AUTOMATIC LANDING SYSTEM

A precision instrument approach to touchdown and, in some cases, through the landing roll out performed by the aircraft autopilot. The autopilot receives position information and/or 

steering commands from onboard navigation equipment and is not dependent on any ground-based aids. ALS approaches are flown in VFR (very high frequency) and IFR (instrument flight rules). These approaches are usually carried out when the weather conditions are less than approximately 600 meters RVR(runway visual range), provided the crews are certified for auto land approaches.

Before landing, the pilot has to perform a landing checklist to ensure that critical items such as fuel flow, landing gear down. Flaps are used for most landings because they permit a lower- approach speed and a steeper angle of descent.The airspeed and rate of descent are stabilized, and the airplane is aligned with the runway centerline.

The phases of ALS are as follows:

1.     Capturing Beam :

The first stage is to capture the beams coming from the Instrument Landing System that includes the Localizer and Glide Slope.

• Localizer radiates the beams for Horizontal Guidance.
• Glide Slope originates the beams for Vertical Guidance.

2.     Decision Height :

In this stage, the pilot has to decide considering the weather and other conditions whether to land or not.

3.     Flare :

The position of an aircraft in which the nose of an aircraft is slightly downward and the tail is slightly upward. The purpose of this "flare" is to reduce the energy of the aircraft such that it "stops flying" and settles onto the runway.

4.     Decrabbing :

Decrab is the phase of the landing with xwind in which the airplane is vulnerable to the gustings and for this reason, all aircraft have to apply cross-controls: left rudder, right aileron (if wind is coming from starboard) meaning rudder and aileron in opposite directions. The nose points into the wind so that the aircraft approaches the runway slightly skewed with respect to the runway centerline (crabbing).

5. Touch Down :

The point where the aircraft lands or touch it's wheels on the runway is called Touch Down.

                        MARKER BEACONS

A marker beacon is a particular type of VHF radio beacon used in an Instrument Landing System(ILS),to provide visual and audible cues to the crew to determine position along an established route to a destination such as Runway.Marker Beacons operate at 75 MHz and radiate approximately 3-4 W of power.

OUTER MARKER:

• It is located between four and seven miles from the runway.
• It transmits Morse code dashes at tone frequency of 400 Hz and illuminates a cyan light ('OM' icon for display).
• It provides the approximate point at which an aircraft on the localizer will intercept the glide slope.

MIDDLE MARKER:

•  It is located approximately 3500 feet from the runway.
• It transmits Morse code of dots/dashes modulated at 1300 Hz frequency and amber light ('MM' icon for display) is illuminated.
• It coincides with the aircraft being 200 feet above the runway touchdown point.

INNER MARKER:

• It is located at a distance of 60 m from the runway.
• The crew recieve Morse code dots modulated at 3000 Hz on the audio system and a corresponding white light ('IM' icon for display) is illuminated.

                            RUNWAY

A defined rectangular area on a land aerodrome prepared for the landing and takeoff of aircraft is called Runway.

Runways may be a man-made surface (often asphalt, concrete, or a mixture of both) or a natural surface (grass,dirt, gravel, ice, or salt).

DIMENSIONS OF RUNWAY:

Runway dimensions vary from as small as 245 m (804 ft) long and 8 m (26 ft) wide in smaller general aviation airports, to 5,500 m (18,045 ft) long and 80 m (262 ft) wide at large international airports.

TYPES OF RUNWAY:

There are three types of Runways.

Visual Runways:

• These runways are used at small air strips and are usually just a strip of grass, gravel, ice, asphalt or concrete.
• Usually, there are no markings on a visual runway, they may have Threshold Markings, Designators and Centerline.
• They donot provide an instrument-based landing procedure; pilots must be able to see the runway to use it.
• Radio communication may not be avaliable and pilots must be self-reliant.

Non-Precision Instrument Runways:

• These runways are often used at small to medium size airports.
• These runways, depending on the surface, may be marked with Threshold Markings, Designators, Centerline and sometimes a 1000 feet (305 m) mark known as an Aiming Point, sometimes installed at 1500 feet (457 m).
• They provide horizontal position guidance to planes on instrument approach via NDB (Non-directional Beacon), VHF Omni directional Range (VOR), Global Positioning System (GPS) etc.

Precision Instrument Runways:

• These runways are found at medium and large size airports.
• These consist of a Blast Pad/ Stop-way (optional for airports handling jets, Threshold, Designators, Centerline, Aiming Point and 500 feet (152 m), 1000 feet (305 m) / 1500 feet (457 m), 2000 feet (610 m), 2500 feet (762 m) and 3000 feet (914 m) Touch-down zone marks.
• Precision runways provide both Horizontal and Vertical guuidance insrument approaches.

SECTIONS OF RUNWAY:

There are runway markings.

• Runway Threshold:  markings across runway denoting the space designated for normal Landing and Take-off conditions.
• Runway Safety Area: cleared, smoothed and graded area around the paved runway which is kept away from obstacles.
• Runway: surface from threshold to tthreshold features threshold markings, centerline and numbers.
• Blast Pads: stopways overrun areas at the end of runways as emergency space to stop the planes when landing or takeoff goes wrong. They are marked with yellow chevrons, not allowing takeoff or land on it except emergency situations.

                          AIR FIELD LIGHTING:

Approach Lights: A sequence of high-intensity lighting at length of 900 m before the runway actually begins.The pilot can see from these lights whether his aircraft is centered correctly.

Runway Edge Lights: are used to outline the edges of runway during periods of darkness or restricted visibility conditions.

Runway End Identifier Lights: are used to provide rapid and positive identification of the approach end of a particular runway.The system consists of a pair of a pair of synchronized flashing lights located laterally on each side of the runway threshold.

Runway Center Lights: are located along the centerline of the runway.The lights are located from the threshold to the end of longitudinal spacing of approximately 15 m.

Touchdown Zone Lights: define the landing portion of the runway.Touchdown Zone Lights are white in color & shall extend from the threshold for a longitudinal distance of 900 m.

Taxiway Edge Lights: blue colored lights define the outline of taxiway during low visibility conditions.

Taxiway Center Light: green colored lights define the centerline of the taxiway during low visibility conditions.

                                                                                    PAPI & VASI LIGHTS

Precision Approach Path Indicator: is a visual aid that provides guidance information to help a pilot acquire and maintain the correct approach (in the vertical plane) to an airport or an aerodrome. It is generally located beside the runway approximately 300 meters beyond the landing threshold of the runway.

Visual Approach Slope Indicator:  is a system of lights on the side of an airport runway threshold that provides visual descent guidance information during approach. These lights may be visible from up to 8 kilometres (5.0 mi) during the day and up to 32 kilometres (20 mi) or more at night.