Hier nog een stukje met uitleg van de canadesen.
Ik ben dus nog steeds op zoek naar de officiele uitleg....
For those in the know, the standard dimension considered for protected airspace is the airway width. For a VHF airway, based on VORs, the width is 4 NM either side of the centerline out to a distance of 50.8 NM from the facilities. From there, lines that splay out from the centerline at an angle of 4.5° either side are joined to cover the area between the facilities. It kind of looks like a diamond sitting on top of a bar. For the VHF airway, airspace planners assess obstacle clearance within this polygon, plus they add a 2 NM buffer on either side of this shape. So over a VOR facility itself, and within 50.8 NM of it on the airway, the Minimum Obstruction Clearance Altitude, or MOCA, will consider 4+2 NM either side of the track, for a total width of 12 NM. DMRs are considered for airway MOCAs, so the added altitude above the highest obstacle is 1,000, 1,500 or 2,000 feet as applicable. Please note that the MOCA and the Minimum Enroute Altitude (MEA) on airways are separate entities. The MOCA ensures obstacle clearance, while the MEA ensures obstacle clearance and signal coverage from the NAVAIDs on which the airway segment is based. ATC will not normally issue clearance below the MEA for an aircraft flying the airway, but may approve it if requested for reasons such as icing, turbulence, etc. This may become important in areas with long airway segments overlying low land for which the MOCA may be low but the MEA is high due to the distance between facilities. Another area where this can be important is where terrain obstructs NAVAID signals forcing a high MEA (there is at least one airway in western Canada with an MEA over FL180) even though the MOCA may be substantially lower. This is becoming more and more important with the advent of more sophisticated NAV systems that don't rely on ground-based NAVAIDs. GPS, for example, can take you anywhere. How do you know how low you can go if there is no published altitude for your area? Often it's best to use airway tracks, even if navigating "GPS direct" as it's often called, simply because there are altitudes published on these tracks. In such a case, ATC may clear you to maintain a lower altitude (one that's at or above the MOCA) since he knows you're not relying on the signal coverage (and hence requiring the MEA) to get you there. Here's a diagram, hopefully enough to clarify what I described above.
Heb het dus nog steeds niet in Jeppesen gevonden......
