Flight Levels and Directions of Flight

by Lluis del Cerro 

On land, persons and vehicles normally use the roads to move between two points. Both roads and highways are built on the ground, physically visible and we move around following them.

Most aircraft move across the air following paths called airways in a similar way as cars move on the highways. These airways, however, are invisible and depicted in the enroute charts by means of navaids and intersections. These navaids are, basically, VORs and NDBs, while intersections are normally points defined by distances and radials to navaids or geographical co-ordinates.

Many official aeronautical administrations have public web sites where all real charts can be obtained for free. You may find most of them in the VATSIM Chart Center. Also, from the EUROCONTROL∞ web site you can access the EAD site and obtain ALL the charts for all the European Union states and other not belonging to the EU as well. Just to clarify, the United Kingdom also belongs to the common european airspace and all what you will read in this article is to be applied for this State.

Some countries (like France) include all information in the enroute charts while others (like Spain or Germany) only display the directions of flight for the airways. Nevertheless, the flight levels can be found in other documents providing even more detailed information. For those charts where all data are depicted, we must look at the legend to find out the abbreviation used for the even and odd flight levels.

The enroute charts themselves are coded and grouped under ENR 6 section, normally for lower and upper airspace separatedly. The documents containing the information on the levels of flight are coded and grouped under ENR 3 - ATS ROUTES section and the airways are listed there in alphabetical order. Thus, in those cases where the flight level is not depicted in the charts, the first thing we should do is find out the basic designator (see next paragraph) and then search for it alphabetically in the ENR 3 documents.

Airways may cross a continent completely and have portions or segments of variable length which run between navaids or intersections. Each airway has a name or code (also known as BASIC DESIGNATOR) formed by a letter and a number between 1 and 999. The basic designator may have a prefix, U for example, to indicate that it belongs to the upper airspace, and an additional suffix letter, F for example, to indicate that it is an airway where only flight information service is provided, for example UG600F.

Within a FIR/UIR, the border between upper and lower airspace is normally at FL245. Thus, lower airspace is below FL245 until land or sea level (AGL/MSL) whilst the upper airspace is above FL245 until UNL (unlimited). Within Flight Information Regions (FIR) we would find the so-called Control Areas including, among others, the terminal areas (TMA) and airways (AWY).

For the purpose of this article, an airway may exist in the lower airspace, upper airspace, or both. The lower limit of an airway (in lower airspace) is given in the Aeronautical Information Publications (AIP). For an airway in the upper airspace, the upper limit would be FL460, which is usually the limit of controlled airspace. There may be airways in both airspaces with the same route, but their respective basic designators will be different depending on the airspace where they run through.

An airway may be single or double direction of flight (along all its length or part of it), which means that an airway may be flown in one direction (only west to east, for example) or in both. In the enroute charts it is depicted whether an airway has single or double direction of flight, as in case of single direction we may see an arrowhead pointing the correct direction of flight, whilst the double direction airways do not display any arrowhead.

Lets deal now with the flight levels of the airways:
Normally, all airways have a MEA (Minimum Enroute Altitude), which is the minimum allowed level to fly that airway ensuring clearance over obstacles and radio reception which is determined by the MRA (Minimum Reception Altitude). The MRA warrants that the navaids will be received at least 500 below the MEA. The altitude of the MEA is normally depicted in charts in a different color (usually blue) and is placed beside some segments of the airway.

Also, and for Europe, we can see "E" and "O" or "A" and "B" to show the levels of flight, and the symbols ">" and "<" which are used to display the directions of flight. According to this, "E" or "B" stands for EVEN flight levels (for example: FL240, FL260, FL280, FL300, FL320, etc.), whilst "O" or "A" stands for ODD flight levels (for example: FL250, FL270, FL290, FL310, FL330, etc.). The symbols "<" and ">" are usually depicted beside the "E" or "O" and are used to indicate whether that segment of the airway must be flown on even or odd flight levels in the direction pointed by the symbol. However, this is only an example and depends on the authority who publishes the information. What we have to do is look at the legend to find out the way it is depicted in the chart (if it is).

What to do when the flight levels are not depicted in enroute charts?
Unfortunately, this is the most common situation. As far as I know, only French enroute charts (by far the best of Europe, in my opinion) have all information in it.

When the enroute charts obtained from section ENR 6 of the AIP do not depict the flight levels, we must go to section ENR 3 of the AIP. Normally we will find two types: Lower ATS routes and High ATS routes. Depending on the airspace we are flying (Low or High), we will choose one or the other. We may also find a single document entitled "Area Navigation (RNAV) Routes", which will be the ones we will use normally. These documents are text and list the airways in alphabetical order. For each airway all its waypoints (navaids or intersections) are listed following the airway route. These documents are build in columns suplying diverse information about the airway or its waypoints like tracks, distances, minimum/maximum altitudes and, of course, flight level to be flown depending on the direction of flight. For this, just look for a column normally labelled directions and divided into Odd and Even, and follow the indications for filing and flying a realistic flight.

