Have you ever flown somewhere and wondered how the pilot lands that plane when all you can see is clouds, fog, or nothing at all? I have. Through the years, avionics (or aviation electronics) have become much more sophisticated. Through both disaster and necessity, air travel has become safer due to innovative thinking and lessons learned through trial and error. One such innovation is ILS.
ILS stands for Instrument Landing System and simply stated is what pilots use to safely guide their aircraft to the runway when the weather conditions do not allow for a visual approach and landing. Such conditions make it very difficult or even impossible to see ahead, so using radio signals (sometimes coupled with high-intensity lights) the pilot can safely navigate the aircraft through low ceilings, rain, dense fog, snow, and safely land.
The radio signals are ground-based and are sent continuously. Containing two separate parts, the signals guide the pilot (or autopilot) both to the centerline of the runway and down a safe rate of descent, or glideslope.
To use the ILS, the pilot must tune the navigation radio receiver in the cockpit to the appropriate frequency for the runway to be used. Once tuned to the proper frequency the pilot can then guide the aircraft along the path set by the radio frequency by either hand-flying or by using the autopilot. When hand-flying the aircraft, the pilot uses the horizontal situation indicator, or HSI, to follow both the glideslope and localizer. When tuned to the proper frequency, the navigation radio, or NAV, sends a signal to the HSI and two indicators will appear.
The indicators are oriented perpendicular to each other - one oriented horizontally and the other vertically. The pilot maneuvers the aircraft so that the indicators form a "+" in the center of the HSI. When this occurs, the pilot knows that the aircraft is both on the proper glide path and is lined up with the runway. When the pilot decides to use autopilot, the autopilot performs the same task.
As the aircraft approaches the runway, it passes over ground-based radio beacons (typically three) set along the localizer which send signals to the navigation radio telling the pilot approximately how far the aircraft is from the runway. This is accomplished by the navigation radio taking the signals and interpreting them, and producing an audible tone for the pilot.
The beacons are called 'markers', and each marker has a different tone to distinguish distance from the runway but each marker of the same type will be uniform. The 'outer marker' has a low-pitched tone and a long duration. It lets the pilot know that the runway is approximately 3.5 to 6 nautical miles from landing and is indicated by a blue light. The 'middle marker' has a higher tone than the outer marker and the tone repeats more frequently (two each second) and lets the pilot know that they are near the missed approach point in low-visibility conditions and has a corresponding amber light. Finally, the 'inner marker' indicates that the arrival at the runway threshold is imminent. It is distinguished by a white light and morse 'dots' at 3kHz.
Using this technology has increased the safety of travelling by air significantly and will continue to prevent air disasters while improving the efficiency and flow of aircraft both in and out of airports around the world.