Some minor comments on Kemo's post...
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Originally Posted by KemoSabeNada
The LOC will give horizontal guidance down to centimeter accuracy, the GS will give a vertical profile down to the touchdown point. The idea and principle is that the aircraft lands on the centerline and at or near the threshold of that runway.
The planned touchdown point (and the target of the GS) is a at the 1000' foot point. If on a standard 3 degree glideslope, the aircraft will cross the runway threshold at 50'. When flying in VMC conditions, especially to a shorter runway, it's not unusual for the flying pilot to announce "I'm visual, going below the glideslope" to let the other guy know that he's intentionally ducking below the glideslope to land before the 1000' point. Most pilots have an innate distaste for wasting runway; the sooner you get it on the ground, the shorter the rollout. It's important to announce that you're doing this, otherwise the non-flying pilot has an obligation to tell you that you're getting low on the GS. Also, the plane's GPWS might even bark "GLIDESLOPE!" at you. Ducking a little low was a common move in my Shuttle flying in the MD-88, going into 7000' runways at both LGA and DCA.
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Each country/state/airport have their own procedures on when the ILS is supposed to be intercepted (or used). I can imagine in Vegas that they use it far out as the approach always seem very long there!
Most airports now have Standard Terminal Arrival Routes (STARs) which are RNAV routes that feed into the ILS. Once we are cleared for the approach, we can arm the Localizer to intercept, and once we've honored all of the altitude restrictions on the STAR we can arm the Glideslope.
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The ILS is generally rock solid and the only way for pilots to experience issues with it is basically ground interference or onboard equipment filtering issues. A typical ground interference issue will be that the ATC are holding aircraft too close to the LOC or GS, or aircraft are taxing inside the beam forming areas when another aircraft is on final.
This is why many airports have two separate "Hold Short" lines for runways. In VMC weather, aircraft routinely hold just short of the active runway until receiving clearance to takeoff or to taxi onto the runway. When the weather dictates using the ILS, there is a more restrictive hold short line so that aircraft don't interfere with the LOC or GS transmitter. (If I recall correctly, this is any time the weather is below 800' ceiling and 2 miles visibility.)
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I have not heard of any incidents where lives were lost due to ILS malfunctioning.
And don't believe that
Die Hard movie (#2?) where they set the glideslope low so that airplanes crashed into the ground short of the runway. We don't just blindly follow that signal. We cross check our altitude continuously and we know that we should be 300' high for every mile out. So at 5 miles from the runway, we should be approximately 1500' above the ground. Also, we can cross check the published crossing altitude at the Outer Marker if that ILS is so equipped.
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Today we also count a 3rd component, the marker beacon.
Marker beacons actually pre-date the use of DME. If we have DME, we can just continuously do our 300'/NM mental math. In the days before DME, when there was no way to tell how far out you were, you would check the published altitude when crossing the marker. And many airports had multiple markers, up to 3 of them, called the Outer Marker, Middle Marker, and Inner Marker. Most of those are going away now, probably due to their redundancy and cost of upkeep.
How did you know that you were crossing the marker? They each had a distinctive sound, only audible if you selected the "Marker" audio on your comm panel and turned up the volume. They also each had a distinctive colored, flashing light on one of your instrument displays (sometimes a special set of lights on the panel; other times incorporated into the Nav display).
When crossing the Outer Marker, you would hear a slowly, continuously repeating beeping sound and a flashing blue light. At the Middle Marker (if so equipped), you'd hear a higher pitched, slightly faster beeping and an amber flashing light. Finally, the Inner Marker (usually very close or right at decision height of 200') you'd hear a much higher pitched (kind of annoying) and very frequent continuous beeping along with a flashing white light. Pilots often reach down and select the Marker audio off to eliminate the incessant beeping. (Actually, tbh, most pilots these days don't ever monitor the marker beacons. I'm old school and almost always had them on.)
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Sorry for the hijack W0X0F and everyone else, and the long read.
Thanks for the contribution Kemo!
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Originally Posted by golddog
Kemo's post brought up a question. I'm assuming that DMC, LOC, and GS are sort of 'markers' for "you're on the right track to airport ABC."
What's the time difference to these pickups? 65 -> 25-> 10 nm sounds like a fair amount, but you're flying at speeds I'm not used to operating something. Is there enough time to react to something in-between, are they one right on top of another, or something different?
They're nicely spaced and, as I mentioned above, the STAR usually feeds you right into the final approach course, intercepting the LOC. There are still some "full" approaches at smaller airports which don't presume RNAV capability. These will have an Initial Approach Fix (IAF) which you navigate to and then fly the published procedure. That procedure could involve a procedure turn (often a course reversal with timed legs on certain headings) to intercept the localizer. The procedure will include a minimum altitude for each leg of the approach and these are observed until intercepting the glideslope.
Last edited by W0X0F; 05-07-2021 at 09:21 PM.