Question

How do you perform an ILS approach? What is a localizer and a glide slope? How are the PAPI lights used?


Answer

How do you fly an ILS approach?

An ILS approach is what is called a precision approach and to someone who is not a pilot it can be explained very simple. An ILS approach has lateral and vertical guidance all the way down to the touchdown point. The pilot can follow the guidance given to perform a safe landing even though being in clouds for the majority of the time. In fact some ILS approach have a precision category that allows for “auto landing”. This means that the plane can perform an automatic landing based on the signals from the ground. Both plane and pilots need to be certified to carry out auto landings.

To continue down the simple language road, the ILS signals tell the pilot if the plane is high or low or left or right of the ideal path. At the plane gets closer to the runway the ILS gets more and more sensitive. Image it like flying in a cone.

Guidance on an ILS approach – the localizer and glideslope.

  • The lateral guidance is called “localizer”.
  • The vertical guidance is called “glideslope”.

Here is a picture to illustrate.

As instrumentation varies from airplane to airplane it can be confusion to start looking at instruments. Here is a general principle. The pilot will be able to see on his/ her instruments how much “off” the ideal path the airplane is. Flying regulations dictate how much off the path the airplane can be before the pilot can continue the approach. If the pilot is unable to fly precise enough or there is no visual contact with the runway at a stage called “minimum“- then the approach will be discontinued. To discontinue the approach means that the pilot will reconfigure the airplane from landing config to climbing config. Pilots call this manoeuvre for a “go around”. The decision to try again, will often be dictated by regulations, fuel capacity and alternatives.

How do pilots find the ILS?

When flying into an airport with an ILS, the pilots will refer to what is called a “chart” to find information about the ILS. The chart can be paper but these days most charts have been digitalised. On the chart there will be a lot of relevant information for the pilots regarding the specific details for landing on a particular runway. In order to fly a specific ILS, the pilots need to tune in the frequency for the ILS. On some airplane types this is done manually other types have advanced systems where the pilot can select the airport, runway and ILS allowing the airplane to automatically tune the ILS frequency.

In most cases pilots will get the navigation assistance from Air Traffic Control (ATC) and get what is called a vector. As Air Traffic Control are using radar  – it is called a radar vector. A radar vector is where Air Traffic Control tells the airplane (pilots) to fly a specific track. Vectors can also include height and speed chances.The language that pilots and Air Traffic Control are using is heavily regulated. An example of the language can be found here CAP 413

How are the PAPI lights used?

First of all, let’s explain PAPI lights. PAPI stands for – Precision Approach Path Indicators. These lights are found next to the runway and they provides the pilot with a visual indication of how the airplane is placed in regards to an ideal path. One important thing that the pilot must verify is what angle the PAPI lights have been set to and compare this to the type of approach angle that the pilot is flying. In most cases PAPI lights are set to 3 degrees which also in most cases correspond to the angle of the ILS approach.

Here is a picture of what a PAPI light looks like

And this is an explanation of how the pilot interprets the PAPI lights

This is a picture taken at night where you can see the runway lights, approach lights and the PAPI lights.

The lights follow international standards but there can be minor variations. For example the PAPI’s are in some parts of the world a little different and could consist of only 3 lights and called VASI (Visual Approach Slope Indicator) – the principle is the same though, to provide the pilot with a visual indication of where the airplane is in reference to an ideal path.

We hope that was an easy explanation for your questions about – How do you perform an ILS approach? What is a localizer and a glide slope? How are the PAPI lights used?

Happy landings

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