Ask the Captain: How has GPS changed flight navigation?

— -- Question: Captain Cox, why do airplanes still fly magnetic course when most all navigation is done now with GPS navigation and the ability to fly true course(s)? It seems odd that the governing bodies haven't changed the way airplanes get from A to B.

— submitted by reader tmw66

Answer: Airways that use ground-navigation stations have used magnetic course for decades. Some aircraft still use this as primary means of navigation. As a result, the FAA provides navigation stations for these older airplanes. While GPS navigation could be true or magnetic, realigning the airways would be a massive change in the airspace system which would not really help expedite the traffic flow.

Q: Can you explain what the Required Navigation Peformance approach is? How is it different from the RNAV approach? Do pilots prefer one method over the other?

— greg13

A: You ask two simple questions that have complex answers. To answer your questions, we first have to define the Performance Based Navigation (PBN) used in our National Airspace System (NAS). Throughout the NAS, aircraft are required to navigate to various standards. In the old days, we had to fly from ground station to ground station over a specific ground track. Today we have the capability to fly to a point space (waypoint) where there is no ground radio station.

How accurately can this be done? The more accurately it can be done allows more flexibility in the NAS. The FAA developed navigation criteria based on capability. The better a navigation system performed, the more flexible the NAS. This became known as PBN. There are two components of PBN, Area Navigation (RNAV) and Required Navigation Performance (RNP).

RNAV allows navigation from a waypoint to another waypoint using ground or space-based navigation aids.

RNP is RNAV with the ability to monitor the accuracy of the ground track between waypoints. Using RNP, it is possible to have aircraft safely fly very precise ground tracks and avoid terrain. Juneau, Alaska, is a good example of modern, sophisticated RNP navigation being used to fly down the channel to land at the airport, avoiding high mountains nearby. The ability to precisely monitor the accuracy of the ground track makes this approach safe.

Because RNP has more flexibility, pilots usually prefer it over just RNAV.

My source of information for this answer is the FAA NextGen PBN fact sheet of April 24, 2009.

Readers, please leave your questions for John Cox here, and only leave comments about this week's column below.

John Cox is a retired airline captain with U.S. Airways and runs his own aviation safety consulting company, Safety Operating Systems.