What Is an ILS?

The Instrument Landing System (ILS) is the most widely used precision approach system in the world. It allows pilots to land in conditions where they cannot see the runway until the very last moments — thick cloud, heavy rain, fog, or darkness. Both runway ends at Prestwick (Runway 12 and Runway 30) are equipped with ILS, operating on a frequency of 110.30 MHz.

The Problem ILS Solves

Imagine you are flying an aircraft towards Prestwick at 3,000 feet. It is a winter evening, and the cloud base is at 300 feet — meaning you cannot see the ground until you descend below 300 feet. The runway is 46 metres wide and you need to be lined up with it precisely. You also need to be descending at exactly the right angle so that you arrive at the runway threshold at the correct height and speed.

Without some form of guidance, this would be impossible. The ILS provides that guidance through two radio beams that the aircraft's instruments can detect and display to the pilot.

The Two Components

An ILS consists of two separate transmitters, each providing one dimension of guidance:

The Localiser — Left and Right

The localiser is a transmitter located at the far end of the runway (the end you are landing towards, but beyond the runway). It transmits a signal that tells the aircraft whether it is to the left or right of the runway centreline.

In the cockpit, this is displayed on an instrument as a vertical needle (or bar). If the needle is to the left of centre, the runway is to the left — the pilot needs to steer left. If it is to the right, steer right. When the needle is centred, the aircraft is perfectly aligned with the runway.

The localiser signal is very narrow and precise. At the runway threshold, the beam is only a few metres wide, giving extremely accurate lateral guidance.

The Glideslope — Up and Down

The glideslope (sometimes called glide path) transmitter is located beside the runway, near the touchdown zone. It transmits a signal that tells the aircraft whether it is above or below the correct descent path.

The correct descent angle is typically 3 degrees — a gentle, steady descent. In the cockpit, the glideslope is shown as a horizontal needle. If it is above centre, the aircraft is below the correct path and needs to climb or reduce its rate of descent. If it is below centre, the aircraft is too high.

When both needles are centred — the vertical localiser needle and the horizontal glideslope needle — the aircraft is exactly on the correct path: lined up with the runway and descending at the right angle.

Flying the ILS — Step by Step

Here is how a pilot flies an ILS approach at Prestwick:

1. Tuning the ILS

The pilot (or the aircraft's flight management computer) tunes the ILS receiver to 110.30 MHz. The aircraft identifies the signal by its Morse code identifier — a short series of letters that confirms the pilot is receiving the correct ILS and not a signal from another airport.

2. Intercepting the Localiser

Approach control vectors the aircraft to intercept the localiser beam. The aircraft is typically at about 2,000-3,000 feet and being given headings by ATC. As the aircraft crosses the localiser beam, the cockpit instrument shows the needle moving. The pilot (or autopilot) turns to follow the needle, aligning with the runway centreline.

3. Capturing the Glideslope

Once established on the localiser, the aircraft continues at its current altitude until it intercepts the glideslope beam from below. The glideslope needle starts above the centre and moves down as the aircraft reaches the correct descent point. When the needle centres, the pilot begins descending — following the glideslope down at approximately 3 degrees.

4. Descending on the ILS

Now both needles are centred. The aircraft is lined up with the runway and descending at the correct angle. The pilot monitors the instruments constantly, making small corrections to keep both needles centred. The autopilot can do this automatically on most modern aircraft.

During this phase, the aircraft is descending at roughly 700 feet per minute at a typical approach speed of 140 knots. The runway is still invisible, hidden somewhere in the cloud or murk ahead.

5. Decision Height

At 200 feet above the runway (for CAT I, which is what Prestwick has), the pilot must look up from the instruments and outside the cockpit window. This is called the Decision Height (DH).

• If the pilot can see the runway lights, approach lights, or runway markings — they continue the approach and land
• If they cannot see the runway — they must immediately go around (missed approach). The pilot applies full power, climbs, and follows the published missed approach procedure

There is no negotiation at Decision Height. If you cannot see the runway, you go around. This rule is absolute.

6. Landing

If the runway is visible, the pilot transitions from instruments to visual references for the final 200 feet. They flare the aircraft, touch down in the touchdown zone, apply brakes and (on jets) reverse thrust, and decelerate to taxi speed.

ILS Categories Explained

Not all ILS systems are equal. They are classified into categories based on how low a pilot can fly before needing to see the runway:

RVR stands for Runway Visual Range — the distance a pilot can see down the runway, measured by instruments beside the runway.

Prestwick has CAT I ILS on both runway ends. This means a pilot can descend to 200 feet in cloud and needs at least 550 metres visibility to land. For most weather conditions, this is perfectly adequate. Only the densest fog would prevent a CAT I approach.

How the ILS Looks at Prestwick

Both Runway 12 and Runway 30 have ILS:

• ILS Runway 12 — the localiser antenna is beyond the Runway 12 (southeast) end, and the glideslope transmitter is beside the runway near the touchdown zone. Aircraft approach from the northwest, over the Firth of Clyde
• ILS Runway 30 — the localiser is beyond the Runway 30 (northwest) end, and the glideslope is beside the runway. Aircraft approach from the southeast, over Ayrshire farmland

Both operate on 110.30 MHz, but they are not active simultaneously — only the ILS for the active runway is transmitting at any given time.

Modern Cockpit Displays

In older aircraft, the ILS is displayed on a basic instrument called a CDI (Course Deviation Indicator) — a round dial with two crossing needles. In modern glass cockpit aircraft (including the Boeing 737s that Ryanair operates at Prestwick), the ILS information is integrated into the PFD (Primary Flight Display) — a large screen that combines attitude, speed, altitude, heading, and ILS deviation into one comprehensive picture.

Many modern aircraft can also couple their autopilot to the ILS, allowing a fully automatic approach. The pilot monitors the autopilot's performance and is ready to take over if anything goes wrong, but the aircraft can fly the entire ILS approach hands-off down to Decision Height.

Key Takeaways

• The ILS uses two radio beams — the localiser (left/right) and glideslope (up/down) — to guide aircraft precisely to the runway
• Both Runway 12 and Runway 30 at Prestwick have CAT I ILS on 110.30 MHz
• CAT I allows approaches down to 200 feet above the runway with 550 m visibility
• At Decision Height, the pilot must see the runway or go around
• Modern aircraft can fly the ILS approach automatically, with the pilot monitoring
• The ILS is the primary precision approach system worldwide and the backbone of bad-weather operations