Flying in low visibility? No problem, we’re still landing.

Ever taken a flight where outside, all you can see is grey, with your aircraft’s wing hardly visible through the assault of the rain or dense fog? Felt a bit of terror when landing in these conditions? Wondered just how did the pilots get you on the ground safely? Well, wonder no more. 

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The aviation industry is very high on safety — you can’t even install a coat hanger without spending thousands of dollars in testing and certification. It is not surprising, then, that it would hardly take any precautions in fortifying itself against the most fearsome of forces in the world — nature. When you are landing in low-visibility conditions, whether caused by rain, fog or haze, you are actually pretty safe thanks to the Instrument Landing System (ILS) present at the majority of major airports and inside most modern aircraft. But how does it work?

The localiser array
The localiser array

On the ground, located at the far end of the runway is a set of radio antennas. These emit two narrow beam signals on 1 of 40 ILS channels, one modulated at 90Hz and the other at 150Hz. Together, they make up the ILS localiser (LOC). This system gives lateral (left and right) guidance to the pilots, telling them if they are on the correct course to the runway or how far off they are from it. Of course, to receive guidance from it, the aircraft must have the necessary radios on-board. This LOC transmission is usually in the range of 110MHz, and appears on cockpit instruments most commonly as a horizontally moving diamond below the artificial horizon (attitude indicator) when the aircraft is making its approach to the runway.

The glideslope array
The glideslope array

The other part of the ILS is the glideslope (G/S). This is another antenna array, but unlike the LOC array, it is situated at the side of the runway’s touchdown zone. It, too sends out two signals modulated at 90Hz and 150Hz, and transmitted at around 330MHz. This system tells pilots how accurately the aircraft is following the ideal glide-path (around 3 degrees from the horizontal) and manifests as a vertically moving diamond to the right of the artificial horizon. Both the LOC and G/S frequencies are paired together — the pilot (or aircraft) only has to manually tune the LOC frequency, and the G/S signal is automatically received when in range.

What the pilots see
What the pilots see

When pilots tune their aircraft instruments to receive the ILS signals, they must also ensure that they’ve got it for the correct airport and the correct runway. This identification is done via the localiser antennas, which periodically transmit a 1020Hz morse code signal. This lets users know the facility is operating normally and that they are tuned to the correct ILS.

There are, of course, limitations in the implementation. Due to the hazard posed by obstructions (e.g. buildings) which can bounce around the ILS signals, the approach paths must be cleared of them which can end up being very costly.

The category of ILS available at the runway is displayed on taxiway signs
The category of ILS available at the runway is displayed on taxiway signs

The ILS is also differentiated based on its sophistication. There are three classes of ILS – Category I, II, and III (with sub-categories IIIA/B/C). The higher the category or sub-category, the more automated the landing system is, which means that it has a lower threshold of minimum visibility at the airport required.

And that’s it! So now you know how pilots transport you safely onto the ground even when visibility is extremely low. No need to get scared in the future!

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