What Do You Know About the ILS?
Maybe you’re a new pilot and haven’t really heard much about the ILS. Or maybe you’re a new instrument student who knows about them but want to get a better understanding of what it is and how it works. Shoot, maybe you’re know exactly what one is and just want to read how I describe it. Either way, lets dive in.
What is an ILS?
ILS stands for Instrument Landing System. So that gives you a clue right there, its a system used to land during instrument conditions. Oh, you want more of an explanation than that? OK, fine. Ever wonder how planes are able to be in IMC and as they poke out of the clouds, they are perfectly set up in a position to land? This is done with instrument approaches and one type of instrument approach is the ILS, which is considered to be a precision approach. The ILS is a system of ground based equipment designed to provide an approach path, both horizontally and vertically to get an aircraft in a position to land, even without being able to see the runway.
Components of the ILS
The ILS has many components that help make up the full ILS system. These can be broken up into 3 categories based off of information provided.
Guidance Information
Navigational guidance is give by both the Localizer and the Glide Slope.
Range Information
Range guidance can be given by either marker beacons or DME.
Visual Information
Visual guidance can be given with the approach lights, touchdown lights, centerline lights, and runway lights.
Now lets talk about each of those components.
Localizer
Image 1
The localizer (LOC) gives lateral navigation. This is what gives the pilot navigation assistance to stay lined up with the runway centerline. The LOC antenna is located at the far end of the runway departure end. Looking at Image 1 (ignore my horrible drawing) you can see both the front course and the back course of the transmitting range. We will focus on the front course for now and talk about the back course later.
The normal reliable coverage of the LOC is 35° on either side of the course centerline up to 10nm front the transmitter and then 10° of either side up to 18nm. This applies to bother the front and back course. You can receive a signal outside of these limits, but they it is not considered reliable. This is good for identifying the LOC’s morse code. All LOCs will have a 4 letter code identifiers, all starting with an I. So for example, in KPIE there are 2 ILSs. The LOC for RWY 18 is identified by I-PIE and the LOC for RWY 36 is identified by I-CWT. Both of which have different frequencies and different morse code identifiers.
Glide Slope
The glide slope (GS) is the component of the ILS that gives vertical guidance to keep a proper path down to the runway. It helps keep guide you down from the FAF to the DA. The GS signal emits in the direction of the LOC front course and the frequency is paired with the LOC frequency. A GS is projection is usually adjusted for a 3° glide path so that it intersects the middle marker (MM) at around 200ft and the outer marker (OM) at around 1,400ft above runway elevation. The GS transmitter is usually located between 750ft and 1,250ft from the approach end of the runway and is off set about 250ft to 650ft from the centerline. Looking at Image 2, you can see that GS transmitter depicted off the left of the runway there.
Image 2
Remaining on the GS will ensure obstacle and terrain clearance. It is best not to descend below the GS because you won’t be guaranteed the obstacle clearance and going above the GS will cause you to have a steeper rate of descent to land. Of course you want to do everything you can to remain on GS, but being slightly above it is still safer than being below it. Once flying below the published DH, you should transition to looking outside and following the PAPI or VASI.
False Glide Slope
Due to signal reflection, there can be a possibility of a false GS, which usually will end up being a 9° or 12° degree angle to the runway. Coming in at a 9° or 12° angle would be a pretty steep descent and you should probably make the decision to go around. The best way to avoid this is to intercept the GS from below, which never puts the aircraft in a position to accidentally intercept the false GS. If for some reason you do intercept the false GS, there are ways you can recognize it. The first clue is that you will be on a much steeper descent, so your rate of descent will be more than 318ft per nm. Second, check your altitude with the altitude you should be at when you are crossing the FAF.
Marker Beacons
Marker beacons are used to give range data along the approach course. There are usually 2 marker beacons associated with an ILS, the outer marker (OM) and the middle marker (mm), however some will also have an inner marker (IM). If there is a back course LOC approach, there will probably also be a back course marker (BC) to indicate the FAF.
Image 3
The OM will normally indicate a location along the LOC course where you will intercept the GS, as long as you are at the appropriate intercept altitude. This is usually around 1,400ft above the runway elevation, which is about 4-7nm from the threshold.
