so tell me what is an instrument Landing system well it's a system we use for landing that uses instruments oh really that's quite interesting but how does it work let's find [Music] out hi I'm Grant and welcome to the sixth class in the radio navigation Series today we're going to be taking a look at the instrument Landing system or ILS this is a short- range bit of Kit we use when the weather is bit less than ideal to help us fly safely down into the runway and it gives us guidance both horizontally and vertically so we can do that nice and accurately ilss are Precision approach navad meaning they give both vertical and horizontal guidance these parts are separated out into the localizer for the horizontal guidance and tracking and the vertical guidance is given by the Glide slope and some older systems may also still feature marker beacons to tell you how far away from the runway you are the ILS localizers use frequencies in the VH band VHF band from 30 to 300 MHz more specifically between 108 and 11.95 MHz this overlaps slightly with the VR frequency range so to differentiate between the two the localizer frequencies are always all decimal places or all decimal places plus 0.05 for example 108. 6 is a VR but 108. 7 and 108. 75 would be localizer frequencies you notice I said localizer frequencies instead of ILS frequencies this is because the Glide slope frequencies are separate and they are in the UHF band between 329 MHz and 335 MHz these are always paired with the VHF localizer frequency so that when we tune the VHF frequency the UHF frequency for the Glide soap is automatic tuned as well if it is available and just like with all radio beacons we've looked at so far an ILS will need to be identified with an audible Morse code signal or that ident signal can be interpreted by an onboard display to show the two three or four letter code of the ILS the ident is sent on the localizer as a 1020 HZ amplitude modulated toad usually transmitted once every 10 seconds the localizer is a pretty simple system to understand while I think it least you have a localizer transmitter that is aligned with the runway set about 300 M off the end of like the far end of the runway so for 27 it's going to be 300 M off the end of the sort of 09 side the transmitter sends out two VHF signals one is amplitude modulated at 90 HZ which is the one on the left and one is amplitude modulated at 150 HZ which is the one on the right the the two signals overlap in along the center line of the runway and when the aircraft receives the 90 HZ signal and the 150 HZ signal at exactly the same level than the aircraft displays that we are on the Cent line and we're flying right down the middle of the runway if we have more of the 90 HZ signal then we get a fly right indication to get us back towards Center Line then if we have more of the 150 HZ signal than the 90 HZ signal the aircraft gets a fly left signal and that's proportional to how far away from the center line we are the localizer is a sensitive piece of equipment we use when very close to the ground so it's important we only use it in the areas that is tested to be reliable and provide good information within this as standard is a 10° either side of the center line out to 25 na miles and then at 17 na miles The Arc widens out to 35° either s side this is the normal coverage but it can be different depending on terrain and these different coverage areas should be displayed on the charts and you should only use ILS within that coverage range which is specific to that airport the Glide slope works on a similar principle to the localizer the Glide so transmitter is set up at the touchdown Zone which is about 300 M down the runway after the threshold or the start of the runway surface with the Glide soap it's exactly the same principle except that the two UHF signals are modul modulated and not VHF signals this time the 90 HZ modulated signal is the one on top and the 150 HZ signal is the one on the bottom so we have more if we have more of 150 htz we get a fly up signal and if we get more of a 90 HZ signal we got a fly down signal if we have equal 150 and 90 HZ then we are on the correct Glide slope and we should have no signals generated telling us to fly up or fly down Glide slopes and glide paths you might hear me say interchangeably are normally set to 3° as this is quite a reasonable rate of descent but they can go up to uh as high as like 6° in places uh I think London city has about a 5 six degree path coming in and some airports might have a you know 3 and 1 half degree could be a 2° that kind of thing but the standard is normally 3° as with the localizer we only want to use the Glide slope within the safe area that it has been tested within which for a Glide path looks something like this we have the angle Theta normally three and the safe lower limit is 0.45 times that angle and the safe upper limit is 1.75 times that angle so for the 3° um we would say that it's probably usable between 1.35 de and 5.25 de and the Glide coverage will extend out to 10 n m distance and it's actually 8° wide either side of the center line This sort of means that we get like a narrowing cone