Long-Range and Oceanic Navigation
Great circles and charts
A great circle is the shortest path between two points; its initial and final tracks differ, and the highest latitude reached is the vertex. A rhumb line has constant track but is longer. On a Mercator chart a great circle plots as a curve, so ocean tracks are drawn as straight lines between waypoints with the tracks re-measured at each one; the gnomonic chart shows great circles as straight lines.
Oceanic operation
- Oceanic airspace uses prescribed tracks, normally with 50 nm or 100 nm lateral separation depending on the region and the RNP specification (RNP 10 or RNP 4 in the Pacific, RNP 4 in the North Atlantic).
- Clearance fixes the route, level and Mach; position reports or ADS-C and CPDLC satisfy the reporting requirement.
- Strategic Lateral Offset Procedure (SLOP) offsets the aircraft to the right of track, reducing the risk from an opposite-direction traffic error.
- Contingency: if a weather deviation is needed, offset, advise ATC and return to track when able; loss of RNP capability requires informing ATC.
- Equal time point and point of no return calculations use groundspeed, not TAS, in each direction.
Worked example
A and B are 1000 nm apart. Outbound groundspeed is 400 kt, return 500 kt. ETP distance from A = 1000 x 500 / (400 + 500) = 556 nm, reached after 556/400 = 1.39 h, about 1 h 23 min from A. Beyond that point the return takes longer than continuing.
Traps
Using TAS instead of groundspeed in ETP or PNR; assuming the return wind equals the outbound; applying the wrong RNP value to the airspace; and forgetting that a track offset must be reported and re-established correctly.