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The control areas are class A airspace (IFR only) above FL055 and class G (uncontrolled) below. There is no upper limit to the class A volume. Flights are routed in these regions using either the Organised Track System (OTS), or a Random Track which is a custom route used when OTS cannot be used. Flights in OTS tracks are at FL285 to FL420. If a flight is diverting, it can do so by first descending to FL280 or below to avoid aircraft in the tracks.

The OTS consists of eastbound (evening departure) and westbound (morning departure) tracks, called the North Atlantic Tracks or NATs, which change daily due to weather, winds and other factors. Gander Center issues the evening tracks which are used for eastbound traffic which will cross the 30W meridian between 0100 and 0800 UTC. Shanwick Center issues the morning tracks which are used for westbound traffic crossing 30W between 1130 and 1900 UTC. The NATs are published in a daily NAT message which can be viewed here: https://pilotweb.nas.faa.gov/common/nat.html.

Flight Simmers may appreciate this site, which publishes the NAT message and adds links to weather information and useful documents: http://blackswan.ch/nat/. You can also look here: http://jetvision.de/nattracks.shtml.

The track message might look a little confusing if you havent see it before. Heres part of a message from Gander (CZQX) FIR:

X VIXUN LOGSU 49/50 50/40 51/30 52/20 LIMRI XETBO
EAST LVLS 320 330 340 350 360 370 380 390 400
WEST LVLS NIL
EUR RTS EAST NIL
NAR N63B N67B-

The X is the name (or label) of the track. Since this is from Gander, its an eastbound track. Such tracks are labelled starting at Z and going in reverse alphabetic order. Westbound tracks start at A and go in alphabetic order. VIXUN and LOGSU are track entry waypoints, while LIMRI and XETBO are the exit waypoints. The points in between are lat/long positions. We can visualize this track by typing the following into skyvectors flight plan text box (eg. change 49/50 to 4950N):

VIXUN LOGSU 4950N 5040N 5130N 5220N LIMRI XETBO

The result looks like this:

Since the above track is current while Im writing this, I can also enable the tracks layer in skyvector to see that the above waypoints line up with skyvectors track depiction:

we can also view the winds and significant weather conditions (SIGMETs) by turning on those layers:

The stem of the wind barb points in the direction of the wind, and the flags indicate the speed. Trianglular flags are 50kt, the longer sticks indicate 10kt and shorter sticks indicate 5kt. Track X has mostly 70kt to 80kt tailwind. The tracks are generally positioned to take advantage of such winds.

The East LVLS part of the message gives the valid flight levels for the track. Note the separation is 1000 feet. The OTS airspace uses Reduced Vertical Separation Minima (RVSM), so aircraft need to be approved for flight in such conditions. The requirement is to have dual redundant systems for altitude measurement which are crosschecked before airspace entry (the dual systems must agree within 200 feet), and also a Mode-C SSR transponder (SSR meaning additional data is provided by the transponder to report identity and altitude). There are other restrictions such as vertical speed limitations for altitude changes, alerting devices and so on. Aircraft should be using a TCAS as well.

NIL means Not Applicable, so in the above message there are no westbound levels or European Routes (EUR RTS, or NERs), but there are North American Routes (NARs). The identifiers such as N63B designate preferred routes to or from the NAT endpoints. The best document I found which describes them in detail is here: http://www.united-virtual.com/files/documents/UVA_WOM1.pdf. In this document we see N63B is described as follows:

N63B TUSKY (Direct 078�/271� 622 NM)VIXUN

So we can simply go direct from TUSKY to the NAT entry point. Both NARs and NERs can have a common and non-common portion. The common portion will connect to the NAT and the non-common portion connects to the common portion and depends on the origin or destination. For example, many westbound NARs direct you to ALLEX (common portion), and from there if you were going to KJFK you would then fly to ENE and then Parch Arrival (non-common portion).

In addition to RVSM separation, the airspace also uses Minimum Navigation Performance Specification (MNPS) rules. This provides lateral and longitudinal (tip-to-tail) separation. As with RVSM the aircraft must be suitably equipped and approved to fly in this environment. For longitudinal separation the aircraft must use Mach number for speed control and maintain a 10 minute separation with other aircraft in the same track. Aircraft flying in this space send out position reports, so if another aircraft reports at you next expected waypoint, you can check your forecast arrival time (clocks must be synchronized with UTC to a standard source such as GPS), and make adjustments to maintain separation. Lateral separation requirement is 60nm.

Heres another important part of the NAT message, in the remarks section:

1.TMI IS 364 AND OPERATORS ARE REMINDED TO INCLUDE THE TMI NUMBER
 AS PART OF THE OCEANIC CLEARANCE READ BACK.

The TMI is the Track Message Identifier. The Julian day of the year is used as the TMI. Since the track message expires, the TMI is read back to the controller during the clearance to assure the current message is used for the flight.

Some of the other remarks are self-explanatory, but this part is worth mentioning:

3.CLEARANCE DELIVERY FREQUENCY ASSIGNMENTS FOR AIRCRAFT OPERATING
 FROM KENKI TO TALGO INCLUSIVE:
 KENKI TO GRIBS 132.02
 MIBNO TO LAKES 134.2
 MOATT TO LOACH 128.7
 SCROD TO CARPE 135.45
 YAY TO YQX 135.05
 VIXUN TO COLOR 128.45
 BANCS TO TALGO 119.42

Before entering the OCA, clearance must be obtained (this is in addition to domestic ATC clearances). For track X entry at VIXUN, the pilot monitors on 128.45 on the second COMM radio. When we start receiving Gander Center, they can be contacted for clearance with an estimated arrival time for VIXUN (UTC or Zulu time).

When creating the flight plan we also need the following information:

• Alternate airports. These include destination and ETOPS alternates. More about ETOPS later.
• Weather: Useful things to check are METAR and TAFS for origin, destination and alternate airports.
• High Level SIGWX information: http://aviationweather.gov/products/swh/ and satellite: http://aviationweather.gov/obs/sat/intl/.
• Charts: Aerodrome and procedure charts including SIDs, STARs and IAPs
• NOTAMS for origin, destination and alternates: https://pilotweb.nas.faa.gov/PilotWeb/.