Due to the heavy traffic flows in Europe, Standard Air Traffic Control and Preferred Routes have been brought in to make the system more easier to plan. Aircraft are expected to comply with these routes when flying between airports  the only variation allowed is joining and leaving for Take Off and Landing. Aircraft cannot jump on and off of these routes , except of course in an emergency. They have to stay on one route throughout. 

These routes have been renamed RAD routes now

In addition to that there are Traffic Avoidance Routes in operation. This is normally where hostilities are ongoing. Normally where NATO or the  UN  need the airspace, so they can do their job without endangering civil traffic etc. An example of this would be the East Coast of Italy down towards Corfu and Bosnia Etc.

Pilots therefore have to be very skillful in flight planning now. A list of these routes are contained in Aerad Europe and Middle East Supplements. Airlines normally have a planning department that do the flight plans for the pilot but the Captain is responsible for checking it.

 ARS = Avoidance Routing  System

Pilots on take off fly the Standard Instrument Departure  (SID) onto an airway which is 10 miles wide. This airway may or may not be part of the RAD or  ARS system but eventually will lead on or off of it. Then when the pilot  gets near his destination he will fly the Standard Arrival Route, and then the approach pattern. At busy airfields radar headings are given by Air Traffic Control who steer the aircraft onto the centre line for landing.

It should be noted that North and Southbound routes do not  always follow the same track. Lower Airspace routes are not always the same as higher airspace routes. For example Traffic from  Portugal  to Manchester could route Northbound from waypoint Kathy( near  the Isle of Wight) To Ockham VOR then Brookmans Park VOR then Trent VOR for the Dayne Standard Arrival .  Southbound Traffic would use a Monty or Niton  Standard Instrument Departures , This  would route out via Brecon VOR then Berry Head VOR near Exeter then direct to Santiago VOR on the north west tip of Spain.

These routes change on weekends and off peak times and are not always the same routes as the weekday peak times.

Why is it Needed ?

Imagine all charter aircraft from UK ,Holland, Germany ,France, Austria and Scandinavia wanting to go to the Canary Islands or Cyprus, all at the same time.  The direction is South West to South East. Then add to that the Traffic in Italy, Switzerland, France wanting to go North West to the UK or over flying over the UK to the Atlantic tracking system for America should give you the general picture.

The UK and Paris/ Brussels areas are  bottleneck areas, hence this system was devised to ease the flow. Slot times are often given to aircraft. This means aircraft have to take off at the time given or go to the back of the queue again if it is missed. A flight would have to be accepted by every Air Traffic Control Centre that handles  the aircraft on the route before Air Traffic Control can give the pilot his clearance. So emergencies or bad weather can soon have a knock on effect to aircraft.  If things are running late , gates do not become available for aircraft at airports which delays the aircraft arrival. This then affects the turn round time for the aircrafts next flight departure time. Coupled with that pilots can go out of hours and flights could be cancelled if a fresh crew is not available . This could cause problems for airports not in the affected areas as flights then start becoming delayed due to the late return of the previous aircraft off another route.  This aircraft may be  flying next to the airport in the unaffected area. It  therefore takes off late and then causes problems on arriving  late at the destination as a gate may not be available.
Welcome to the real world of aviation !

The Aerad supplements are  at a reasonable price well worth getting. They list all the airports ICAO codes and entry and exit points for flying over various countries of Europe. They are also available for others areas of the world. 

The RAD system has a link below. These routes are changed from time to time you need the acrobat reader to read the RAD. Maps