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Learning hub · Radio Navigation · Lesson 3

Instrument approaches: ILS and GNSS

As a student pilot you will fly by visual references, but instrument approaches shape the airspace and the traffic around you, and they are an escape route you may meet in training. This lesson explains in plain terms what an ILS gives, how GNSS approaches work and what the minima mean.

About 12 minutes, plus the quiz. Part of Radio Navigation. It builds on VOR, NDB, DME and GNSS, which explains the basic aids.

1. Why a VFR pilot should know this

An instrument approach is a published procedure that lets a pilot descend to a low height in cloud and then land, using instruments only until a certain point. You are not learning to fly one here: that needs an instrument rating, practice and a suitable aeroplane. There are still good reasons to know the basics.

  • At many aerodromes, aeroplanes on instrument approaches arrive from a long straight line. You will need to stay out of their way and understand the radio calls you hear.
  • The ILS has protected areas on the ground. At some aerodromes you may be told to hold short of the runway at a point further away in poor weather.
  • Instrument flying is the logical next step for many pilots, and it makes the principle of a stable, planned approach easier to understand.
  • Weather can close in. Knowing what an approach offers helps you decide early to divert, and helps you understand an instructor or controller who offers help.

None of this lets a VFR pilot fly into cloud. See illusions and spatial disorientation.

2. What an ILS gives you

The instrument landing system (ILS) gives guidance in two directions. It has three ground parts and the pilot flies by two needles.

  • The localiser is a VHF transmitter (in the 108 to 112 MHz band, the same band as VOR) placed beyond the far end of the runway. It gives lateral guidance: left or right of the extended centre line. The needle is much more sensitive than a VOR needle, so make small corrections.
  • The glide path (often called the glideslope) is a UHF transmitter beside the runway near the touchdown zone. It gives vertical guidance along a descent path, normally about 3° to the horizontal.
  • Distance along the approach comes from DME paired with the ILS, from marker beacons (older installations, which pass over the aeroplane as a light and a tone) or, today, often from GNSS.

You tune the localiser frequency (the glide path is paired automatically), identify the Morse code, and fly the two needles to keep them centred: “fly the needle towards the centre” with a small correction. A 3° path means about 318 ft of height per nautical mile (tan 3° × 6,076 ft). That means 3 NM from the threshold the path is about 950 ft above it. A groundspeed of 90 kt gives a rate of descent near 480 ft per minute.

The ILS also has categories, which say how low it may be used. At a high level: Category I allows the lowest point (the decision height) to be no lower than 200 ft; Category II and Category III allow lower minima, down to no decision height at all for the best systems. They need special ground equipment, an aeroplane and a crew that are all approved for that category. For you as a student, the point is that the weather limit is set by the equipment, crew and procedure together. Check the AIP for any particular runway.

3. GNSS approaches

Satellite navigation (see the earlier lesson) can also guide an approach. In Europe, these are mainly RNP APCH procedures, and the chart shows the lines of minima you can use with your equipment:

  • LNAV: lateral guidance only. The descent is stepped or flown to a minimum altitude, as in a classic non-precision approach.
  • LNAV/VNAV: lateral guidance plus a vertical path, usually calculated from a barometric altimeter or satellite data.
  • LPV: lateral and vertical guidance with a performance close to an ILS. It needs a satellite-based augmentation system (SBAS), which in Europe is EGNOS, and a receiver approved for the procedure. Its minima can be as low as 200 ft above the runway.

The plus side is that GNSS approaches do not need a ground transmitter at every runway, so many smaller aerodromes have one. The downsides are the ones you know from the earlier lesson: signals can be lost, jammed or spoofed, and the aeroplane’s receiver must be approved and the database current. A handheld GPS or an EFB, including VectorEFB, is not an approved approach navigator.

4. Minima: DA and MDA

Every approach has a lowest point to which the pilot may descend using instruments. If the pilot cannot see the required visual references there, they must not continue below it. There are two kinds.