for our reliable signal and we only have one correct line to fly down to end up being exactly on the localizer and exactly on the Glide slope marker beacons are small radio beacons used to cross check the height and distance from the wrongway to make sure you are on the correct Glide path in a modern ILS the marker beacons are not often built because we can use a DME distance to achieve this same effect we can check distances and Heights from DME distance instead of using the marker beacons the marker beacons basically send out fan shaped signals on the VHF band modulated to generate an audible tone in the cockpit or to even make three different lights light up as you pass the markers the three markers are the outer marker which will always light up a blue light and it's always a 4 miles and it's given the morse code Das D Das for o at quite a low pitch the middle marker is always Orange it's at 6 nautical miles with a medium pitch level and the morse code C- do Dash a c because in Morse code m is a single Dash which would cause confusion with the three dashes for zero if it's continuously playing so it's dash dot dash and a middle pitch the inner marker is white at the threshold of the runway and it's high pitched and it's an i for inner dot dot and that be uh her just as you pass over the threshold so when you're flying down the Glide soap you see a blue indicator low pitch you know you're at 4 all miles from their WR way and can check your altitude against as a chart to see that you're on the correct Glide slope ILS installations can be different quality which means that some of them are more accurate than others the receivers on aircraft can also be different qualities as well for Price reasons a Cessna 172 for example isn't going to have the same state-of-the-art equipment that a brand new a350 will when it comes fresh out the factory for example so the aircraft and the airport will have certain capabilities in terms of the category or the level of ILS they can use which is shown in this table here so you got category 1 2 3 A and C um and you've got this thing here which is the MDA SL DH MDA stands for minimum descent altitude and DH stands for decision height that's basically how far you can fly uh how low you can fly sorry before you have to be able to see the runway or some elements of lighting for the runway and rvr stands for Runway visual range it's a very precise measurement of how visible the air is if that makes sense visibility is a strange it's basically a measure of how see through the air is how opaque it is um so obviously zero would be very thick fog and progressively less thick fog if that makes sense so for Cat 1 approach for example we can fly down to 200 ft off the ground before seeing the runway and we have to have an rvr which is given in AIS information that kind of thing of 550 M to give you an example the 77 800 is cat 3A certified meaning that it can land in the lowest rbr of 200 M and it has to make a decision of whether it can see the runway or not uh at between 0 and 100 ft so even if the airport has the equipment for cat 3B approaches the 737800 would not be able to land at it and when using the ILS down to such low altitudes and bad visibilities it is important to make sure that the signal is reliable obviously this is done by creating a safe area around the ILS transmitters where airport vehicles and other aircraft cannot enter when the ILS is in use vehicles at aircraft are generating electrical fields which may disrupt the signals so every ILS will have a critical area surrounding the transmitter for the localizer and the Glide slope where planes and vehicles can enter when the eyelas is being used and outside of this area there's a larger sensitive area where planes and vehicles may cause disruption so the movement of the vehicles and planes is controlled by air traffic control this sensitive area will change with the level of sensitivity being used the category um that so basically for a cat one approach we might have this sensitive area but when the weather conditions degrade to the point where cat 3A levels of sensitivity are needed to keep the accuracy high enough the sensitive area maybe push back a bit so you'll often see holding points at runways that have cat 2/3 holding points to keep aircraft further away from the runway in case of low weather conditions to keep that signal reliably good for the low visibility approaches so there's a few ways we can display ILS information within the aircraft the simplest way is to use an omni bearing indicator this is very similar to how an Obi is used for VR tracking check out the V video for more details on that and basically what happens is the vertical deviation represents the localizer and the horizontal position in space and the horizontal needle represents the Glide slope and our vertical position in Space the needles show our deviation and give us correction information so for this example that would mean we have to fly up and slightly to the right to get back on the localizer and the Glide slope the Obi when used for uh V navigation has full scale deflection which is 10° so in this example if we were using a