[0:01]An aircraft descends through a thick layer of overcast clouds. The altimeter indicates the ground is getting closer and closer. Suddenly, approach lights appear out of the mist leading to a wide runway and the aircraft makes a smooth touchdown. It's one of the most satisfying actions a pilot can make, flying an instrument approach to minimum altitudes and it's made possible by the instrument landing system or ILS. The ILS uses highly directional radio transmitters on the ground to provide an approach path for exact alignment on an aircraft's descent to land. Let's talk about how the ILS works by looking at an example. This is the approach chart or plate that the FAA publishes for the ILS approach to runway 31 in Sue City, Iowa. We won't get into all the details of the approach plate in this video, we just want to visualize the approach and how the ILS works. Private pilots know that they can use an array of red and white lights called Pappy or Vazi to determine if they are too high or low on approach. On this Vazi here, the aircraft is on the proper approach path or glide slope, when the pilot can see a line of red lights over a line of white lights. Get too low and now the lights are red over red, get too high and it's white over white. The pilot can adjust the aircraft's descent angle so that the indication on the Vazi is red over white, meaning we're back on the appropriate glide slope. Vazi works by each light sending out both red and white indications directionally and they're oriented so that on the proper glide slope, the aircraft sees red on top and white on bottom. This overlap, which creates the glide slip, can be represented on the chart with this feather symbol. Now, we can tell our descent angle by looking at lights located at the runway. But what do we do if the weather limits our visibility and we can't see the lights? Well, we can replace lights with radio transmissions, whether we use lights that get picked up by our eyes or radio waves, which is just another form of light energy that get picked up by our antennas, the result can be the same. Here, we'll use two radio transmissions at different frequencies, one at 90 Hz and another at 150 Hz. We can think of this as broadcasting two different colors, just like the Vazi does, after all, color differences are just a result of different frequencies of light energy too. These transmissions will also be oriented such that they'll overlap along the proper glide slope. So, when our equipment on board picks up both signals perfectly overlapped, it'll indicate to us that we're centered on the approach. Too much of the 90 Hz signal and it'll indicate too high and too much of the 150 Hz, it'll indicate too low. These transmissions originate from a series of antennas that look like this and are situated at the side of the approach end of the runway. They're right next to the Vazi lights, indicated by the V in the black circle, which makes sense because as we said, they're doing basically the exact same job.
[3:13]A typical glide slope on an ILS approach is set up to be 3 degrees off horizontal. This diagram isn't quite to scale with that to make it easier to depict. Now, we use our VOR receiver to interpret the signals from the glide slope. Here we are dead center on the glide slope, so the indication on our receiver has the horizontal needle centered up. If we were also looking at the Vazi lights, we'd see red over white. If we climb above the glide slope, the horizontal needle starts to move below center incrementally and the Vazi will eventually indicate white over white, we're too high. If we over correct and drift below the glide slope, the needle will move the opposite direction and float above center and the Vazi will show red over red, we're too low. Notice that the needle moves incrementally, giving us an idea of how far off center we are, though the Vazi can only indicate too high, too low or just right. We make corrections off these indications by chasing the needle, just like we do for VOR navigation. Here the needle is above center, so we need to climb or otherwise slow our descent and we do so until the needle centers and we adjust to keep it there. One limitation of the glide slope antenna is that because objects can reflect signals from the ground, false glide slopes can be created, which have 9 or 12 degree angles to the runway instead of the typical 3. For this reason, it's typical to fly an ILS approach by intercepting the glide slope from beneath it, so as to avoid these false glide slopes. Here are the symbols on the actual approach plate. You can see the glide slope feather just as we've drawn it out, as well as the approach path having the aircraft start at an altitude below the glide slope and intercept it from below. The glide slope covers vertical navigation, but the ILS also provides for horizontal or lateral guidance in much the same way the glide slope works. Here again, we see two signals transmitted out from the runway, one on the left at 90 Hz and one on the right at 150 Hz. These two signals are directed such that they intercept on a course, guiding the aircraft down the center line of the runway. This transmission is called a localizer and it's also represented by a feather symbol. And again, we can also use our VOR receiver to track it. Here we see that it works much the same as a VOR does. If we drift right of center, the needle swings to the left and vice versa. In order to stay on the localizer and thus the extended center line of the runway, we need to chase the needle again. The localizer antenna looks like this. It's situated at the opposite end of the runway from the approach, so its signal starts at the far end of the runway and then widens out as it gets further away from it. By the time the signal reaches the threshold of the approach end of the runway, it's 700 ft wide or 350 ft on either side of center line. This means that an aircraft just 300 ft off center at the runway threshold would see full deflection on the needle, so it's quite sensitive at this point. Because of this fixed width at the threshold and the fact that the localizer transmitter is set up at the far end of the runway, the angle of the transmission varies with the length of the runway. For example, a much shorter runway would have a localizer that would look more like this in order for the width at the threshold to remain at 700 ft. This means that the width of the localizer course at given distance away is not fixed, it's dependent on the length of the runway and the only constant value is the 700 ft width at runway threshold. Besides the localizer and glide slope, many ILS approaches provide distance information using marker beacons. Here the football symbol indicates an outer marker. This is typically between 4 to 7 miles from the runway and indicates where the aircraft will intercept the glide slope at minimum altitudes. This is the point where the aircraft normally should be configured for the approach. When passing over the outer marker, the aircraft with the proper equipment will receive this indication in the cockpit.
