FAQ: Flight Paths

Q: How is a path specified?

A: A path consists of two more more locations, separated by dashes. For example, ORD-SFO specifies a path from ORD (Chicago O'Hare International Airport) to SFO (San Francisco International Airport).

Q: Can multiple paths be specified?

A: Independent paths can be placed on separate lines or separated by commas or semi-colons. A single path can consist of more than two locations for multi-hop routings. For example, LAX-JFK,LAX-TUS-FTW-BNA-LGA compares the contemporary non-stop flight from Los Angeles to New York to the 1941 vintage route of American Airlines' Flight 004, the "Limited Mercury," which was flown using a Douglas DC-3.

Q: Why do actual flight paths differ from what is shown on the maps?

A: Eastbound flights often fly a path that's closer to the equator (more southerly in the northern hemisphere) than the great circle path in order to hitch a ride on strong tailwinds. These tail winds result in a shorter flying time for the longer ground path.

Various other factors may also dictate a flight path that is longer than the great circle path. A track system is used across the North Atlantic to efficently and safely manage the larger number of flights, even though this may result in a less efficient path for a given flight. There may also be a need to avoid certain areas, including areas too far from a suitable landing point (see the ETOPS section of the FAQ) or over unfriendly countries.

Q: What does specifying the ground speed do?

A: If a ground speed is specified the distance table will include a time computed from the distance and speed. This assumes the entire trip will occur at the selected speed, ignoring acceleration and deceleration.

Q: How is Mach used to calculate trip time?

A: A Mach number is a fraction of the speed of sound. The speed of sound varies as a function of type of gas (dry air is different from humid air!), pressure, and temperature, so there isn't a straightforward conversion of Mach to distance/time.

The Great Circle Mapper assumes the speed of sound is 574 knots (about 660.5 miles per hour or 1063 kilometers per hour), which is a close approximation for the lower stratospheric altitudes from 36,000 feet to well above 65,000 feet where transport jets usually fly, assuming a temperature of -70°F. More information is available from the following resources:

• Speed of Sound (NASA)
• Earth Atmosphere Model (NASA)
• Speed of Sound & Standard Atmosphere (Aerospaceweb.org)

Q: Why are statute miles and miles/hour the default instead of nautical miles and knots?

A: The Great Circle Mapper serves a diverse community of users. Some are pilots and would prefer nautical miles and knots. Others large groups of users, frequent fliers being one example, are less likely to understand the distinction between nautical and statute miles and might be confused or annoyed if the default were different. Hence, the site uses defaults which are least likely to cause confusion.

Perhaps a better answer would be if you could choose your own default. That feature is coming soon, but isn't quite ready yet.

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