v we might be about 4° off to the desired track off to the left of this V but with an ILS the scale is actually tightened up it's reduced a bit because we're flying close to the ground and the Precision of our flying needs to be increased so the maximum deflection on the scale horizontally repres represents 2.5 de and for the vertical deviation the maximum deflection is 0.75 de this higher Precision allows for more accurate flying which is obviously safer when close to the ground and when flying on an ILS you have to be accurate to within half scale deflection if you fall outside of this accuracy a go around should be flown and another attempt may be needed or possibly a divert to another airport a horizontal situation indicator and HSI is similar to the Obi except we get a live input of the heading information fed into the display so the compass row moves with us as we turn the localizer information is represented as the same way as it is for a VR we get sort of this arrow with the bar off to the side in this case would be sort of two dots off and that would represent 4° on a v but again that scale is tightened so this only represents one degree deflection off to the left of of the localizer in this case the Glide slope information however is displayed slightly differently it's off to the side in this area here this works the same way as the deviation needle in the Obi but we have this little Diamond that represents this needle instead if the diamond is above you like this then you need to fly up a bit and if it's below the center you need to fly down a bit it's nice and easy it's quite a simple way to display and on Modern primary flight displays you'll just basically get two of these diamonds one representing the localizer and one representing the Glide slope on your fancy digital display with your speed tape your altitude tape your um Artificial Horizon that kind of thing you'll just get the two diamonds that tell you oh in this case you need to stay where you are on the Glide soap you're pretty bang in the middle but on the localizer you need to fly left a bit so there are a few errors in the ILS system but not very very many making them pretty reliable bits of Kit so the first one is just signal disruption and it can come in many forms such as the reflection and bending of the beams caused by vehicles in that sensitive area or the critical area which is right beside the ILS instulation um or reflecting and bending of the localizer and glide soap beam from terrain which is why it's always important to use the localizer and glide soap within the normal protected area or the airports slightly different specif ifed area which takes into account all these mountains and things like that all ILS installations should have comprehensive monitoring systems to check the reliability of all the radio beams being produced by the system this way if they fall below the required standard they can be switched off quickly and another type of approach can be floen or offered by air traffic control instead one of the most common errors is false Glide slopes so false Glide slopes form when essentially side lobes are formed almost like Echoes either side of the desired Glide slope but they're much weaker in strength the ones below are not really an issue because they Echo into the ground so it's unlikely you will ever see uh one below the ones above though are the real issue and if you follow them you'll have a much steeper descent path the first one you're likely to encounter will be uh double the normal Glide path so for instance a 3° Glide the first false Glide path will be at 6° if you follow this six Dee one you'll have a much higher descent rate and likely a much higher speed than you want for an approach as well so to counteract these false slopes we always try to intercept the uh Glide path from below it because there's no false paths below the correct one so if we do it this way we always hit the correct slope first rather than hitting one of these false ones another thing we can do is check the altitude and distance that's why those marker beacons were invented all those years ago uh and we can do this with nowadays with a DME distance so basically you can do a rough eror check or check the chart for instance at uh four KN miles you should show about 1,300 ft on a 3° Glide path if it's a sea level um airport for example and you could do that check the chart it might be different obviously depending on the altitude of the airport another error is back course approaches while they're not really errors they are not used in Europe for approaches as they can cause confusion so back course approaches are error prone let's say rather than an error of the system so basically when the localizer signal is generated an opposite signal is sent out the other side of the transmitter in the opposite direction which is the back course the 150 HZ and 90 HZ lobes are now the opposite way around so if we're flying towards the runway using this back course then the fly left and Fly Right signals are the wrong way around remember more 90 htz means fly right but in this case we get more 90 HZ when we are already to the right of the