Quote:
Originally Posted by ArcticKnight
Hi WOXOF
When discussing the neat video posted ITT showing a 747 overtaking a 737 on the exact same flight path (but different FL, obv), you mentioned that the use of GPS for Nav is the reason for the tight tracking.
I live in central Alberta and there are a ton of overflights to see. Quite often I will see an aircraft take what appears to be a pretty significant correction in path (let's say more than 5 degrees), as if it was the old days flying over a VOR and then commencing on a new heading.
Am I imagining this, or in some airspace are these types of inefficient flight paths still occurring? It would seem sub-optimal for both fuel and time..
It's not your imagination. Although most planes are now equipped with GPS, the underlying airways are built as segments between discrete points. Those discrete points can be VORs (actual physical ground-based NAVAIDs) or simply points defined by their LAT/LON. At these points, the heading usually changes resulting in the turn you're observing.
For aircraft not equipped with GPS or some other form of long range nav (such as IRS), the LAT/LON defined airways are not available and they are limited to the standard Victor airways (below 18,000') or Jet airways (above 18,000').
Quite often, we also file point-to-point rather than using defined airways. It would be very unlikely that all the points are co-linear. Although I haven't had this happen recently, I used to routinely get cleared direct to the Wilkes-Barre VOR (LVP, located in Pennsylvania) after contacting Los Angeles Center late at night on a red-eye from Las Vegas to JFK. This kind of direct routing would not be offered to a plane not equipped with long range nav capability. (ATC knows our equipment and nav capabilities from codes we use on our filed flight plan. You can see a list of these codes
here.)
As for being inefficient, "direct to destination airport" would always be the shortest path but it's just not practical for the amount of traffic we have. Using defined airways brings some order to the flow.