glide path. Joining procedurally, you arrive from a hold, a racetrack or a DME arc and the chart gives the altitudes.

1. Before the approach: tune and identify the ILS by its Morse ident, set the inbound course, set QNH, and brief the DA, the missed approach and the RVR (runway visual range) you need.
2. Hold the intercept heading. When the localiser bar comes alive and starts to move, begin the turn so that you roll out on the centreline without overshooting.
3. Once established, tell ATC and track it with heading changes of about 5°, finding the heading that keeps the bar still.
4. Stay level at the platform altitude, on speed. As the glide path pointer moves down to centre, lower the nose to the descent attitude and reduce power in the same movement.
5. Set the missed approach altitude, check altitude against the DME table at the first check point, and complete the final checks.

Capture the glide path from below. The ILS also radiates false glide paths above the true one, at about two and three times its angle (6° and 9° for a 3° path), some of them with the sense reversed, and they can give convincing but wrong indications. Coming at the path from above also means chasing it with a high rate of descent, which is the start of an unstable approach.

Try it: Fly the “ILS from radar vectors, 8 NM final” lesson on the glass cockpit

Flying a stable approach: power, attitude and small corrections

Attitude holds the path, power holds the speed. Set the attitude and power that give the right rate of descent, then make small, early corrections to both. A half-dot error on the glide path wants a pitch change of a degree or so, not a dive. Chasing the needles makes them swing.

By 1000 ft above the runway in cloud the approach should be stable: on the localiser and glide path within a dot, on speed, in the landing configuration, with a steady rate of descent of no more than about 1000 ft/min. If it is not, go around and set it up again. Airlines write this into their procedures; flying single-pilot it is simply good discipline.

Decision altitude, CAT I minima and the approach ban

The decision altitude (DA) is read on the altimeter against QNH; the decision height (DH) is the same point measured above the threshold. Set the altimeter wrongly and both are wrong (see altimeter settings). For a Category I ILS the DH is never below 200 ft, and it is higher where the chart’s obstacle clearance height demands it. At Jersey the ILS obstacle clearance height for a light aircraft is under 200 ft, so the DH is 200 ft and the DA about 470 ft.

The runway visual range you need depends on the DH and on how much approach lighting the runway has. In the UK and EASA table, a 200 ft DH needs 550 m with a full approach lighting system, 750 m with intermediate lighting, 1000 m with basic lighting and 1200 m with none. The real runways in the course show the difference: Jersey runway 26, with over 900 m of approach lights, needs 550 m; Exeter runway 08, with about 330 m, needs 1000 m; Jersey runway 08, with only 165 m of lights, is treated as basic (the AIP lets operators do so), so it needs 1000 m.

In the UK you may start an approach in any weather. On a private flight (Part-NCO), if the controlling RVR is below 550 m you may not continue below 1000 ft above the aerodrome. This is the approach ban. Before 30 October 2025 the ban for private flights was set by the approach’s own minimum RVR, and for commercial flights it still is. If the RVR drops after you have passed 1000 ft, you may carry on to the DA. Jersey is not the UK and has its own rule: under Jersey law the ban is still the approach’s own minimum RVR, so at Jersey runway 08 an RVR below 1200 m bans the approach. US rules for private flights have no approach ban, but you still need the published visibility to descend below the DA and land.

At the DA: visual references, the go-around and the missed approach

Continue below the DA only if you can see enough of the runway environment to judge your position and fly a normal landing: parts of the approach lighting, the threshold with its markings or lights, the PAPI (precision approach path indicator), the touchdown zone or its lights, or the runway edge lights. If you can see none of them at the DA, go around. There is no waiting to see whether it improves.

The go-around is power, attitude, climb: full power, pitch up to the climb attitude, clean up as the flight manual says, and follow the missed approach on the chart. Tell ATC you are going around once the aircraft is climbing. Fly the published missed approach track and altitude unless ATC give you something else.

When the glide path fails: the localiser-only approach

If the glide path is out of service you can still fly the localiser for track. The approach becomes a non-precision approach with a higher minimum descent altitude (MDA) and, usually, more RVR. Fly it as a CDFA (continuous descent final approach): start down at the final approach fix at the rate for your groundspeed, check altitude against the DME table every mile, and treat the MDA as a decision altitude. Without the required visual references, go around there, or at the missed approach point if you reach it first. The older way, levelling off at the MDA and flying on to the missed approach point (“dive and drive”), is where many controlled-flight-into-terrain accidents happen, and UK and European rules now require commercial aeroplane operators to use CDFA, with few exceptions. At Jersey runway 08 the localiser-only MDA is 590 ft, about 120 ft above the ILS DA, and the RVR needed rises to 1400 m.

Try it: Fly the “Localiser only: glide path unserviceable (Jersey RWY 08)” lesson on steam gauges

The same module has two more real approaches: the Jersey runway 26 ILS joined from the hold and the Exeter runway 08 ILS joined from a DME arc.

What the IR skill test looks for

The tolerances

• Localiser and glide path: no more than half-scale deflection (one dot on an HSI or PFD).
• Altitude at the final approach point: within 100 ft.
• Airspeed: within 5 kt once the descent is established.
• The go-around at the DA: nothing below it, no more than 50 ft above it.
• No continuing below the DA without the required visual references.

Practising against them

The course lessons fly the approach on the autopilot in approach mode while you handle the set-up, the power, the missed approach altitude and the decision. Each starts with a weather check so you can say “weather checked, no approach ban”, and pauses at the DA with a line saying what the cloud base and visibility let you see. Afterwards a debrief marks the approach against the tolerances above, and adds the rate of descent below 1000 ft. Switch the lesson from Guided to Examined to fly it as the test, without help.