Navigation - Lesson Two
- Enroute: Airways and Intersections -

Route Network:

While the blue sky might seem wide enough for aircraft to fly from one point to another via the shortest possible route, in a straight line, real airliners never do this. This is because there's an invisible road network made of so-called Airways and Intersections. When enroute to their destination, airline pilots must use these invisible roads. The airway system ensures the smooth flow of traffic on the crowded skies. If every aircraft flied whatever direction he wanted there would be a chaotic mass of traffic that Air Traffic Controllers would be unable to handle. By flying along pre-defined routes, both the pilot and the controller knows exactly where the aircraft should be throughout the flight.

Obviously, we would like to fly on the shortest possible route but airways seldom go in straight lines between where we are starting from and where we are going. So, we have to piece together the segments of airways which will take us to our destination. This is what we call flight planning. In the good old days pilots spent several hours bending over their Enroute Charts to devise a flight plan. Nowadays the guys at the airline's navigation department do it for them with the aid of computers. Fortunately, you, virtual pilots, do have a navigation department and a computer too. You can either download flight plans ready to use from the FS SUPPORT section or use FS' built-in Flight Planner or a 3rd party addon, like FS Navigator that you can download from here.

This doesn't mean that you can miss preflight altogether. The ATC can amend your routing at any moment, so you better get prepared. Study your flight plan over and over again until you are familiar with every waypoint on your route, as well as nearby airports and navaids, so that you can avoid nasty surprises. Remember: the instructions of the controller have precedence over your flight plan.

Enroute charts are pieces of paper showing all VORs, NDBs, DMEs, Airways, and Intersections in a certain geographical area. Flight planning software like FS Navigator do the same, but on your computer screen, not on paper. Click here for an example.

Airways:

Airways are simply imaginary paths following radials between VORs or GPS waypoints. There are two kinds of them: High Altitude and Low Altitude airways. In the UK and Ireland, we use Low Altitude airways from FL100 up to FL245 and High Altitude airways above FL245. This is different in other countries, however! In the USA, for example, they use High Altitude airways above FL180.

Tracking an airway is easy: you just follow the VOR or GPS course that's appropriate for the airway segment you are on. There are two things you must keep in mind, however. Motorways don't always go in a straight line from one junction to the other. Neither do airways! Airways can bind actually anywhere, even between two VORs, so you don't always fly on your present course until the next VOR. Many a time I was annoyed at one-way streets in the city that compelled me to take a long way round. I thought I left them behind when I sit into the cockpit, but I was wrong. For whatever reason, some airway segments are one-way! If your flight planner ever calculates a much longer route from B to A than from A to B, it is because of these 'one way streets'.

E.G. our example shows the High Altitude airways around Wallasey (WAL) VOR, one of the airway junctions in the Manchester area. You can see several lines of different colour with letters superimposed on them. They are airways starting off from WAL or just crossing the general area. The solid black line represents our route as we arrive from Dublin on the airway UL975 and continue towards London on UB3.

Intersections:

Just like motorways, airways cross over each other, creating intersections. Intersections are a little bit more than that, however. They are important pre-defined waypoints that we use for flight planning or reporting our position to the ATC. They are much more numerous then VORs and NDBs, so they allow a much more accurate navigation. Actually, we can find intersections everywhere, even where there are no intersecting airways and no radio beacons in the vicinity. They are still called intersections out of nostalgia but in fact they are nothing but standard GPS waypoints.

There are three different methods to find an intersection. The first one only applies to "real" intersections: it requires us to follow a certain VOR radial until we reach another radial of a different VOR. The second method is to follow a VOR radial until we reach a certain DME distance from that VOR. This method requires a VOR/DME station. The third way, that we can use anywhere in the world, is the GPS way.

E.G. if you look at the map once again, you will see several little triangles with short names under them. They represent intersections. NOKIN in the middle is a typical intersection, where 3 airways meet: UN862, UY98, UY99, while MONTY to the SW is just a waypoint on UA25 defined by a radial and a DME distance.

Flight Planning:

As we've mentioned before, flight planning is an important part of preflight. Now, being familiar with airways, intersections, and flight levels, you are ready to create a flight plan. But there is one thing we have overlooked so far. You might have already noticed, that you can specify your route either by waypoints or by airway segments in between. Which one should we use? Both, but for different purposes!

In order to be able to fly your route or program your FMC, you need a flight plan that contains all the waypoints, since neither your VOR reciever nor your FMC can recognize those imaginary airways. Air Traffic Controllers, however, are much more intelligent - and much more busy - so they don't bother with scores of waypoints. They use SID, STAR, and airways designators instead. So the flight plan that you submit to the ATC when flying online should be made up of airway and SID/STAR designators.

Example Flight: Dublin - Heathrow using SIDs and STARs in ATC environment

• Flight Plan containing Navaids (VORs, NDBs, ISECs) only:
  EIDW - OE - DUB - LIFFY - GINIS - NATKO - LYNAS - ROLEX - WAL - HON - WCO - BNN - EGLL

• Flight Plan containing Airways and SID/STAR designators:
  EIDW - LIFFY 2A - UL975 - UB3 - BNN 1A - EGLL

Now, you know the principles of enroute navigation and flight planning. When you fly in ATC environment, create your flight plan with every precaution and submit it in the right format to make ATC amendments unnecessary. A perfect flight plan is a sign of professionalism and may result in better service. If the controller still suggests you a revised routeing or a different altitude, please accept it without arguments (if you can), as the last word is always the controller's word.