  • Decision altitude (DA) (or decision height, DH), used for approaches with a vertical path such as an ILS or LPV. At the DA you must decide: if you have the required visual references, continue; if not, start the missed approach right away. Because you are descending on a path, you may dip slightly below the DA in the go-around and that is allowed for in the design.
  • Minimum descent altitude (MDA) (or height, MDH), used for approaches without a vertical path, such as a LNAV or VOR approach. You descend to the MDA and then level off and continue to the missed approach point. If you have the visual references, you may descend to land; if not, you go around at the missed approach point. You must not go below the MDA without seeing the runway.

“Altitude” is measured above mean sea level (QNH), and “height” above the aerodrome or threshold. The visibility (or runway visual range) needed is on the chart too. The missed approach is always part of the plan, and a good pilot briefs it first.

This lesson is general background for student pilots. It is not instrument rating training and does not make anyone current or approved to fly an approach. Each procedure, minimum, and category comes from official charts, the AIP and the flight manual. VectorEFB is a training and planning aid and is not certified as navigation equipment.

5. Try it yourself

Work each one out on paper first, then open the answer to compare. The numbers are invented.

  1. An invented aeroplane flies a 3° ILS glide path. Use about 318 ft per NM. How high above the threshold should it be at 4 NM, and what rate of descent keeps it on the path at a groundspeed of 100 kt?

    Show the answer
    1. Height at 4 NM = 4 × 318 = about 1,270 ft above the threshold.
    2. At 100 kt the aeroplane covers 100 ÷ 60 = 1.67 NM per minute.
    3. Rate of descent = 1.67 × 318 = about 530 ft per minute.
    4. Check: a quick rule is groundspeed × 5, which gives 500 ft per minute. Close, so the numbers agree. Remember that a stronger headwind lowers the groundspeed and so lowers the rate needed.

  2. An invented ILS has a decision altitude of 420 ft and the threshold elevation is 220 ft. What is the decision height? You reach the DA and see only cloud. What do you do?

    Show the answer
    1. Decision height = DA − threshold elevation = 420 − 220 = 200 ft.
    2. At the DA the rule is to decide. Without the required visual references, you begin the missed approach straight away.
    3. You do not wait for a better view, and you do not carry on in the hope of a gap.

  3. An invented non-precision approach has an MDA of 600 ft and the aeroplane levels off at the MDA. At the missed approach point the runway is still not in sight. What do you do, and how is this different from a DA?

    Show the answer
    1. You cannot see the runway at the missed approach point, so you go around and fly the missed approach procedure.
    2. With an MDA you descend first, level off and then continue to a fixed point. The decision is taken at the missed approach point, not at a height.
    3. With a DA you decide as you reach a height while still descending on a path.
    4. In both cases, if you cannot see the runway, you do not continue below the minimum.

6. Watch

A video on the localiser and glideslope, which goes with section 2. Check what it says against your course material; the channel may follow American practice, which differs in places from European (ICAO/EASA) practice.

Open on YouTube if the player does not load. Not made by VectorEFB; check what you learn against your flying school’s material and your instructor.

More to watch

A video on the GPS approach lines of minima, which goes with section 3. This video is American, so wording and some rules differ from European (ICAO/EASA) practice. It uses the American WAAS system, which is the American counterpart of Europe’s EGNOS.

Open on YouTube if the player does not load. Not made by VectorEFB.

7. Check yourself

Five original practice questions. Answer them, press Check answers and read each explanation.

1. The localiser of an ILS gives:

The localiser gives left and right guidance. The glide path gives vertical guidance.

2. A normal ILS glide path angle is about:

An ILS glide path is normally about 3° to the horizontal, which is roughly 318 ft of height for each nautical mile.

3. At the decision altitude (DA) without the required visual references, you:

The DA is a point where you decide. If the references are not there, you go around.

4. An approach with only lateral guidance (such as LNAV) normally has:

Approaches without a vertical path use an MDA: you level at it until the missed approach point.

5. Which statement about an LPV approach is correct?

LPV uses GNSS with SBAS (EGNOS in Europe) and requires an approved receiver. A phone or an EFB app is not approved navigation equipment.

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For training and situational awareness only. Not certified for navigation. Always use official charts, the AIP, NOTAMs and a proper weather briefing for real flights.