How an instrument landing system(ILS) can autoland aircraft.

At the end of a flight, there is always that feeling of excitement. The flight was pleasant enough, with some nice views along the way, but now you are ready to get off and rejoin planet Earth. You look out of the window and glimpse the layout of the world you are about to become part of, perhaps seeing some expected landmarks.

Now that you are lower it becomes more interesting as there is more to see close up. The usual hubbub of activity and announcements as the crew prepares the cabin for landing is going on around you as you continue to enjoy the view. Gradually, however, more clouds are appearing and your view becomes more and more interrupted until eventually, all you see are variations of white and grey. You wonder to yourself, can the pilots see any better than I can? How are they going to land this thing without any visibility? I assume this aircraft uses an instrument landing system(ILS) to get us down. Maybe even autoland.

We've all no doubt heard about instrument landing systems and the ability to autoland an aircraft. But what does it mean?

Why do we need ISL and autoland?

Some parts of the world have a greater need to be able to autoland an aircraft. For example, the United Kingdom and much of North-West Europe are very prone to thick fog. This is mainly due to the cooler air condensing over the warmer waters of the Gulf Stream. In the U.K. the thickness of these fogs, famously the "London Fogs", was further exacerbated by soot in the air and visibility was literally down to a few feet.

This has improved since the Clean Air Act came into force which forbade the burning of smoke-producing fuel. Driven by these conditions the U.K. Government created a unit to investigate the feasibility of an autoland system in the mid-1940s. The flight delays and cancellations caused by these weather conditions were very disruptive and costly.

The Sud Aviation Caravelle was the first aircraft to be certified to Cat III autoland on 28 December 1968. These categories are determined by the number of backup autopilots an aircraft has. Generally, three are required as backups in case one or more fail.

So what do we know about how an aircraft can be brought to the ground safely using technology? Well, we need several different pieces of technology on the aircraft as well as on the ground. Let's look at the ground first.

At most major airports, some or all of the runways will be equipped with an ILS or Instrument landing System. This system very basically comes with two radio beams. One of those beams is located beyond the far end of the runway and is responsible for sending a signal directly down the centreline of the runway. This beam is used for directing the aircraft horizontally left or right until it is lined up with the runway centre line.

The second of the two beams is located next to the touchdown point of the runway, so the point where the aircraft wheels should first settle on the runway. This beam is directed up at an angle of 3 degrees, as this is the angle at which aircraft approach a runway to land. This beam is the benchmark for letting aircraft know whether they are too high or too low in their approach.

The Hawker-Siddeley HS.121 Trident followed the Caravelle with a (CAT IIIA) and CAT IIIB in 1975.

How does ILS work?

You are perfectly correct if you observed no beams in the sky on your last approach in the fog or any other time for that matter. That is why pilots have an instrument on their panel called a VOR or Very high-frequency Omni-directional Range.

This is a very important instrument and one of its many uses is to show a representation of the ILS radio beams as two needles. The horizontal needle will move up and down to represent the horizontal 3-degree glideslope. The other vertical needle will move left and right to represent the aircraft's position relative to the runway centre line. The pilots can then orient the aircraft correctly for the approach and "fly the needles" for a safe landing.

In addition, some airfields may have marker beacons along the approach path to the runway so as to back up the ILS information as far as the centreline and distance to go is concerned. There is usually an outer, middle and inner marker and these will light up on the instrument panel to confirm the progress toward the runway.

The ILS is a great system for getting aircraft lined up and approaching at the correct angle and heading for landing. If a pilot is flying the aircraft, however, they will expect some visual clues to start presenting themselves while the aircraft is still at a height where last-minute corrections can be made or the landing can be aborted.

Put yourself in the pilot's seat of a Boeing 747 on approach to Heathrow. It's foggy, and Tower Bridge was the last thing you saw before you popped into cloud and then fog. You dutifully fly the needles nicely lined up and as your altitude decreases you slow the aircraft, drop the undercarriage and run out all the flaps. The aircraft's manoeuvrability now takes on the attitude of a breeze block, so you need to be sure you are on the centreline and glideslope as the ability to correct your position is severely diminished.