The MM will indicate a location along the GS when you are about 200ft above the threshold. Most ILS minimums are around 200AGL (around 2,400 to 4,200ft from the threshold), so the middle marker is a perfect signal to indicate you’ve reached your DH.
The IM is rare and most GA pilots will never see one on an approach. On a Cat II ILS approach, the minimums will be lower than your average 200ft AG. The IM will usually indicate a point between the MM and the runway threshold that an aircraft has reached his DH, as long as he is on the GS.
In order to receive the signal from any of the marker beacons, you will need a marker beacon receive, like the ones depicted in Image 3. On the receiver, the O is for the OM, M is for the MM, and of course the I is for the IM.
Image 4
When you pass over one of the marker beacons, you will see the appropriate light illuminate and hear an audio morse code. The morse codes are depicted below in Image 4. That way when you are flying, you will have both an audio and visual indicator that you have reached that marker beacon. You can think of them as an electronic crew member giving you callouts at each point along the approach. The following video shows you how the receiver lights up and sounds out the morse code as it passes over each marker beacon. Look at Image 3 while the plane passes over each marker beacon and you can match it to the proper morse code.
DME
DME stands for Distance Measuring Equipment and is used in lieu of the marker beacons to establish other fixes along the localizer course. The DME can be installed with the ILS or it can be used from separate navigational aids. For example, if you look at the ILS in KCGZ, the DME is from the VOR.
Approach Lighting System (ALS)
The approach lighting system is a component of the ILS that helps enhance visibility and guidance to the runway doing low visibility. They are meat to help the pilot transition instrument flight to visual flight and help find the centerline. Image 5 below has all the precision and non precision approach lighting systems. This helps when descending below minimum under FAR 91.175.
Image 5
Failures
With so many components to one system, what happens when some of those components are not working properly?
If the localizer is out of service or fails, the ILS approach is not authorized.
If the glide slope fails, the ILS approach is not authorized, but you can do the localizer approach.
You should look at the inoperative component table in the Terminal Procedures Publication (TPP), for adjustments to minimums due to inoperative airborne or ground system equipment.
Localizer Back Course
So, what is a localizer back course (LOC BC)? If you scroll back up to Image 1, you will see my awesome drawing where I depicted the back course on the other side of the runway where the localizer is. The LOC transmits both in front of the antenna and also behind it. A LOC BC approach will be very similar to a LOC approach, but there are a few things that are different. One difference is that on a LOC approach, the localizer antenna is on the departure end of the landing runway. However, as you can already imagine, on the LOC BC, the antenna will be on the arrival end of the landing runway and will be transmitting from the back of the localizer antenna. So, you can expect the MAP (missed approach point) to be more than a half mile from the LOC antenna. As you fly closer, the needle will more and more sensitive and hard to remain on course. So you won’t be able to fly it all the way to the approach end of the runway. Depending on wether you have an HSI (horizontal situation indicator) or a CDI (course deviation indicator) with an OBS (omni bearing selector), you will have a different way of following your instruments.
Flying LOC BC using a CDI with an OBS
When using a CDI with an OBS to fly a LOC BC, you will get a reverse sensing on the CDI. Meaning, you will fly away from the needle instead of flying to the needle. As the AIM says it, “when flying inbound on the back course it is necessary to steer the aircraft in the direction opposite the needle deflection when making corrections from off-course to on-course. This "‘flying away from the needle' is also required when flying outbound on the front course of the localizer.”
Flying LOC BC using an HSI
Flying a LOC BC with an HSI is so much easier because the heading indicator and the CDI needle are combined. You will set your course needle on the course for the front course. So, if you are flying a front course LOC for runway 9 and your final approach course is 090, then you will set your HSI course needle to 090. Now lets say you are flying the LOC BC for runway 27 and your final approach course is 270, you will set your course needle to 090. This will create normal sensing instead of reverse sensing.
With all of that being said, never try to do a LOC BC approach unless there is a LOC BC approach procedure published for that particular runway and the approach is authorized by ATC.
Hopefully this helps you better understand all the different components of the ILS and how they all work together to guide an aircraft down to the runway when in IMC. If you still don’t understand something or maybe want to discuss the ILS in further detail, head down to comments and keep the conversation going. Also share any cool ILS plates you find to be pretty awesome.