Without a doubt, ILS is the most famous aid aeroplanes have in approach. Although we generally refer to it by its acronym, its full name is Instrument Landing System.
Through ground stations and onboard equipment, ILS provides pilots with vertical and horizontal guidance a few meters prior to touch down, which increases safety in severe weather conditions.
When referring to the vertical plane, it is termed glideslope or glide path and when we refer to the horizontal plane, we call it localiser.
As mentioned earlier, ILS is formed by two signals, one is for vertical guidance and the other one, for horizontal guidance.
The infrastructure for ILS on ground comprises an aerial which provides guidance in the horizontal plane, normally this is 300 metres from the runway end and on the other hand, a second aerial for the vertical plane which can be found in the header of the runway.
Localiser aerials emit between 108.1 MHz and 111.975 MHz and are modulated so that the aeroplane can detect its position with respect to it. If the aeroplane receives a tone of 90 MHz, it will be to the left of the localiser, whereas if it received a tone of 150 MHz, it’ll be on the right.
The aerial that provides information about the glideslope (vertical plane) emits the information at frequencies between 328.6 MHz and 335.4 MHz.
The ILS transmission frequencies emitted always have odd number decimals: 109.9, 110.3, etc. This is due to ILS and VOR sharing a range of frequencies so that it is easier for pilots to identify them.
As well as the localiser and the glide path, ILS can offer distance through DMEs or marker beacons.
From a localiser, we can find up to three beacons that allow us to establish the distance at which the aeroplane is on the runway.
It can be found at between 4 and 7 miles, corresponds to blue and its identifier is two dashes per second.
It is situated 0.6 miles from the runway, is associated with orange and its identifier is dash and dot.
Its placement is more optional, it is at 0.24 miles from the runway and its colour is white. Its identifier is dots.
At present, marker beacons are being deactivated, as the majority of ILS have incorporated DMEs. We recommend not missing our post regarding VOR to find out about what DME is.
As mentioned above, ILS allows pilots to descend to a certain height before landing and this is known as a ‘minimum’.
Once minimums are reached, if the runway or its lighting system are visible, landing can continue. If, in contrast, they cannot be seen, pilots would then commence a frustrated approach.
Minimums change according to the airport’s certification as well as the aircraft’s. In some cases they can even be 0 feet; this is what is known as autoland, or the aeroplane landing automatically.
Although it looks like ILS will go on being used in years to come because of the precision and reliability they offer, airports with more complex orography will be substituted by RNAV approaches, as they are more flexible.
We hope you have learnt a little more about ILS and precision approaches and if you’d like to carry on learning, we recommend our post on LVP or low visibility procedures as we are convinced you’ll find it interesting.