Using the ENR 3 documents is very accurate but may take some time in long flights involving several countries. Even in one country (Germany, for example) the same airway may change its flight level when reaching one of its waypoints.

The situation is different in the U.S.A. and Canada where the charts depict an arrowhead when an airway is for a single direction or without the arrowhead if it has a double direction (which is normally the case). Similarly, the flight levels in the USA and Canada are assigned in the upper airspace (above FL180) in the following way:

Levels below FL290:

  • Course 360 to 179: F190, FL210, etc.
  • Course 180 to 359: FL180, FL200, etc.

Levels above FL290:

  • Course 360 to 179: FL290, FL330, etc.
  • Course 180 to 359: FL310, FL350, etc.

Flight Level Conversion Chart∞ to be used ONLY when no enroute charts available

Directional Airways
Although we are talking about airway segments, only a few of them change their flight level along its path. Thus, for a certain airway, we will normally fly the same flight level during all its length (even or odd). Also, normally, when an airway ends and starts a new one, the latter uses the same levels of flight as the former one.

Let's see an example of a simple flight:

We are now going to prepare a flight plan from LFPO (Paris Orly) to LEBL (Barcelona El Prat). A real route may be the following (Check Vroute∞):


Now you should get your enroute charts for France from the official SIA website∞ or from EUROCONTROL∞ EAD page. In this case, better get the enroute chart for high airspace from the French site. 

ERIXU is the SID (Standard Instrumental Departure) waypoint where we will join the first airway (UN860) of our route. As you can see in the French enroute chart, the airway UN860 has a grey arrow pointing south in our direction of flight. As you can see in the chart legend, this means that this airway is for Odd flight levels in that direction.

At ETAMO we leave the airway UN860 to join the UN855. Also there we see the grey arrow point south in our direction of flight. Thus we must maintain an Odd flight level in this segment.

We have an altitude restriction to cross PPG VOR. It is shown in our flight plan route as PPG/N0410F250. This is to improve traffic flow and aircraft destination LEBL should be at FL250 or below at PPG VOR. We have almost reached the end of our journey at ALBER, the last mandatory reporting point between Marseille (LFMM) and Barcelona (LECB) FIRs. Normally, shortly before reaching this point we will be transferred (handoff) between LFMM_CTR and LECB_CTR, being the latter facility which will issue the appropriate STAR (Standard Terminal Arrival Route) via ALBER to our destination, as this is the last fix we have in our flight plan route and a valid STAR. At ALBER we leave the airway to start the sandard instrumental arrival procedure which will end with a safe landing on the active runway at Barcelona (LEBL).

This has been an example on the right way to fly a flight plan route using airways and following correctly the directions and flight levels for each airway.

Now another example a lot more complicated::

We will now fly from Perpiny (LFMP) in France to Vilnius (EYVI) in Lithuania. This flight is 1.275 nm long and the route is the following:


Our SID waypoint is FJR VOR and our STAR fix is OMETA. Depending on LFMP_TWR or the winds at our departure airport, we will choose the appropiate SID. Now, let's prepare the correct flight levels to fly our route:

We get the ENR 6 chart for High French airspace and locate FJR VOR in south France, close to the Mediterranean sea, where we will join the airway UM731. Looking at the enroute chart, we can see a grey arrow pointing southeast, our direction of flight, what means that this part of our route is for ODD flight levels. We note this in the route in the following way: FJR/N0300F330. This means that over FJR we expect to fly at a speed of 300 knots and at FL330. The speed is merely an indication and does not need to be precisely followed. The important point here is the flight level notation.

Our next waypoint in UM731 is DIVKO, where we will join the airway UM984. As this is still French airspace, we look at the enroute chart to choose the correct flight level. The airway UM984 has a grey arrow pointing southwest, meaning that Odd flight levels are in that direction, the opposite to ours. Thus, here we must change to an Even flight level. Thus, we note this in our route as DIVKO/N0300F340.

Our next waypoint in airway UM984 is PADKO where we will join airway UT24, still in French airspace. The airway UT24 has the grey arrow pointing in our direction of flight. Thus, we need to change our flight level to an Odd one. We note this in our route as, for example, PADKO/N0300F350.

The next waypoint is ADITA where we will leave UT24 to join airway UN870 to MAXIR. Following the same procedure as before, we must change to an Even flight level noting it in the route as ADITA/N0300F360.

Going ahead planning our route "as it should", we reach MAXIR where we will leave UN870 to join airway UN853. Having a look at the French chart, we see that the grey arrow points southward. As we are flying the opposite direction, we must fly an even flight level. As we were already in an even flight level at ADITA, nothing must be changed in our route.

Flying the airway UN853 we will reach MOBLO where we will enter Swiss airspace. The wonderful French enroute chart is no longer useful. As the Swiss enroute charts do not include the flight level indications, we must unse the ENR 3 documents. For this we use the ENR 3.3 document entitled "Area Navigation (RNAV) Routes, as we are flying a brand new aircraft fully equipped to fly any RNAV procedure.

