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FAQ: Enroute altitude changes Print
Often airway requirements force airplanes to change flight level while enroute. Some of these changes are part of route description, and are required for route to pass the validation process. For instance, in absence of high level airways leading to your final destination, you might want to descend earlier to a low level airway. This article explains how altitude changes are encoded in route description.
An enroute flight level change (a "switch") is encoded with the route text as a combination of two numbers, one for speed and one for altitude. Each number is prefixed with an indicator of unit. Two most common unit indicators are N for knots (nautical miles per hour) and F for flight level (100s of feet above standard altitude sea level). For example, N0400F340 means 400 knots at Flight Level 340 (34.000 ft).
"Switches", or in other words pairs of speed and altitude values can exist in route text at the beginning of it, setting the initial cruise speed & altitude, or behind a specific waypoint name, separated by "/", indicating that these new values will be valid from this particular waypoint onwards.
While switches are an industry standard, vroute uses one non-standard extension. Since vroute's flight plans are often valid for a range of flight levels, 
N0430F290 CIV UN872 KOVIN UY317 RESMI/N0430F290+ UN857 DIRMO UN855 BGR

This route contains two switches:
  • N0430F290 tells us that the flight will begin with true airspeed of 430 kts and at flight level 290.
  • RESMI/N0430F290+ informs that after RESMI waypoint we would like to climb to our target cruise flight level, which is FL290 or higher ("+").
 This is how it will look on vertical profile (in this picture, target cruise FL is 340):
Some other, less frequently used units are:
  • M for Mach number (i.e. M082 is Mach 0.82)
  • K for Kilometers per hour (i.e. K870 is 870 km/h)
  • S for meters (i.e. S1160 is 11600 meters)
 Please visit our support forums if you have any questions or comments.