Tracking Direct?
Scenario
An aircraft tracks through way-point A towards way-point B. ATC instruct the crew to turn right to way-point C for expedition. The aircraft turns through the expected track and establishes on a track between where the crew received the revised route and way-point C (see diagram).
ATC expected the aircraft to turn until it tracked directly towards way-point C; the shortest track to the way-point.
I observe this situation frequently. I understand the difference between fly-by and fly-over but this should not effect the tracks in these cases.
Possible Effects
- Controllers ask crews to confirm that they are going the way-point as instructed; this wastes RT time. The over-turn could be because the crew are turning to track to a wrong way-point so ATC need to check. The manoeuvre can alarm controllers because the of the large and unexpected heading differences.
- Controllers may need to re-evaluate and revise their tactical plans.
- Controllers delay level changes for the aircraft concerned and others around it. The aircraft uses more airspace. I regularly observe lateral displacements of two to three nautical miles in terminal airspace where the minimum radar separation is three miles.
- Controllers intervene by issuing headings or stopping a climb or descent to avoid the effects of the different track on other aircraft.
- Aircraft travel further and therefore burn more fuel increasing operator costs. Delayed level changes may add to costs.
- If the controller uses assured separation techniques then the manoeuvre will not immediately compromise safety. If however he or she uses an anticipated track to provide lateral separation then the manoeuvre may erode separations.
Discussion
I've discussed this with a few pilots and they recognized the issues and knew how to manage the aircraft navigation to avoid the problem. It happens frequently so some pilots are either not aware of the FMS management or situational awareness issues, or they do not appreciate the effects. If ATC ask you to confirm where you are tracking to then that may indicate that you have not managed the aircraft track as they expected. I suggest that the situation is unintended and need not happen; just accepting the effects wastes time and resources.
Solutions
Pilots can pro-actively manage the aircraft FMS to fly the shortest path.
ATC can issue headings first and then release the crew on their own navigation after the aircraft is tracking towards the next way-point. This is more work for both ATC and pilots and mitigates the problem rather than eliminating it at source.
I'd appreciate any comments or advice from pilots or other controllers about this article.