As you pop out of the cloud, you expect to see the Hatton Reservoir ahead of you but there it is over to the right. You are nicely lined up for the Southern Perimeter Road, landing on which is not approved. So you go round and try again.

So what happened there? Well, two things really. Firstly, as you get closer to the ground, the accuracy of the radio beam signals is diminished due to ground and other factors. The second was the fact that the aircraft flying more slowly near the ground is far less manoeuvrable as the lessened effect of flight controls due to less air passing over them. The controls become sloppy and need more exaggerated movements to achieve the same result. Any last-minute corrections to go over further to the right would be met with disaster as there would be no time to get the aircraft correctly lined up again in a stable descent.

So this was the problem. ILS is a great system, but not good enough to land in near-zero visibility conditions.

This brings us to the aircraft systems that are used in concert with the ILS signal to achieve a safe landing in visibility conditions that in the past would have required the pilot to seek out their alternative landing airport.

An older-style analogue VOR instrument shows the needles neatly crossed in the middle to show the aircraft is in the correct space horizontally and vertically.

Autoland and the radio altimeter.

Autoland is a system that takes control of the aircraft's approach and landing using autopilot. During the autoland process, the autopilot will still use the ILS as described above to fly the needles, however, in addition, it will also reference the radio altimeter. What is that I hear you ask. Well, as you probably know the standard altimeter is little more than a barometer measuring air pressure. The higher up we go the less air pressure becomes at a fairly uniform rate, so we assign different heights with air pressure. This works fairly well when you measure in thousands of feet, however, you need something a little more accurate when you are flying near the ground.

A radio altimeter is very much like a boat or submarine depth sounder, it sends down a radio signal directly beneath the aircraft and then listens for the remnants of the signal to come back and then measures the change of phase between the sent signal and the returned signal. This gives the height above the ground directly below the aircraft. The radio altimeter is only used at the beginning and end of the flight as beyond 2,500 feet it becomes ineffective. Above that height, the instrument will have a flag pop up saying Off so that it doesn't confuse pilots during flight.

On approach using autoland.

So back to our intrepid crew landing a 747 at Heathrow. Tower Bridge disappears behind us again for our second attempt.

This time you've made an early decision that autoland might be the way to go. You select this setting on the autopilot panel and as speed reduces you run out the flaps again and dangle the Dunlops. You now have the best seat in the house while you watch the aircraft line up precisely on the needles, horizontally and vertically. At 2,500 feet the radio altimeter leaps into action and your co-pilot starts reading out the steadily decreasing altitude. You picture Kew Gardens below you and start further reducing speed. By Hounslow, you need to be in a steady flight configuration at around 1,000 feet.

ILS coupled with autoland enables aircraft to continue operating in all types of weather.

This is where the radio altimeter earns its right to be on the instrument panel.

Because of its high accuracy, you will be depending on it to flare the aircraft for a soft landing at the correct moment. The flare is when the aircraft nose is raised just prior to touching down. This serves to arrest its downward movement to something that the landing gear can cope with when slammed down onto the ground.

This is one of the trickiest parts of flight and one that pilots train and train for. Flare too early and you can stall over the runway and come down like a ton of bricks. Flare too late and well, the same effect really. Being able to flare at the correct moment is made much easier if you can see the ground.

We won't be able to see the ground which is why we are using our trusty autoland.

Through 1,000 feet and you constantly scan your instruments to check for anything that looks out of place. Meanwhile, you are subconsciously taking in your co-pilot reading off the altimeter readings.

Still pea-soup out there. Approach speed now we are on short finals. Then you hear your co-pilot calmly call out THIRTY FEET! Right on queue, the aircraft nose raises higher to bleed off the speed for the touchdown.

As the spoilers pop up and brakes come on with reverse thrust the autoland will still use the ILS to maintain the aircraft on the centreline of the runway until such time it is manually disconnected.

Another flight managed to operate on schedule regardless of the visibility conditions.