Instrument Landing System (ILS)

The basic navigation aid for the pilots for safe landing because in civil aircrafts there are approx. 200 lives are on stake of death per aircraft, so if any error occurs during landing approach or in landing can cause a hilarious danger with the loss of lives in along with the economical loses of aircraft and airport. In modern aviation industry pilots are trained to take full benefit of ILS facility on the airport as well as Manual Landing after the crash of Air France 447 330-230air bus, and it is made a standard by ICAO for declaring international airport should have the complete facility of ILS. ILS system comprised of the instruments made necessary by the ICAO:

•  Localizer
•  Glide Slope
• Marker beacons
•  Precision Approach Path Indicator (PAPI)

Localizer (LOC):

LOC is an instrument comprised of antenna array placed on the other end of runway which provide guidance in horizontal plane to the aircraft onboard.

PCAA have a frequency band allocated 108MHz to 112MHz which is quite a narrow band, however the frequencies of the localizer are placed on odd decimals. The signal transmitted by the localizer consists of two vertical balloon-shaped patterns that overlap, at the center. They are aligned with the extended centerline of the runway.

§  The right side of this pattern, as seen by an approaching aircraft, is modulated at 150 Hz and is called the "blue" area.

§  The left side of the pattern is modulated at 90 Hz and is called the "yellow" area.

Functioning of LOC is to help in setting up the runway center line guidance to relate aircraft’s virtual center line, resulting aircraft will not deviate from the actual runway center line.

Glideslope:

Glideslope provides vertical guidance to the pilot during the approach. Glide slope consist of a ground-based UHF radio transmitter and antenna system which actually give 2.8 degree angle information of aircraft relative to the runway while landing. Like the localizer, the glide slope signal is also consists of two overlapping beams modulated at 90 Hz and 150 Hz. However, these signals are aligned above each other and are radiated primarily along the approach track. The thickness of the overlap area is 1.4º or .7º above and .7º below the optimum glide slope.

Precision Approach Path Indicator (PAPI):

PAPI are led lights placed on the runway in the form of making markers for visual navigation of aircraft during landing. PAPI’s are usually zero visibility condition aids which can be seen by the pilot through eyes, while landing when uncertainty conditions among the instruments happens. These systems have an effective visual range of at least 3 miles during the day and up to 20 miles at night.

MARKER

Marker serve to identify a particular location in space along an airway or an approach to an instrument runway. This is done by a means of 75 MHz transmitter which transmit a directional signal to be received by aircraft flying overhead.

Marker has three types:

OUTER MARKER

The Outer Marker, which normally identifies the final approach fix (FAF), is situated on the same course/track as the localizer and the runway center-line, four to seven nautical miles^[1] before the runway threshold. It is typically located about 1 NM (2 km) inside the point where the   glideslope intercepts the intermediate altitude and transmits a 400 Hz tone signal on a low-powered (3 watts). The system gives the pilot a visual and aural indication.

MIDDLE MARKER:

A middle marker works on the same principle as an outer marker. It is normally positioned 0.5 to 0.8 nautical miles (1 km) before the runway threshold. When the aircraft is above the middle marker, the receiver's amber middle marker light starts blinking, and a repeating pattern of audible Morse code-like dot-dashes at a frequency of 1,300 Hz in the headset.

Inner Marker:

Similar to the outer and middle markers; located at the beginning (threshold) of the runway on some ILS approach systems (usually Category II and III) having decision heights of less than 200 feet (60 m) AGL. Triggers a flashing white light on the same marker beacon receiver used for the outer and middle markers; also a series of audio tone 'dots' at a frequency of 3,000 Hz in the headset.