One of the most complex stages of flight includes approaching the runway and landing at the end of it. Ensuring you do this properly guarantees the safety of passengers and air crew alike. However, there are some visual limitations that prevent pilots from taking advantage of any external visual cues. Luckily for us, the Instrument Landing System (ILS) aids pilots in achieving safe approaches and descents towards the landing area in any given situation.
An Instrument Landing System is a group of navigation or precision approach assistance to safely guide the aircraft towards the runway. Considered a precision aviation instrument, an ILS is based on radio beams that provide a pilot lateral and vertical guidance during approach and landing stages of flight. Furthermore, it is composed of several essential components, each of which take on specific roles. Generally, these components are categorized in navigation, range, and visual groupings.
The ILS localizer is considered the primary component of the ILS approach as it provides lateral guidance, allowing pilots to align the aircraft with the runway properly. It does this by transmitting two narrow beams: one that goes moderately to the right and one that veers to the left of the centerline of the runway. These beams intersect in that centerline, and this intersection is called the “on LOC” signal or indicator. Through the ILS localizer, the pilots can receive information about the aircraft’s distance from the centerline via airborne equipment situated at the opposite end of the runway threshold.
ILS Glide-Slope for Vertical Guidance
The ILS glide-slope is utilized for vertical guidance and helps the ILS approach system provide information about the altitude for pilots to land the aircraft safely. Similar to the localizer, the glide-slope airborne equipment uses two narrow beams that intersect to create an “on GS” signal. It is important to note that the beams move above and below the standard vertical profile that is established for a safe approach and landing. This is possible because the GS airborne equipment provides a crossing height of about 50 feet at the runway threshold and a standard angle of 3 degrees. The angle may vary according to the airfield since there are different constraints like obstacles and ground formations.
On some ILS systems, there are marker beacons which belong to the category of range assistance as they allow pilots to approximate how far they are from the runway. Marker beacons work by flashing colored lights and morse code tones, both of which are either positioned in the outer marker or middle marker beacon. The outer marker beacon is usually the first beacon located 3 ½ to 6 NM from the threshold within 250 feet of the extended runway centerline so that the pilot can make a positive position fix on the localizer. On the other side, the middle marker beacon is positioned 0.5 to 0.8 NM from the threshold on the extended centerline of the runway, and it crosses the GS at about 200 to 250 feet above the runway elevation.
Distance Measuring Equipment (DME)
The DME is an alternative of navigation assistance that can be added alongside the localizer course, and it helps the pilot learn the distance from the runway by providing slant distance to the aircraft regarding touch-down point.
Approach Lighting System (ALS)
This component belongs to the visual category, and its main purpose is to make the runway as visible as possible, even in low visibility conditions. Other common lighting systems include Sequenced Flashing Light (SFL), Touchdown Zone Lights (TDZ), and Centerline Lights (CLL).
Runway Visual Range (RVR)
The components in this range act as a limit for the pilot to complete the final approach and land. They take the form of altitude levels wherein the minimum visual reference must be available. As such, the RVR depends on the type of precision instrument approach procedure utilized or the category of the ILS signals. In general, there are two altitudes based on the types of approach.
Firstly, there is the Minimum Descent Altitude (MDA), which is utilized for a non-precision approach or an approach that lacks the full ILS with LOC and GS included. Prior to starting the approach to land, the MDA defines the minimum altitude for the pilot to level off and stabilize the aircraft. Secondly, there is the Decision Height (DH) which is utilized in precision approaches using GS. The DH defines the minimum altitude for the pilot to determine if the runway environment is visible enough to carry out the final approach course and landing. When a pilot identifies the visibility below the minimum established in the approach chart, a Missed Approach Point (MAP) is used for the DH in these cases.
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