Inner markers are located right at the runway threshold. Inner markers (white light, continuous dots) are found with ILS approaches that can be flown to lower minimums, such as Category II ILSs, where decision heights can be as low as 100 to 150 feet agl. If you don't see the runway when the middle marker goes off, it's probably time to execute a missed approach. In other words, being at the middle marker often means that you're at 200 feet agl and one-half mile from the runway threshold. Frequently, activation of the middle marker nearly coincides with the decision height (DH - the altitude at which you must decide to land or not) and published minimums for the runway to which you're flying. The middle marker is usually about 3,500 feet from the threshold, causes an amber beacon light to shine, and has a dot-and-dash identifier.
ILS VS LOCALIZER CODE
It illuminates a purple light on your airplane's marker beacon display and gives a three-dash Morse code identifier that you can hear through your airplane's audio switching panel or cabin speakers. The outer marker is usually 3.5 to seven miles from the ILS's runway threshold. Broadcasting on 75 MHz and sending up an elliptical, nondirectional beam, markers are great for giving you range information along the ILS. Many times, marker beacons are used in sequence. Keep the localizer and glideslope needles centered as you fly the approach and you'll fly "down the pipe" to the runway.Īlong the way, there will be intermediate fixes along the ILS approach path. But when you fly an ILS, you're really following two signals: a localizer, for lateral guidance and a glideslope, for vertical guidance. Like VORs, ILSs also use VHF signals to provide course information. Because ILSs are so common, we'll concentrate on them in this article. Who could ask for more than a precise, stabilized, straight-in approach on a three-degree glidepath, ending with a landing on a specially prepared and approved runway? That's what you get with a properly flown ILS or PAR approach. And when you break out of those rotten conditions, there's the greater assurance that the landing runway will be right in the center of your windshield. It's this kind of accuracy that permits descents to altitudes as low as 200 feet agl in visibilities as low as one-half mile. They also give you information on your range from the runway, provide you with target altitudes along the approach path, and keep you well-aligned with the landing runway's extended centerline. First and foremost, precision approaches provide pilots with extremely accurate lateral as well as vertical guidance to relatively long and wide runways.
Why do pilots lean toward precison approaches? Several reasons. The terms slightly and well are used for advisories (e.g., "you're slightly left of course and well above the glideslope.") If pilots on PAR approaches receive no communications for a period longer than five seconds, then they must execute a missed approach. On a PAR approach, controllers constantly issue lateral and vertical commands to keep the pilot on the proper approach path. PARs involve controllers essentially talking pilots down to minimums. Precision approaches are those with vertical guidance, and they include Instrument Landing System (ILS) and Precision Approach Radar (PAR) approach procedures. Instrument pilots generally agree: If ceilings and visibilities are heading downhill, then a letdown through the clag on a precision instrument approach is the only way to fly.
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