Introduction
This file specification is not official and is only an interpretation of the format. However, I have yet to find an occurrence in which this specification fails to work in X-Plane v8, v9 or v10. This is a preliminary version of a work in progress, if you find anything obviously incorrect then please contact me.

Throughout this specification I will be using the following file for examples, clearly this is not a real plan and is only here to highlight as much of the specification as possible:
						I
						3 version
						1 
						4 
						1 EDDM 0.000000 48.364822 11.794361 
						2 GL 1000 57.083820 9.680093 
						3 KFK 2000 38.803889 30.546944
						11 DETKO 0.600000 28.097222 49.525000 
						28 +13.691_+100.760 0.000000 13.691230 100.760811
					
Header
						I
						3 version
						1 
						4 
					
The first two lines cover the source computer OS and the version of the specification the file is built to. A letter 'I' represents a file built on a PC, whilst a letter 'A' represents one built on an Apple computer; there is full compatibility between the two, so either will work across either OS. All FMS files I have come across for X-Plane 8 and 9 use the version 3.
						I
						3 version
					
The following two lines each contain an integer value. The first number is almost exclusively a '1' and the second number varies with the number of waypoints and appears to be number of waypoints minus 1, although I have not seen any adverse effects from using a different number here.
						1
						4
					
Waypoints
The remainder of the file consists of a number of waypoints that mark out the route for the FMC. Each row is made up of five segments that define the location, altitude, type and ID of the waypoint. The elements within each row can be either tab or space delimited.
						1 EDDM 0.000000 48.364822 11.794361 
						2 GL 1000 57.083820 9.680093
						3 KFK 2000 38.803889 30.546944
						11 DETKO 0.600000 -28.097222 -49.525000
						28 +13.691_+100.760 0.000000 13.691230 100.760811
					
Type
The initial integer on each row defines of what type the waypoint is. The available options are as follows:

1 - Airport ICAO
2 - NDB
3 - VOR
11 - Fix
28 - Lat/Lon Position

These are the only waypoint type values seen in flight plans. Other values do not work with the FMC and are not intended for use in .fms files.
						2 GL 1000 57.083820 9.680093               NDB
						3 KFK 2000 38.803889 30.546944             VOR
						11 DETKO 0.600000 -28.097222 -49.525000    FIX
					
ID
This element contains the ID for the waypoint and defines what title shows up on the FMC. For all waypoints except the lat/lon type it should consist of only alphanumeric characters.
						1 EDDM 0.000000 48.364822 11.794361 
						3 KFK 2000 38.803889 30.546944
					
The lat/lon type is special in that it has a strict predefined format and also makes use of the +,-,. and _ symbols. The format used is as follows, each '0' character represents a single digit that must be present; the first set of zeros corresponds to the latitude value and the second to the longitude. The '+' character can be substitued for the '-' character for waypoints in the southern or western hemispheres, although positive numbers cannot omit the '+' symbol.
						28 +00.000_+000.000 0.000000 0.000000 0.000000

						+12.345_+009.459      Correct for a waypoint at 12.345°/0.459°.
						-28.478_-056.370      Correct for a waypoint at -28.478°/-56.370°.
						12.366_040.101        Incorrect: '+' or '-' symbol is required, even if values are positive.
						-16.982 +112.006      Incorrect: '_' symbol is required to separate the two values.
						-98.283_+200.554      Incorrect: the latitude numbers must be in the range -90 to 90.
						                                 and the longitude values must be in the range -180 to 180.
						+6.080_-9.412         Incorrect: the leading digits are required even when they are zero.

						28 +13.691_+100.760 0.000000 13.691230 100.760811
					
Altitude
The altitude value set here is displayed on the FMC and can additionally be used to set the autopilot altitude through VNAV. The units are feet and the maximum value is 99999; any number greater than this should still work but will cause issues with the display of the value in the FMC and on the A/P. The value may also have up to 6 decimal places, but these will be rounded to the nearest integer when displayed in X-Plane.
						11 DETKO 0.600000 28.097222 49.525000       Will be rounded to 1
						11 DETKO 1000000 -28.097222 -49.525000      Will not display properly, but should still work
					
Location
The final two values are the latitude and longitude values that make up the location of the waypoint. They are in decimal degrees and support up to six decimal places. As with the ID values, the latitude value should be in the range -90° to 90° and the longitude value should be in the range -180° to 180 °.
						1 EDDM 0.000000 48.364822 11.794361
						2 GL 1000 57.083820 9.680093
						3 KFK 2000 38.803889 30.546944
						11 DETKO 0.600000 -28.097222 -49.525000
						28 +13.691_+100.760 0.000000 13.691230 100.760811
					
Padding
After all of the waypoints are defined, the file may contain a number of blank lines with the following format. These can be omitted or included at your discretion.
						0 ---- 0.000000 0.000000 0.000000