We press the "search" button of Adobe Reader or we browse the document. Keep in mind that the airway designators are in alphabetical order and it is not so difficult to find what we are looking for. Also be careful when using the "search" button as some countries write the airway in a single part (UZ662) and others in two parts (UZ 662). It may happen that the Adobe Reader cannot find the designator depending on how it is written.

Alright, now we found our airway UZ662 and we see that in Swiss airspace it runs from MOBLO to KORED and column number 5 has two subcolums labelled "Odd" and "Even". If we move down to our UN662 airway, we see that the arrow points from MOBLO to KORED in the subcolumn "Even", meaning that we must fly even flight levels between these two intersections. As we were already flying an even flight level, nothing must be noted in our route.

At KORED we leave airway UZ662 to join the UN871 still in Switzerland airspace. The same procedure explained in last paragraph is to be followed in this case. Thus UN871 between KORED and KUDES is for even flight levels. No change must be done in the route.

At KUDES we enter in German airspace to join airway UN851. As German enroute charts do not depict the flight levels, we must use again the ENR 3 documents. We will find this airway in ENR 3.3 again for RNAV routes. Once we have located it in the document, we wil see the same columns (same European airspace, same notations) as we saw for Switzerland. In German airspace and airway UN851 we will fly from KUDES to LOHRE. Having a look at colum 5 we see that the arrow goes from KUDES to LOHRE in the subcolumn for even flight levels. Thus, no amendment must be done in our route.

At LOHRE we are joining the airway UN746, still in German airspace and thus using the same ENR 3.3 document. We search for this airway and in column 5 we can see the arrow going from LOHRE to ESOBU and in the subcolumn for Odd flight levels. Thus, at LOHRE we must change our flight level to an odd one and note it in the route as, for example, LOHRE/N0300F370.

Well, we are reaching the end of our flight planning and only remains the segment of airway UN858 between ESOBU and OMETA. However, this is a long one and running through Germany, Poland and Lithuania.

The UN858 runs in German airspace between ESOBU and SUI VOR (look at the "Waypoints" tab in VRoute∞ detail for this route). Thus, in ENR 3.3 document for Germany we see in column 5 that this segment must be flown in odd flight levels. No notations must be done in our route.

At SUI VOR we enter in Polish airspace. Thus, we look for ENR 3 documents for Poland. In this case, the RNAV routes for High Airspace are in document ENR 3.4. We search in it for UN858 and we see that our route runs in this airspace from SUI to BOKSU. The notation in this document is slightly different but easy to understand: we have a column labelled "MNM FL" and two subcolumns in it with arrows pointing the direction of flight. If we look at the segment of our interest, we see that the column for even flight levels is darkened and cannot be used whilst the column for odd flight levels says "minimum FL290". This means that we must fly at an odd flight level above FL290. As since LOHRE we were at an odd flight level, nothing must be changed in our route.

Finally, betwen BOKSU and OMETA we are in Lithuanian airspace. Although the ENR document for this airway in Lithuania says that we must fly it in odd flight levels, likely we had already started our descent so nothing else should be done. At OMETA we will fly the STAR to the active runway for landing where we expect a safe arrival.

We have done our flight planning correctly and now we only have to load the flight plan form from VATSIM Flight Plan management∞, fill the information in the boxes and in number 8. "Route of flight" type the following:

FJR/N0300F330 UM731 DIVKO/N0300F340 UM984 PADKO/N0300F350 UT24 ADITA/N0300F360 UN870 MAXIR UN853 MOBLO UZ662 KORED UN871 KUDES UN851 LOHRE/N0300F370 UN746 ESOBU UN858 OMETA

End of this complex example.

When should we apply the semi-circular rule when filling the "flight level" box of our flight plan? This rule should only be used when neither the pilot or the ATC have enroute chars to find out which is the right flight level. This rule may also be used when flying between islands (like in the Canaries) where distances are too short as to join any airway. In any case, we should always keep in mind that the semi-circular rule is a secondary way of choosing the flight level. Thus, do not be surprised if once you have requested a cruise level according to this rule, the ATC on duty amends it according to the flight levels of our filed airways. For more information about this issue, check lesson on "Direct vs. Airways" in this Pilot Resource Centre.

Final note:
When I control, sometimes I find pilots asking me why I change their flight levels and whether this is a particular case for Spain. Normally what happens is that those pilots still do not know that the European airspace is a single one for all its States since 2004 and that the semi-circular rule should not be applied here. As said, this rule is only applied in USA, Canada and a few other countries as normally the European system here explained is widely used around the world.

I am specially indebted to Francisco Urquia and Pepe Morales, real ATCs, for their comments, contributions and suggestions, and also to Joaquin Blanco for providing me with the information regarding the enroute charts for U.S.A. and Canada.

I hope that this article will be useful to make more realistic our simulation experience and keep always in mind that the best we perform our role in VATSIM skies, the more the others will enjoy our company.