Sachin Walls
The localizer is a fundamental component of the Instrument Landing System (ILS) that provides lateral guidance to pilots during an approach to land. It helps pilots align their aircraft with the runway centerline by transmitting radio signals. The localizer is used to line up precisely with the runway, and the pilot attempts to manoeuvre the aircraft to keep the indicators centered while approaching the runway. It is a critical tool for landing in low-visibility conditions.
| Characteristics | Values |
|---|---|
| Definition | A localizer is a component of an Instrument Landing System (ILS) that provides lateral (horizontal) guidance to pilots during an approach to land. |
| Function | It helps in aligning the aircraft with the centerline of the runway. |
| Use case | The localizer is used to line up precisely with the runway. |
| Signal | The localizer is an ILS radio signal directed down a runway. |
| Signal frequency | The localizer signal is in the 110 MHz range, with frequencies between 108.10 MHz and 111.95 MHz. |
| Signal identification | The localizer provides for ILS facility identification by transmitting a 1,020 Hz Morse code identification signal, always starting with the Morse code letter "I". |
| Indication on cockpit instruments | In modern cockpits, the localizer is seen as a colored dot (usually a diamond) at the bottom of the artificial horizon. In older cockpits, it appears as an arrow in the gyro compass below the artificial horizon. |
| Accuracy | The localizer is only guaranteed to be accurate up to 10 degrees on either side of the runway up to 18NM. |
| Safety | The localizer enhances safety by providing precise alignment with the runway centerline. |
| Operational efficiency | Efficient use of the localizer streamlines approach procedures, minimizes airspace congestion, and enhances airport capacity. |
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What You'll Learn
Localizer as a component of the Instrument Landing System (ILS)
The Instrument Landing System (ILS) is a radio navigation system that provides precision guidance to aircraft approaching a runway. It has been used for nearly 100 years and is one of the few instrument approaches that can guide aircraft to the runway in near-zero visibility and low cloud ceilings.
The ILS relies on multiple ground-based and aircraft equipment, with the localizer being the primary component. The localizer is the lateral component of the ILS and provides horizontal guidance to help guide aircraft along the axis of the runway. It is used to line up precisely with the runway and keep the aircraft on course. The localizer antenna is positioned at the far end of the runway and transmits signals on odd-tenth frequencies between 108.1 MHz and 111.95 MHz.
The localizer works in conjunction with the glideslope, which operates on a vertical plane. The glideslope transmits two signals, one at 90 Hz and the other at 150 Hz, which are interpreted by the ILS receiver. These signals are directionally transmitted above and below the desired approach path, and the aircraft's GS equipment interprets the signals' amplitude to determine its vertical location above or below the path.
The localizer and glideslope indicators are displayed on the aircraft's instruments, often as additional pointers in the attitude indicator. Pilots attempt to manoeuvre the aircraft to keep these indicators centred while approaching the runway. The localizer and glideslope signals become more sensitive as the aircraft gets closer to the runway, as the course width of both decreases.
The ILS also includes an Approach Light System (ALS), which consists of a pattern of lights installed at the runway's approach end. This helps pilots transition from instrument-guided flight to a visual approach to landing, aiding in identifying the runway's centreline.
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How localizers guide aircraft to the runway
An Instrument Landing System (ILS) provides precision guidance to aircraft approaching a runway. The localizer is a critical component of the ILS, responsible for horizontal guidance. It ensures that the aircraft is aligned with the runway's centreline during landing.
The localizer is a radio signal directed down the runway, allowing the aircraft to fly along it and align with the runway. The localizer antenna is positioned at the far end of the runway and transmits signals in the frequency range of 108.1 MHz to 111.95 MHz (odd tenths only). The transmitted localizer beam is typically directed in the heading of the runway extension, ensuring the aircraft is on the correct path.
As the aircraft approaches the runway, the localizer and glideslope signals become more sensitive, with the course width decreasing. This requires small corrections from the pilot to stay on course. The localizer is usually represented on flight instruments as a green line or triangle, with the glideslope as a green diamond or triangle.
In the cockpit, the localizer is displayed as a coloured dot, usually in the shape of a diamond, at the bottom of the artificial horizon. If the aircraft is located on the intended glide path, the dot will appear in the middle of the scale. If the aircraft deviates from the beam, the dot will move accordingly, indicating to the pilot that they need to adjust their heading.
The localizer is an essential tool for guiding aircraft to the runway, especially in low-visibility conditions. It has been in use for nearly 100 years and continues to be a vital component of aviation navigation systems.
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Limitations of localizers
An instrument landing system localizer, or simply localizer (LOC, or LLZ prior to 2007), is a system of horizontal guidance in the instrument landing system (ILS) that guides aircraft along the axis of the runway. It is the lateral component of the ILS for the runway centreline when combined with the vertical glide path. The localizer provides horizontal guidance, while the glideslope provides vertical guidance.
While localizers are useful tools for aircraft landing, they do have some limitations. Firstly, they require both a transmitting airport runway system and receiving cockpit instruments. Older aircraft without an ILS receiver cannot utilize ILS facilities, and modern aircraft cannot use their ILS instruments at runways lacking ILS facilities. This is particularly relevant in parts of Africa and Asia, where large airports may lack transmitting ILS systems.
Another limitation is that localizers are susceptible to hazardous reflections that can affect the radiated signal. To mitigate this, ILS critical and sensitive areas are established, but this can result in delays and increased separation between aircraft. Additionally, localizers may not always be directed in the heading of the runway extension, as seen in some exceptions like Innsbruck, Austria, and Macao.
