"We ask passengers to return to their seats and secure their restraints. We will be landing in 10 minutes."

The landing is sometimes the most anticipated and anxiety-inducing phase of the flight. Nothing compares to the shutter of passengers landing in a cross-wind, or skidding to a long stop on an icy runway.

Yet when the plane comes to safe stop, it is a testament to the skill of the pilots who control the aircraft — even if they could not see the runway at all.

For various reasons, the pilots might not be able to accurately see the tarmac and ground signals of the airport, and in these cases must plan a descent, approach and landing completely based on instrument readings. Heavy rain, snow, low clouds, fog or nighttime, or some permutation of those, can easily obscure the runway from the most sharp-sighted of pilots.

Often, flights are delayed or canceled for inclement conditions, but sometimes the plane is already in flight. For many reasons, aircraft need reliable orientation systems that can see where the pilot cannot. This prompted aircraft designers to develop the necessary technology to assist the flight crew in maintaining control of the aircraft during difficult landings.

Instrument Landing Systems (ILSs)

The ILS guides pilots with precision, providing detailed information on maneuvers to align the aircraft with the centerline of the runway, and at the correct descent rate to make sure that the aircraft lands near or exactly at the touchdown marker. This process is only possible using both ground-based and aircraft-based radio systems

Figure 1. A localizer antenna. Source: Super Dominicano/CC BY-SA 2.5A ground-based system provides directional guidance by transmitting VHF and UHF signals. The localizer transmits the VHF signal from two directional antennas. If the aircraft is veered left of the runway centerline, a 90 Hz frequency becomes stronger; if veered right a 150 Hz frequency is stronger. This equipment is typically a large antenna array normally located at the end of the runway. This ensures that the signals provide aircraft with accurate guidance to the middle of the runway.

Figure 2. Glide slope antenna. Source: Herr HK/CC BY-SA 3.0The glide slope operates in the UHF signal, but uses a similar orientation mechanism. The UHF band means a smaller antenna array, so the glide slope is located on the touchdown side of the runway. The glide slope signal is transmitted at 3° from the runway, with a relatively narrow beamwidth. If the aircraft strays above the signal, the 90 Hz frequency becomes stronger; if it strays below, the 150 Hz signal is stronger. Some airports may have different descent angles, which can be affected by local terrain or regulations.

Paired with the ground-based system is the aircraft-based system. This system includes antennas, computers, and monitors that receive the signal transmitted from the ground and process it to provide auto-pilot reference data, or displayed for the pilots when manually controlling the aircraft.

Located inside the nose radome, just below the weather radar are the localizer and glide slope antenna. These antennas receive the signals transmitted by the ground base equipment and convert them as markers to be displayed in display units (DUs) located in front of the flight crew.

The information displayed on the DU is also sent to the flight data acquisition unit (FDAU), which processes the information to be digitized. Afterward, it is forwarded to the digital flight data recorder, aka the “black box” of the aircraft.

Another important component that allows the automatic flight control of the aircraft during an approach in an ILS is the auto flight control system (AFCS) computer. The information received by both localizer and glide slope antennas is forwarded to the AFCS. The AFCS reads the transmitted signal and creates a command to the hydraulic system to provide pressurized hydraulic fluid to actuate the autopilot actuators, which in turn moves the flight controls such as the aileron, rudder and elevator. The movement of the flight controls ensures that the aircraft adjusts and maintains its flight path to follow the desired lateral and vertical position.

The movement of the flight controls and all the subsequent operation of related components and computers are also transmitted to the FDAU, which again digitizes the data before being transmitted to the flight data recorder. This technology allows full recording of all the movements and processes the aircraft undertakes, for review if an accident or incident occurs.

Simulation

Imagine being aboard an aircraft that is about to land. The flight crew will first make contact with the ATC to request for the coordinate wherein they can intercept the signal transmitted by both the localizer and glide slope. Once successfully intercepted, they can continue to fly the aircraft either manually or through autopilot.

When the autopilot is engaged, the localizer and glide slope signals received by the antennas will be forwarded to the AFCS to process and give a command to actuate the necessary flight controls. All of the above processes are recorded by the black box.

However, when the aircraft is flown manually by the flight crew, they would need to observe the markers located on their display unit as a reference on what lateral and vertical direction they would veer the aircraft to obtain the correct flight path. In this scenario, the AFCS is not engaged thus it will not provide any automatic command to the flight controls. Nevertheless, all of the inputs made by the pilot on the controls are also recorded by the black box.

Less workload

Ultimately, the ILS provides an accurate landing location for aircraft with poor visibility. It also reduces the flight crew’s workload significantly, as the aircraft position is located to the display unit, together with the other important flight parameters such as speed and altitude.

However, not all airports are equipped with ILSs, and others may only have ILSs on some of its runways. Its further ubiquity will only promote aviation safety