How do pilots align planes with runways

When tourists land by plane on the small Greek island of Samos, the pilots will have landed the plane in the classic way from the outside: A radio beacon guides the planes close to the island and as soon as the airfield comes into view, the pilots fly a kind of traffic pattern Landing. Nothing other than what private pilots or flight students do with their small aircraft at many airports in the country for exercise. What if the weather is bad? Then it is not possible to land on Samos!

How do planes find the runway when visibility is poor?

Philip I.

So that pilots do not have to rely on the outside view, the first procedures for flying according to instruments were introduced around 100 years ago and thus also procedures for instrument approaches. This is how the planes find their way in the sky, even when clouds are in the way.

In essence, all types of approaches are about getting the pilot safely to a position from which he can see the runway and land successfully. The potential inaccuracies in this process must become smaller and smaller, the lower the altitude and thus also the distance to the runway.

Advantages and disadvantages of funk

The simplest version is the option in Samos: A radio beacon guides the crew roughly in the direction of the airport - a point from which the crew can look for their own way to the runway. This may well contain one or the other curve at a very low height. Setting up such a procedure is very easy and cheap for the airport, but it also has many disadvantages.

Also read: How do planes find their way in the sky? Answers from the cockpit

For one thing, the weather has to be good for an airplane to land. On the other hand, it is not certain how exactly the aircraft will fly from visual contact with landing. This means that air traffic control can only ever send one aircraft on approach, which causes traffic jams when there is a lot of traffic. In this respect, these methods are only used where no other methods are possible.

Non-precision approaches - an established classic

Aircraft can be guided to the runway much better with so-called non-precision approaches: A radio beacon is located directly next to the runway or in the extension of the landing direction in front of / behind it. This radio beacon is selected in the cockpit and steered on a certain course - ideally it is exactly the landing course.

Since the display accuracy of these radio beacons is not that great, the pilots must be able to see the runway or parts of the approach lighting at a height of at least 250 to 300 feet in order to be able to make corrections if necessary. But this is not the only reason for the name "Not Precision" - it comes from the fact that there is no vertical guidance: Trusting that the navigation is correct, the crew must leave the altitude at a certain point and begin the descent.

During the descent there is only the altimeter and a table with altitude values, depending on the distance to the runway, which is checked with the distance values. If there is a radio system to determine the distance ...

The aircraft slowly descends in the direction of the runway until the crew can see it or the approach lighting no later than 250 feet above the runway height. But what if the clouds are too low? Then the crew has to take off. This is the only safe option. Unfortunately, attempts to go deeper in order to still be able to see something have repeatedly led to serious accidents.

The instrument landing system

For years, an instrument landing system has been the method of choice for better and safer approaches. The so-called "ILS" offers the pilot two beams: The landing course transmitter shows exactly the way to the runway. The glide slope transmitter - and that is the big difference - provides height guidance and guides the aircraft down precisely so that they can touch down exactly in the landing zone.

The accuracy of the ILS system is now so high that it is possible to have aircraft land automatically. In extreme cases, the pilots do not have to see a few lights of the center line lights until they have touched down. In order for this to be possible, the airport, the weather, the airline, the crew and the aircraft have to meet very high technical requirements. You can only land automatically if all of these requirements are met.

Conversely, this means that a very large number of airports and airlines cannot offer these procedures. The airport in particular is very limited, as it has to keep large areas free around the runways for this type of flight operation so that the signals can be transmitted undisturbed. It will therefore only create the conditions for automatic landings if it is absolutely necessary.

Does an instrument landing system also have disadvantages?

An instrument landing system has two serious disadvantages: On the one hand, every airport has to carry out complex and expensive installations for every runway. It's not worth it if there is very little traffic or if it is rarely foggy. The second disadvantage is that an ILS can only allow a straight approach for physical reasons.

These disadvantages make use of new approaches: The so-called RNAV or RNP approaches require little or no infrastructure on the ground and can be flexibly set up. The basic principle is quickly explained: Such an approach consists of arbitrarily positioned waypoints and an altitude guide. The aircraft flies off the chain of waypoints and follows the vertical guidance alone. So curves can easily be built into the approach.

A new factor is critical here: How exactly can the aircraft follow this approach path? The abbreviation RNP stands for "Required Navigation Performance" and replaces the conventional RNAV approaches (= Area Navigation). The more precisely the aircraft can navigate, the more flight routes and lower cloud bases are possible. Prominent examples of approaches that were only possible with these modern means are the approaches through the mountains of New Zealand to Queenstown Airport.

A special form is an approach that is basically based on GPS, but with a local correction transmitter it becomes so precise that it is in no way inferior to a simple ILS in terms of accuracy - and can therefore be flown up to a cloud height of only 200 feet and visibility of 550 meters. These so-called GLS approaches (Ground Based Augmentation System, GBAS, Landing System) have the advantage that they can be guided in curves. However, so far only a few types of aircraft have been equipped with the necessary technology as standard.

About the author

In the series "Answers from the Cockpit", airliner pilot Nikolaus Braun regularly answers questions on pilot topics relating to aviation technology and flight operations. If you also have a question, write to [email protected]

Nikolaus Braun is a pilot with a large German airline and currently flies on the Airbus A330 / A340. The studied Dipl-Ing. (FH) for aviation system technology and management also advises part-time with his company Nikolaus Braun Aviation Consulting (NBAC) on projects in research, development, legislation and teaching.