Question: How are transoceanic routes determined, since there are no navigation aids such as VOR, NDBs in the middle of the ocean? What if the flight's GPS system fails?
— Samuel, Louisville
Answer: GPS is the primary means of overwater navigation today, but there are backup inertial units. Most flight management systems combine information from the GPS and Inertial Reference Units (IRU) and display a blended position to the pilots. If a GPS system fails, there is usually a second GPS receiver and at least two (usually three) IRUs.
The routes are published with latitude and longitude coordinates for each waypoint. In some heavily used over-water areas such as the North Atlantic, the routes are adjusted daily to take the greatest advantage of wind conditions. These are known as tracks.
Q: How does the FMS (flight management system) navigate over open ocean, since there may be no waypoints the radios could pick up?
— Shelby Tate, Prescott, Ariz.
A: FMS use inputs from GPS receivers to determine the position from satellites, or from an inertial reference system that calculates the position by very accurately measuring the acceleration (lateral, vertical and longitudinal) forces applied to the airplane. The result is that an FMC can fly to a waypoint (a predetermined point in space) accurately without receiving navigation signals from the ground stations.
Q: On overseas flights, what are mandatory reporting points and to whom and what do you report?
— Joe Holt, Richland, Wash.
A: It depends on the specific flight plan route. Position reports are required at mandatory reporting points, these are depicted on charts; if on tracks, the coordinates of the track points are used. Oceanic air traffic control takes the reports, there are several around the world. Using long-range High Frequency (HF) radios or via Satellite Communication (SATCOM) pilots report passing the point, the estimate of the next point, the point after that, and any remarks.
Q: Can pilots change altitude during a transoceanic flight when out of radar contact if turbulence is bad?
— Deb, no location given
A: Yes, over every ocean is an oceanic control. A pilot can call a variety of radio stations to have them relay a change of altitude request. It can sometimes be slow in getting the climb or descent clearance, but there is a process for changing altitude in oceanic airspace.
Q: Why aren't waypoint names used on transoceanic flights instead of latitude and longitude for position reports?
— Stephen, Montreal
A: The track system moves every day to take advantage of the wind, so the use of latitude and longitudes is easier than having hundreds of named waypoints. This is a tradition that dates back to the early ocean-crossing days, so there is some feeling that this is a proven system and there is not a need to change it.
John Cox is a retired airline captain with US Airways and runs his own aviation safety consulting company, Safety Operating Systems.
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