Furthermore, the interpretation of localizer signals in the cockpit has evolved over time. In older cockpits, the localizer appeared as an arrow in the gyro compass below the artificial horizon, with the top and bottom of the arrow indicating the current heading. However, modern cockpits present the localizer as a coloured dot, usually in the shape of a diamond, during the descent and approach to the selected runway. This dot may not always appear in the centre of the scale, and pilots need to adjust their heading accordingly.
While localizers aid in horizontal guidance, they do not provide vertical guidance, which is the role of the glideslope. Localizers work in conjunction with glideslopes to ensure precise alignment with the runway.
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The history of localizers
The localizer is a system of horizontal guidance in the instrument landing system (ILS), which is used to guide aircraft along the axis of the runway. In aviation, a localizer is the lateral component of the ILS for the runway centreline when combined with the vertical glide path. The ILS is a radio navigation system that provides precision guidance to aircraft approaching a runway.
The history of the localizer can be traced back to the development of the ILS in the early 20th century. The first tests of the ILS were conducted in 1929 in the United States, with Jimmy Doolittle becoming the first pilot to take off, fly and land an airplane using instruments alone, without a view outside the cockpit. A basic system, fully operative, was introduced in 1932 at Berlin-Tempelhof Central Airport (Germany) named LFF or "Lorenz beam" after its inventor, the C. Lorenz AG company. The Civil Aeronautics Board (CAB) of the United States authorized the installation of the system in 1941 at six locations.
Over time, the localizer has become an integral part of the ILS. It is used to provide horizontal guidance to aircraft during the approach and landing phases of flight. The localizer transmits signals on frequencies ranging from 108.1 MHz to 111.95 MHz, with odd-tenth values. These signals are received by the aircraft's cockpit instruments, which interpret the signals to determine the aircraft's position relative to the runway centreline.
In modern cockpits, the localizer is typically represented as a coloured dot, usually in the shape of a diamond, at the bottom of the artificial horizon. This dot appears during the descent and approach phases of flight, provided that the navigation radio is set to the ILS frequency of the specific runway. If the aircraft is located on the centreline of the runway, the localizer dot will appear in the middle of the scale. If the aircraft deviates from the centreline, the dot will move accordingly, indicating the necessary adjustments to the heading.
The localizer has undergone several iterations and improvements since its inception. Older cockpits may feature a localizer scale below the artificial horizon or a localizer indicator shown below the Attitude Indicator. The very first generation of localizer gauges had a different cockpit interface and was not included in the artificial horizon or any compass but was a gauge of its own. Today, the localizer is an essential tool for pilots, providing critical guidance and navigation assistance during low-visibility approaches and landings.
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Localizers and glide slope
The Instrument Landing System (ILS) is a system that supports visual landings and enables safe landings in conditions where visual approaches are impossible. It uses two directional radio signals: the localizer and the glideslope.
The localizer provides lateral or horizontal guidance, allowing pilots to align with the runway. It operates at a frequency of 108 to 112 MHz and is more sensitive than VORs, with full-scale deflection at 2.5°. The localizer is the lateral portion of an instrument approach. Pilots must line up with the centre of the runway to receive both a 90 Hz and 150 Hz signal simultaneously. If a pilot is too far to the left, they will receive only the 90 Hz signal, and vice versa if they are too far to the right.
The glideslope provides vertical guidance for the correct descent angle. It operates at a frequency of 329.15 to 335 MHz and requires an Ultra High Frequency (UHF) receiver. The glideslope advises pilots when they are off course vertically. If a pilot is on the glideslope, they will receive both a 90 Hz and a 150 Hz signal simultaneously, which will cancel each other out. If a pilot is flying above the glideslope, they will receive only the 90 Hz signal. Flying below the glideslope is indicated by receiving only the 150 Hz signal. The standard glide slope angle is 3°, but this may vary depending on the terrain near the runway.
The localizer and glideslope work together to provide navigational guidance that pilots need to land safely. If the localizer is inoperative, the airfield is not authorized for ILS landings. However, an inoperative glideslope transmitter still allows pilots to fly a non-precision localizer instrument approach.
To use the ILS, pilots must tune to the ILS frequency and verify its Morse code identifier. They must then intercept the localizer within 30° of the published course to avoid false signals and intercept the glideslope from below at the specified altitude, ensuring a smooth descent path. Pilots must follow the ILS approach path indicated by the localizer and descend along the glide path to the decision height, which is typically 200 feet. At this height, the pilot must have an adequate visual reference to the landing environment to decide whether to continue the descent or execute a missed approach.
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Frequently asked questions
A localizer is a component of an Instrument Landing System (ILS) that provides lateral (horizontal) guidance to pilots during an approach to land.
Localizers emit radio signals along a specific azimuth, creating a signal corridor known as the "localizer course". Pilots intercept and track this course using onboard navigation receivers, allowing for precise lateral positioning relative to the runway centerline.
Pilots engage the localizer function on their cockpit instrumentation to receive lateral guidance information during instrument approaches. They interpret course deviation indications and make necessary flight control inputs to maintain alignment with the runway centerline.
The localizer is a critical component of the Instrument Landing System, offering essential lateral guidance to aircraft during instrument approaches and landings. By providing precise alignment with the runway centerline, the localizer enhances safety, facilitates operational efficiency, and enables aircraft to conduct precision landings in diverse weather conditions.
No, localizers are used to guide aircraft along the axis of the runway during landing, not for locating an airport. However, the Instrument Landing System (ILS) as a whole can be used for airport identification by periodically transmitting a 1,020 Hz Morse code identification signal, that always starts with the Morse Code letter "I".
