Approach procedure - Landing

The approach procedure – simply explained

Tim Takeoff
20.08.2019
4 pictures
6 minutes

Every second countless aircraft are taking off or landing worldwide. Thunderstorms, cold, fog or high temperatures must – ideally – not affect the tight schedule. An old saying in aviation always applies: Every take-off is voluntary, but every landing a must. To ensure this works smoothly, there are many different approach procedures. Today we would like to introduce the most important ones.

The visual approach or “seat-of-the pants approach”

The simplest approach is naturally under windless blue skies and sufficient daylight: the visual approach. This does have a slightly more professional ring to it than what is dubbed in aviation as the “seat-of-the-pants approach” – in other words, the pilot’s trained eye, gut feeling and confidence in the aircraft.

Ultimately, it’s a matter of finding the runway without any assistance other than the instrument readings and planning the flight path by oneself. Particularly in the USA, this is a technique often practiced in commercial aviation in good weather and on starry nights.

The simplest version: A light

In the early days of aviation, it was enough just to find the runway using simple aids, if one already had an idea where the journey goes. A simple beacon, or rotating beacon, helps the pilot enormously to find the destination, even in poor visibility conditions. This principle is still used today at smaller airports.

Beacons with radio waves: The radio beacon

The radio beacons, which we already introduced in our article about navigation, were developed from this idea according to the same principle. The non-directional beacon (NDB) could even emit radio waves as a simple radio transmitter; these are picked up by an instrument on board which then finds its position. For the approach, the pilot only had to set the frequency of the NDB and fly the right course. This had already fulfilled the lateral part. Now it`s important to be at the right distance and at the right altitude. For this purpose, exact approach charts were created. So, the goal was to stay on course and adjust the distance in accordance with the altitude. This is still practised today at various remote airfields, or as an emergency solution at a commercial airport.

Radio beacons with timing

An advanced version of the NDB approach is the VOR approach. A VOR (VHF omnidirectional radio range beacon – read more about this in our article on flight navigation) does not only transmit circular signals, it also features a kind of “timer” which always transmits a signal when the rotating radio beacon exceeds a certain point. Now the on-board instrument only has to measure the time elapsed between this point and the reception of the main signal in order to know exactly which course it is on. In aviation, this is referred to as a “radial”. This is a fixed course on the compass rose, starting from a radio beacon.

Dense fog

However, since the VOR-DME method is still not fully equipped against measurement inaccuracies, and the pilot still has to compare the altitude and distance by himself, this system can be used only to a certain extent in bad weather conditions. To be able to land properly in dense fog up to shortly before the runway, or even use automatic landing, even more precise technology was needed.

The instrument landing system (ILS)

As history has often shown, the development of such a system during the Second World War was initially driven by military requirements. Beside Germany and Switzerland, the Americans also ultimately developed the instrument landing system (ILS). Several signals are effectively transmitted. Laterally, the “localizer” is used, a type of guiding beam that is aligned conically in the direction away from the runway and keeps the pilot, with increasing precision, in line with the extended midline of the runway.

Even more exciting is the vertical path, where a similar type of guide beam is used, the “glideslope”. These radio transmitters are mounted directly on the runway. It shows the pilot an exact approach angle of 3 degrees to the runway. If the approach starts at an altitude of 3,000 feet (914.4 meter), the correct distance to the runway should be about 9 nautical miles (16.66 kilometres). The localizer is positioned behind the runway to be able to stop the aircraft even after an automatic landing on the middle of the runway. The Glideslope transmitter is located directly adjacent to the touchdown zone of the runway pad. Both signals can be displayed similarly in the cockpit.

The simpler, the safer

To achieve greater accuracy, a DME (distance measuring equipment) is also required to check the exact distance. The instrument landing system (ILS) has technical aids to monitor itself and give an alarm in case of deviations. This means that if the pilot flies towards the ILS with the aid of other radio beacons or assisted by air traffic controllers via radar, he “only” has to follow both signals precisely. The pilot lands in the touchdown zone and can maintain the aircraft’s direction even in dense fog after landing.

Automated landings

ILS technology has been around since the 1950s. Several categories of accuracy were established. This is referred to as CAT I to CAT III, with CAT I representing the most inaccurate variant of the ILS. In this context, “inaccurate” is taken to mean that there is still a minimum altitude below which one cannot go in weather conditions without visibility. Although a fully automated landing is possible, this should only be attempted if visual contact to the runway has been established. This is determined by the minimum cloud base in combination with the measured visibility distance at the airport.

The ABC of the ILS approach

This only changes with an approach of the category CAT IIIb. CAT IIIb requires only minimum visibility after landing to allow taxiing off the runway. In London tests are already underway with CAT IIIc, in which even taxiing on the apron without visibility is automated. However, since the technical effort is too high and cost-intensive, CAT IIIc procedures are currently not in use.

Requirements on board

Of course, an instrument approach system does not consist of ground-based assistance only. Numerous requirements must also be met inside the cockpit. At least three autopilots monitor each other while controlling the aircraft. If one autopilot switches* off, the other two functioning autopilots can still land the aircraft safely.

Of course, the personnel are also trained, and the crews sent to the simulator once a year (read about it this WingMag article). The instruments must be adjusted precisely, and there are exact specifications as to what should happen in the event of the failure of various components. In some cases, even a failed engine may be allowed to land automatically. Other problems result in a take-off manoeuvre. If the weather does not improve or if further errors are made, another airport with intact equipment or other weather conditions must be approached.

Approaches without ground support

Aviation is coming to rely ever increasingly on satellite-based navigation; by the same token, this development does not stop at approach procedures. It brings many benefits, as there is hardly any need for costly, high-maintenance equipment at the airport itself, and a GPS procedure can be set up at almost any airport.

This is referred to as “area navigation”, or RNAV for short, which you can find in the collection of approach charts. The RNAV procedure basically only describes the fact that it can cope without ground support. RNAV-GPS even declares the approach as a purely GPS-based approach.

Precision approaches

Here, too, there are higher approach minimums than for an ILS approach, and for this reason the RNAV approach is still classed as one of the “non-precision approaches”. Only the various types of ILS are assigned to the category “precision approach” and allow automated landings. Of course, there is a strong development trend towards the view that on account of its simplicity and ever-increasing accuracy, the RNAV system can also “mutate” into a precision approach in the future.

If you want to know more about navigation in aviation, just read our article!

You can learn more about how an instrument landing system works from Captain Joe:

by Tim Takeoff

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