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The ILS

Most pilots with instrument ratings would probably agree that when it comes to an uneventful passage through haze, gloom, or dark of night and back to Mother Earth, an ILS is a much better deal than a VOR approach. Given a choice between the somewhat more relaxed progression of a non-precision descent profile and the relatively more rapid cross-checking required to remain within the allotted confines of a precision approach path, when the chips (and the ceilings) are down, the precision approach is definitely the better of the two. Until there are many more GPS WAAS approaches besides the few now coming online, for a while yet at least, the odds are that if you have to get down through a layer of low clouds, the bases of which might be as low as 200 feet, you’ll be flying an ILS. (Of course, the PAR or precision approach radar that I wrote about last year also qualifies as a precision approach, but I’ll concentrate on the ILS here.) So what is it that makes an ILS so special?Most pilots with instrument ratings would probably agree that when it comes to an uneventful passage through haze, gloom, or dark of night and back to Mother Earth, an ILS is a much better deal than a VOR approach. Given a choice between the somewhat more relaxed progression of a non-precision descent profile and the relatively more rapid cross-checking required to remain within the allotted confines of a precision approach path, when the chips (and the ceilings) are down, the precision approach is definitely the better of the two. Until there are many more GPS WAAS approaches besides the few now coming online, for a while yet at least, the odds are that if you have to get down through a layer of low clouds, the bases of which might be as low as 200 feet, you'll be flying an ILS. (Of course, the PAR or precision approach radar that I wrote about last year also qualifies as a precision approach, but I'll concentrate on the ILS here.) So what is it that makes an ILS so special?

An ILS of course provides very accurate vertical as well as lateral guidance to the extended centerline of what are always relatively long, wide, specifically well-marked, well-lit, and well-maintained runways. (When I say very accurate, I should quantify that. The vertical thickness of an ILS glideslope is but 1.4 degree, top to bottom, and the width of its horizontal component, the localizer, can be as little as three, up to about six degrees.) The runways having an ILS also must have certain unique and easily identified approach light configurations. Another piece of information provided by the various components of an ILS is distance remaining to the runway, as well as target altitudes at certain points along your approach path. Unlike VORs though, an ILS actually uses two signals: a VHF signal (which means anywhere between 30 to 300 MHz) for the localizer, and for the glideslope, an automatically paired and dedicated UHF frequency. (UHF generally means 300 MHz to 3 GHz.) Associated with these, each ILS installation also features so-called 'marker beacons' at precisely located intermediate positions along the descent path. These broadcast an upward-directed (but essentially non-directional) elliptical VHF signal at 75 MHz, and each provides its own unique audible as well as visual indication in the cockpit when they are being over-flown, so you have additional information to confirm where you are at that moment. In addition, various forms of approach lights are also part of each ILS installation.

An ILS can have three such beacons in succession, each progressively closer to the runway, although most general aviation pilots flying under Part 91 will usually make use of ILS approaches having only the first two. The first one, the outer marker, is usually located just inside where the glideslope is first intercepted, which can be between 3.5 and seven miles from the runway threshold. Assuming you remembered to turn it on, you would see a purple light on your marker beacon display, and you would also hear a repeating three-dash Morse code identifier (the letter 'O') at about 120 Hz (although the actual 'continuous wave' sound, if you are musically inclined, would be at about G above middle C, or around 400 Hz). At most single engine approach speeds, you would hear it for perhaps 20 seconds as you flew through the signal. You would hear (and see) the next one, the middle marker, at about 3500 feet from the runway. This would appear to you as a flashing amber beacon with alternating Morse code dots and dashes at the slightly more urgent frequency of about 190 per minute, with the audio tone at a much higher 1300 Hz or so (around an E, two octaves above middle C on the piano). The middle marker is usually activated as you reach decision height about a half-mile from the runway. (Contrary to what you may have hoped, the dots and dashes could be either the letter 'A' or the letter 'N'; the Morse code identifier for the letter 'M'-as in middle-would be hard to distinguish from the three dashes for the letter 'O'. But as it turns out, the Morse code assignments are in fact logical, because the Morse signal for the inner marker, usually located right at the runway threshold, is a frantic series of dots at 360 per minute accompanied by a white light and a much higher attention-getting audio frequency of about 3000 Hz, close to an F#, three octaves above middle C.)

There are also other navigational fixes that tell you where you are along the approach which are possible with an ILS. The outer marker might be a so-called locator outer marker (LOM), which means that it also has an associated NDB signal (from about 190 to 530 KHz). There can also be distance information from charted intersections of the localizer with a particular radial from a nearby VOR, and some localizers also have DME capability. And as I mentioned, an ILS-equipped runway also must have approach lighting and easily identifiable runway markings for its threshold, touchdown zone, centerline, edges, and some have a blazing movie marquee of sequenced flashing lights, literally pointing at the runway through the gloom.

