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I am reading the NORTH ATLANTIC OPERATIONS AND AIRSPACE MANUAL and I came across the following statement more than once during my reading:

The activation time of the westbound OTS shall be published as 1130z to 1900z at 30W.

The activation time of the eastbound OTS shall be published as 0100z to 0800z at 30W.

You can find this quotes at page 175 of the link provided above.

Here is my question: if the activation times are expressed as ZULU time (therefore as UTC), why is it specified at 30W? As far as I know a ZULU time is referred to the Greenwich meridian and therefore at 0W. To me it sounds quite contradictory to express an UTC time by specifying a longitude different than 0.

What am I misinterpreting? What is the right interpretation of the quotes presented above?

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  • $\begingroup$ yes sorry about that $\endgroup$ – Federico Gentile May 18 '17 at 8:43
  • $\begingroup$ does this mean that if I cross for example 30W at 11.29 I will not be flying on a westbound track for the rest of the flight (according to the flight plan)? $\endgroup$ – Federico Gentile May 18 '17 at 8:54
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Think of 30W as a kind of traffic marker for flight planning. If your planned route is going to cross 30W before the tracks activate (that is, ahead of the traffic flow), then you can file "random route." If you're crossing 30W during the active period, then you have to file on one of the tracks unless you can climb above them before crossing the entry fix. This also works going against the flow: if you're crossing 30W in the opposite direction during the active period, you'll have to route either north or south of the tracks unless you can get above the NAT FL's before your entry fix. As far as Zulu goes, it's just the common reference everyone uses (therefore, "Universal Coordinated Time").

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Overall facts behind the NAT OTS:

  • There are strong winds in altitude over the Atlantic Ocean (jet streams), including the northern polar jet stream over the North Atlantic Ocean, which is blowing from west to east.

  • Airlines try to take benefit of these tailwinds when flying from NA to Europe, and they try to find routes with the weakest headwind velocity when flying from Europe to NA.

  • The result is that at peak times, which are different for each direction, there are concentrations of aircraft at the same location.

  • To maximize traffic, airlines are using a collaborative system, the NAT-OTS where routes are optimized each day for winds and peak times, in each direction.

  • Due to passenger preferences for traveling, peaks happen at known times, and at 30W longitude (the middle of the Pound), therefore the estimated time for a flight crossing 30W meridian is a first importance.

  • The OTS activation periods you mentioned are these peak periods, expressed in UTC, for the two traffic directions when aircraft are crowded together over 30W locations.

  • Shanwick and Gander Oceanic Centers have a common border at 30W, as visible here:

    enter image description here


From the elements above, "The activation time of the westbound OTS shall be published as 1130z to 1900z at 30W" can be understood this way:

  • The westbound OTS is activated for the westbound peak traffic over 30W longitude.

  • The westbound tracks are active (in this example of track message) between 11:30 UTC and until 19:00 UTC.

  • These tracks are optimized furthermore to deal with the spatial peak occurring at 30W longitude.

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