EXPECT ILS APPROACH RUNWAY 27 LEFT

I confess that in 1901, I said to my brother Orville that man would not fly for 50 years.
Wilbur Wright

He had proved himself wrong. The Wright brothers invented something in 1903 which went on to revolutionise communication across the globe forever.

Today it’s raining hard outside. The window has turned hazy. I am trying to see through the mist but everything is a blur. All of a sudden, a strip of black tarmac speeds past underneath. The seat gives a jolt and voila, the plane has landed.

I believe everybody who has flown in an aeroplane has wondered how the pilot manages to land the aeroplane at the correct place, let alone perfectly. In 1929, Dr. Kramer and Dieckmann had their eureka moment when they invented the Instrument Landing System, or commonly known as the ILS. This simple, yet elegant way of tuning of frequencies does wonders in the safe landing of an engineering marvel on the ground.

Moreover, the ILS is a boon for aircrafts during approach with reduced visibility. Imagine yourself inside an aeroplane when it is landing at Kochi airport during heavy monsoon or at Delhi airport on a very foggy night. Doesn’t it send chills down your back thinking of how the plane will land when the pilots can see no more than you do outside the windows? But ILS has evolved a lot since its inception and modern-day technologies have made landing possible with no visibility at all!

So let me take you on a flight where I’ll talk about the ILS.

Fasten your seatbelts. We are cleared for take-off on 21R… Full throttle… V1… Rotate!!

The ILS is the most reliable and most used of all the landing systems operational today. The ILS consists of approach lights, runway markings and above all, radio frequency(RF) signals. The RF signals are the most interesting of all and this what you’ll get to know on this flight. In essence, it has three components: the localiser, the glideslope and marker beacons.

The localiser is an array of antennae located at the far end of the runway(opposite to the end where the plane lands) which provides horizontal guidance. It operates at a VHF(Very High) frequency of 108.1-111.95 MHz(carrier wave) and helps the aircraft align with the runway centreline. It sends out two lobes of frequencies at 90Hz and 150Hz which are modulated onto the carrier wave.

The ILS receiver on the aircraft interprets these signals. Relatively higher amplitude of either frequency is regarded as a deviation from the centreline (refer to the picture above). The pilot then corrects his path accordingly to equalise the amplitudes.

Is that all?

No. Although the plane might be centred but what about its altitude? It might be too high and overshoot the runway or it might be too low and crash onto the terrain. The glideslope is the answer.

It provides vertical guidance and ensures that the aircraft maintains a 3⁰profile with the runway so that the plane can touchdown smoothly at the correct spot. The glideslope antennae are placed to the side of the touchdown area about 1000ft down the runway. It works in the exact same way as the localiser does, the only difference being both of their signals are transmitted mutually perpendicularly and the glide slope transmits carrier waves at Ultra High(UHF) Frequency of 330.95-334.7 MHz. The modulation frequencies are the same, i.e., 90 and 150 Hz. If you have understood the principle behind the working of a localiser then these two pictures are self-explanatory!

(I sound a bit like the textbook authors, don’t I?)

Let me sow a seed of thought now.

Is there a reason why the glideslope uses UHF instead of VHF?

Well, yes and we have all read about it in our high school physics course. The size of the antenna and the frequency of the transmitted wave are inversely proportional. If it were to use a VHF, the glideslope would have to have pretty large antennae close to the runway touchdown area and that always possesses a problem.

The last component that remains are the marker beacons which are placed on the final approach path. They consist of the outer, middle and inner markers. They transmit vertical signals at 400Hz, 1300Hz and 3000Hz respectively at a carrier frequency of 75MHz. When the aircraft intercepts these signals, the pilot gets to hear sound beeps which confirms a correct ILS approach towards the runway.

Well, there you have it. The aircraft is now moving through an invisible funnel of signals, squeezing its way towards the runway.

This was all in a nutshell about how an infinitesimal part goes into the operations of an aircraft.

Prepare for landing on 27L.

50… 40… 30… 20… 10… Retard… Retard

I hope you have enjoyed your flight.

Over and out.

By Arumay Biswas (Department of Chemical Sciences, IISER Kolkata)

References:

About the Author:

I am Arumay Biswas, a 4th-year Chemistry major from IISER Kolkata. I consider myself to be an aviation fanatic and I feel guilty to find aeroplanes more interesting than Chemistry itself. I hope to travel around the world and experience different food cultures. Apart from planes and food I like studying(sigh!), cooking, listening to Indian classical music and reading Harry Potter.

He has previously contributed to the blog: Life @iiserkolkata!