I mentioned earlier that an ILS is more accurate. Actually, I can quantify that. An ILS localizer is about four times as accurate as a VOR signal. And the closer you get to the runway, the more accurate it becomes. What this means in practice is that navigation by crisis management, i.e., reacting to needle excursions with over-corrections and then adjustments to those corrections, won't work on a localizer; you will usually just precipitate a swordfight. Small corrections are the rule here. At the outer marker, a one-dot deflection on a localizer display equates to being about 500 feet off course laterally. Near the middle marker, that becomes just 150 feet to one side. (And at the inner marker, that becomes just tens of feet.) If you think that's picky, wait until you hear about the glideslope! At the outer marker, a one-dot deviation means you're 50 feet high or low. At the middle marker, that becomes all of eight feet. (whew!)

There are a few traps lying in wait for the unwary or forgetful:

  • The instrument from whose face you will be flying the ILS will likely be the same CDI display you use when tracking a VOR, but there is one important difference (aside from the accuracy, that is): the only use that the omni-bearing selector knob will have for you is as a heading bug. This is because, unlike the omni-directional VOR signal, an ILS localizer signal is unidirectional.

  • Also, its effective useful reception angle is at most 35 degrees off of the direction of the localizer. (If you're more than about 10 miles away, it won't be much more than 10 degrees.)

  • Another weakness has to do with so-called 'sidelobes'. If you tried to intercept the glideslope from above, rather from below (which is the only way to do it reliably), your receiver can be tricked and indicate a false (and much too steep) glidepath. Always fly at a low enough altitude to intercept the glideslope from below, but equally important and perhaps more so, know the published glideslope intercept altitude, which you will see next to the Maltese cross for the FAF on the instrument approach plate. (We call them 'plates', but while some of us may take them as commandments, they're actually printed on the flimsiest of paper sheets.)

  • The last warning might remind you of Lewis Carroll's Jabberwocky, but…beware the back course! ILS approaches also have backwards-pointing signals, known as back courses, some of which are actually used for localizer approaches to the other end of the same runway. (Some even have glide slope information.) If you are flying in on a back course approach with conventional navigation equipment, or for that matter outbound (away from the airport) on a regular ILS front course, remember these three words well: YOU'RE THE NEEDLE! (i.e., instead of 'needle right, fly right' it becomes 'needle right, you're to the right.)

    Some useful techniques commonly used to successfully fly the ILS are fairly logical and easy to remember:

  • The first thing might very well be to familiarize yourself with each approach (which goes for any instrument approach) before you fly it; ad-libbing usually doesn't go as well as a rehearsed performance. Many have minimums of '200 and a half' but nearby obstacles or local terrain can change that.

  • Never intercept a localizer at more than a 20 to 30 degree angle. They're too narrow and easy to blow right through. (Trust me on this one.)

  • Have an idea of the approximate descent rate you'll need to stay on the glideslope. (One rule of thumb is to divide your approach speed in half and add a zero.) This way, you'll be starting with an approximate 'known' profile, instead of guessing what it might be. The usual range of power for an airplane like a Cessna 172 is a setting of about 1700 rpm.

  • A good practice is to approach the Final Approach Fix (which is usually very close to the outer marker) with the first notch of flaps, to help slow you down, and then lower the gear when you intercept the glideslope.

  • In the real world, at least in a radar environment, you will be given vectors to intercept the localizer (well outside the FAF) probably something like 95% of the time. If not, you'll need to intercept the so-called Initial Approach Fix, fly away from the airport about two minutes past the outer marker, get turned around in what's called a procedure turn (which means flying 45 degrees off course for one minute, followed by a 180, and then re-intercepting the localizer) and then fly inbound and descend according to the rest of the approach procedure.

  • Lastly, once you're one of the cognoscenti, don't announce your position on the local CTAF with expressions like 'Smallville traffic, 35 Romeo, procedure turn inbound, ILS 22, Smallville'. A number of the VFR folks in the pattern won't have any idea what that means, which also means that they won't know where or when to look out for you as you pop out of the clouds. Say those words if you want, perhaps for the folks in the tower (if there's a tower) but also add something like '35R, ILS runway 22 inbound at 1700 MSL, about five miles from the runway'.

    You can see a few of the things I've been mentioning on the attached ILS from runway 23 at Frederick, Maryland, the home airport and galactic headquarters of the Aircraft Owners and Pilots Association.

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