![]() Too far to the right is indicated by 150 Hz. If a pilot is too far to the left, they will receive only the 90 Hz signal. A pilot who is lined up with the center of the runway will receive both a 90 Hz and 150 Hz signal simultaneously. To do this, in addition to the primary radio signal, the localizer uses horizontal polarization and a different modulation frequency for each side of centerline. The purpose of the localizer is to provide pilots with horizontal guidance to help them maintain alignment with the runway centerline throughout the final approach and landing. Pilots can pick up the localizer signal from a distance of approximately 18 nautical miles away and an altitude of 4,500 feet above the antenna. The localizer transmits in the range between 108.10 and 111.95 MHz at a power of about 100 watts. The localizer transmits a VHF (very high frequency) signal from an antenna that is aligned with the centerline of the runway. VHF Localizer The localizer is a piece of equipment that is usually located 1,000 feet from the opposite end of the runway so it does not present a collision hazard to approaching aircraft. The FAA created Category IIIc approaches but has yet to authorize them or certify any aircraft to make the approach that would allow a completely blind landing with 0 feet of runway visual distance required. ![]() A Category IIIa approach can have as low as a 50-foot decision height and down to a 700-foot RVR, while a Category IIIb allows an RVR down to just 300 feet. The Category III ILS with its three subclasses is more rarely used and comes with very low minimums. Pilots landing using a Category II ILS have a decision height minimum of not less than 100 feet and 1,800 to 1,200 feet RVR is required. A Category II ILS has additional equipment, facilities, and pilot training which are required since the minimums are lower. Some airlines and corporate flight departments are FAA authorized to use Category II instrument landing systems. Most Instrument Landing Systems are Category I with a decision height of no less than 200 feet and visibility minimums of one-half mile or 2,400 feet of runway visual range (RVR). There are three categories of Instrument Landing Systems, each with their own minimums. ![]() An inoperative glideslope transmitter means that pilots can still fly a non-precision localizer instrument approach. If the localizer is inoperative, the airfield is not authorized for ILS landings. Both the localizer and glideslope communicate with aircraft via radio beams and together they provide the navigational guidance that pilots need to safely land. They are aided by marker beacons and the approach lighting system (ALS). The two key physical components of the Instrument Landing System are the localizer and the glideslope. The FAA describes the ILS saying it provides aircraft with “precision vertical and horizontal navigation guidance information during approach and landing.” They also note that, “associated Marker Beacons and/or Low Power Distance Measuring Equipment (LPDME) identify distance to the runway.” How does an Instrument Landing System work? What is an Instrument Landing System?Īn instrument landing system is one of the primary landing navigation aids used by pilots flying under instrument flight rules (IFR). The ILS has been around and being used to help IFR pilots land for more than fifty years. When you make the leap and decide to earn your instrument certification, one of the first new systems that your CFI will be introducing you to is the ILS, or instrument landing system.
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