localizer so our equipment in the aircraft and our indications are all reversed so when the deflection is out to the right that means we've got to steer to the left to correct which makes no sense in my mind so you get used to flying on the instruments with the fly left fly right indications then you do a back course and they're all completely reversed making it quite confusing and potentially be a bit dangerous what you can do with back courses is fly away from them while accurately tracking this could be used as part of a departure procedure for example if we're flying away from the runway on the back course and we have more of a 150 HZ signal we'll get a fly left indication which is normal for the 150 htz that gives us a fly left to fly back on the localizer if we come over here and we get the fly left that's correct cuz it gets us back on the localizer and keeps us tracking away from the runway correctly to summarize then the ILS consists of two parts the localizer and the Glide slope and the frequency range is the VHF one between 108 and 11.95 MHz sharing its space with VR frequencies so the ILS frequencies are always odd decimal places plus5 at 0 05 yes and the even ones will be uh VR frequencies Glide slope uses UHF frequencies but it doesn't matter we don't have to tune them because they're paired with localizer frequencies so when we tune up the localizer we get the gly SL as well what happens is a localizer produces two loes of signal one modulated at 90 one modulated at 150 HZ if we have more 90 we get told to fly right if we have more 150 we get told to fly left if we've got the same amount then we get no fly left no fly right and we're flying right down the center line the standard safe area to use this is 10° either side of the center line out to 25 nautical miles and then within 17 nautic miles it extends out slightly to 35° either side the Glide slope works on the same principle we've got the two modulated frequencies one at 150 HZ one at 90 HZ in this case we h if we have more 90 HZ signal we get told to fly down a bit if we have more 150 HZ signal we get told to fly up a bit the safe area for this is whatever the angle is Theta the lower limit is 0.45 Theta and the upper limit is 1.75 Theta and that extends out to 10 nautical miles and we have an 8° width either side just like the localizer being 10° this is only 8° either side of the center line older systems might have marker beacons uh we've got a white and orange and a blue the blue is the outer at four KN miles always and it will give a three dashes as a signal the Orange is at 0.6 n miles it will give dash. Dash as the signal and the threshold marker is always white and it will give a double dot a dot dot as a signal to represent I for inner these can be used to check your distance to make sure you are on the correct Glide slope you've got different categories of ILS different sensitivity levels Cat 1 Cat 2 cat 3 a cat 3 B cat 3 C cat one is probably the most common one you'll see anything else below is sort of what you would consider low visibility cat 3C probably unlikely to ever get used because if there's zero meters visibility how are you supposed to tax you anywhere what do you like cat 3B you can sometimes get cat 3 a you see quite a bit um but yeah different visibility levels different Runway visual ranges and different minimum Heights that you're allowed to fly to before being able to see the runway cockpit indications you've got the Obi the HSI and a PFD Obi works the same as it does for VR tracking deflection off to the right means go right deflection up means go up the maximum scale is 2.5 de for the localizer 2.5 de deflection would be fully off to the right or fully off to the left and it's 0.75 de for the Glide slope indication this vertical one here HSI works in the same way it does for vs except for the Glide information is brought outside of the HSI and is represented by this little Diamond if the diamond is above you fly up if the diamond is below you fly down and on a PFD a modern Flight Deck glass cockpit you'll just see two of these diamonds one for the localizer and one for the Glide slope again with that maximum deflection of 2.5 de and 0.75 de errors I've just basically talked about you get signal disruption hence the sensitive and critical areas surrounding the ILS system when you want to fly with higher sensitivities we got false Glide slopes which are basically Echoes of this normal Glide slope there's not any generated below but there are some generated above the first one you'll likely to encounter will be double the angle of the Glide slope so for that reason we always try and intersect the Glide slope from a below and we cross check the distance to go from the runway with our height to check that we're on the correct Glide path and whilst not an error back course approaches are not allowed to be flowing in Europe because they can cause confusion basically your equipment starts working opposite to what you'd expect the fly left indication actually means to fly right and the Fly Right actually means to fly left to get back on the center line but they can be used for departure procedures if you want