[7:30]Marker beacons are used less and less in ILS approaches and distance information is provided through DME, which many approaches incorporate or GPS. If we remove the overlay, we can see what the approach symbols look like on the plate. You can see the localizer feather and the football shape for the outer marker. Notice the shading on the right side, you'll always see the right side of the feather shaded on a localizer front course like this, as opposed to the left side, which would be on a back course, which is covered in another video. An ILS approach is flown by tuning the aircraft's navigation equipment to the correct frequency. The top of the plate lists the frequency for this approach as 109.3. This frequency is for the localizer signal alone, though it's paired with a glide slope signal at a much higher frequency. The navigation equipment will also pick that up automatically. In addition to receiving these two signals, the aircraft will also receive an identification signal, which broadcasts the Morse code identifier for the localizer. Here the identifier is ISUX and the transmission will include the Morse identifier shown here by the dots and dashes, just as you'd find when identifying a VOR. Unlike a VOR though, the localizer and glide slope transmissions aren't broadcast in all directions, they're highly directional, which means that unless you're lined up with them, you can't rely on getting an accurate signal. Proper indications are only guaranteed within 35 degrees of either side of the runway center line out to 10 miles from the localizer antenna. And within 10 degrees of either side of the runway out to 18 miles. What this means is that even an aircraft that's very close to the airport but not inside these coverage areas, as is often the case for flights that are being vectored by ATC onto the approach course, won't receive an indication when tuned to the ILS and an off flag or something similar will be displayed. This will stay until the aircraft enters the coverage area and the receiver will come alive. Here showing that we're left of course and below the glide slope. As we intercept the localizer, the needle will swing toward center. So, putting the localizer and glide slope together, we get a three-dimensional guidance down to the runway. First, we see the localizer beaming a course from its antenna at the back of the runway and the glide slope transmitting a course from closer to the approach end. Where they intersect, the aircraft will be shown as being on center of both needles. Deviating off this three-dimensional center will indicate on one or both needles accordingly. We might also have one or more marker beacons indicating distance and key parts of the approach. Here we have an outer and a middle marker. As we continue inbound, the course gets smaller and so the needles get more sensitive. The middle marker is situated at the decision altitude, typically 200 feet AGL, where we'll decide to either continue the descent down to the runway, or if we don't have any visual cues, we'll start a climb out and execute a missed approach. Putting everything together, here's an abbreviated ILS approach at night time. We can see the glide slope lights, in this case, a Pappy initially indicating four reds, meaning we're too low, as confirmed by the glide slope needle being above center. We'll hold the altitude until the needle centers.
[10:57]You can also see the lights turning from four reds to two whites and two reds. Once it's centered up, we make our descent down to the runway, keeping both needles centered as best we can.
[11:13]Finally, at 200 feet AGL, the middle marker will sound at the top right of the screen, indicating our decision altitude. Since we can see the runway just fine, we'll decide to continue our descent and to land. If this was helpful, please click subscribe so that you can stay up to date on every new training video coming out each Tuesday and Friday and get access to posts and articles that will take your training even further. It just takes one click and it's so worth it.
