North Atlantic Tracks [LONG]

From:         wolpjame@cwis.isu.edu (Jim Wolper)
Date:         22 Jan 96 03:24:11 
Organization: Idaho State University
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There has been some discussion lately on the 'net about the
North Atlantic track system, and since I recently had the
opportunity to cross the Atlantic (as a passenger, alas) I
have found more about the system which I would like to share.
This information is compiled from Jeppesen charts and discussions
with airline flight crews.  The North Atlantic Plotting
Chart and the Atlantic Orientation Chart are the most
useful.

What I am writing here applies to airliners and business jets.
One 'netter crossed the Atlantic in a Cessna 182 a couple of
years ago, and the procedures he outlined were different.
(I think it was Dave Rogers but my memory is fading.  It
was a very interesting article.)

Most of the North Atlantic is classified as MNPS airspace,
that is, Minimum Navigation Performance Specifications.  To fly in
this airspace between FL275 and FL400, the aircraft and
aircrew must be certified to meet certain navigation
performance standards.  In practice, this means two separate
acceptable forms of long range navigation equipment
(INS, Omega, and now certain GPS installations), high
frequency (HF) radios, and the like.  There is one route
which has continuous VHF radio coverage, but this takes you
pretty far out of the way. On the other hand, if you want
to take your J3 to England, this is probably the route you'd
have to take.

Since there is little or no radar coverage, controllers have
to use non-radar procedures and separation standards of
60 miles (laterally) are the rule.  Controllers use a
Mach number technique to keep aircraft separated as well.
The basic idea seems to be not to run up the tailpipe of
the airplane ahead.  Each position report is supposed to
include the aircraft's Mach number, so the controllers
have some warning of any possible loss of separation.

The North Atlantic Track system (NAT) changes daily based
on the weather and winds aloft.  There are two big packs of
airplanes crossing, one eastbound and one westbound, and
I think it is very difficult to go against the flow because
all of the desirable altitudes -- FL310, FL330, FL350, FL370,
and FL390 -- are taken.  (Remember that above FL290 the
usual vertical separation is 2000 feet.)  The packs
travel at different times of day, with eastbound flights leaving
North America in the early evening for an early morning arrival
in Europe, and westbound flights leaving Europe in the late
morning and arriving in North America in the afternoon.

Shanwick control prepares a "route message" each day and
transmits it via VHF radio, teletype, and fax to operators.
This lists the day's routes.  A route typically starts at
a domestic fix at the departure end and terminates at
a domestic fix at the arrival end.  Thus, my flight from
London Gatwick to Cincinnati flew a route through UK
domestic airspace (ACORN DCT BPK [vor] DCT ROBIN DCT POL [vor]
DCT MARGO), crossed, and joined the Canadian domestic airspace system
at an intersection called LAKES.  The oceanic route is specified by
indicating the latitude where it crosses each 10 degree longitude
line, with direct in between.  Thus, coming back to North
America we flew MARGO 59N010W 61N020W 62N030W 62N040W 61N050W
59N060W LAKES.

I have the whole route message for the eastbound trip on 29
December.  It is a little long so I will only mention some
of the highlights.  Track U ("uniform") began at BANCS, a fix
over the Grand Banks of the tip of Newfoundland, and
proceeded via "46/50 48/40 49/30 50/20 50/15" and
terminated at KENUK, a fix just south of the southwestern
tip of Ireland.  Here, "46/50" means "46N050W", etc.   On the
same day, track T ("tango") started at COLOR (the tip
of Newfoundland) and proceeded via "47/50 49/40 50/30 51/20
51/15" to GIPER, which is 60NM north of KENUK.  Notice that
each fix on route T is 1 degree (60 nautical miles) north of the
corresponding fix for U: T crosses 40W at latitude 49N, while
U crosses at latitude 48N.

On 29 December the northernmost route was Q, passing through
54/20, while the southernmost was Z, passing through 44/20.
Not all of the routes are parallel: for example, a flight from
Atlanta to London would not be parallel to a flight from
Atlanta to Rome, which might have used track Y passing
through "31/60 32/50 35/40 37/30 38/20 38/15".  (I am
just guessing here, but Rome is at about 42N, so this
route makes more sense than one through, say,  54/15.
Route W terminated at approximately 41/15, so it is another
possibility.)

However on 13 October track U went from COLOR to DOLIP (the
next fix north of GIPER) via 47/50 49/40 51/30 52/20 and
52/15.  So you see there is a big difference

Emergency procedures are also of interest here.  Since HF
communications are difficult, and since speed is used as
a separation criterion, it is difficult for an aircraft which
has, say, shut down an engine to inform the controllers, and
it is imperative that the aircraft move away from its assigned
track so as not to lose separation with trailing traffic.
The general procedures are to climb 1000' and turn 90 degrees
from the track. Special procedures are needed for supersonic aircraft,
because at Concorde speeds it is not possible to turn without crossing the
next track over.

Aircraft in this airspace leave their transponders on and
squawk 2000.  They are assigned a new squawk code by the
domestic controlling agency after the crossing.

Other oceanic route structures are more rigid.  For example,
southbound flights along the coast of Portugal or Africa follow
a non-varying route structure, as do flights crossing the Pacific.
I'm not sure why the Atlantic route structure has this
flexibility, and I wonder if it is a relic of the Cold War, since
most of the states bordering the Atlantic were NATO members and
thus more inclined to cooperate.


Jim Wolper (Commercial Pilot ASEL AMEL CFII)
Department of Mathematics	<http://math.isu.edu/~wolperj>
Idaho State University