Enroute Altitudes & VFR-on-Top
MEA, MOCA, MRA, MCA and the changeover point, plus what VFR-on-Top actually authorises.
Enroute Altitudes
- MEA — Minimum Enroute Altitude: Guarantees a usable nav signal from at least one VOR defining the airway over the entire segment. If a stretch loses signal it's labeled "MEA gap" — dead reckon until reception is restored.
- MRA — Minimum Reception Altitude: The lowest altitude at which an off-course NAVAID signal can be received to identify a fix. Used when MRA > MEA at a particular fix.
- MOCA — Minimum Obstruction Clearance Altitude: Provides obstacle clearance and nav signal reception within 22 NM of a VOR. May be lower than MEA — use only within VOR reception range.
- MCA — Minimum Crossing Altitude: The lowest altitude at which a fix can be crossed when proceeding into a higher MEA segment.
- MSA — Minimum Sector Altitude: Provides at least 1,000 ft clearance over obstacles within a sector of a 25 NM circle around a navaid. Found on approach plates as an emergency reference.
- OROCA — Off-Route Obstruction Clearance Altitude: Provides 1,000 ft clearance above the highest obstacle in non-mountainous areas, 2,000 ft in mountainous. Does not guarantee nav signal or ATC reception — purely an obstacle reference for off-airway flight.
Changeover Point (COP)
The point along an airway segment between two adjacent VORs where the pilot should switch the navigation receiver from the station behind to the station ahead. Established to prevent loss of nav guidance and frequency interference.
- Usually depicted at the midpoint of a long segment, but not always — published COPs handle non-standard cases
- Switch is made at the COP, not before — you may not have reception of the next station yet
- "Pilots are urged to observe COPs to the fullest extent."
VFR-on-Top
An ATC authorization for an IFR aircraft to operate in VFR conditions at an appropriate VFR altitude. Combines IFR routing with VFR cruising flexibility — useful when a layer obscures the surface but cruise altitude is clear.
- Comply with VFR visibility and cloud clearance criteria (the IFR rules don't relax just because you have a clearance)
- Comply with minimum IFR altitudes (MEA, MOCA, etc.)
- Altitude rule: add 500 ft to the corresponding IFR cruising altitude
- Magnetic course 0–179°: odd thousands + 500 (3,500 / 5,500 / etc.)
- Magnetic course 180–359°: even thousands + 500 (4,500 / 8,500 / etc.)
- References: AIM 4-4-8 (IFR Clearance VFR-on-Top), AIM 5-5-13 (VFR-on-Top)
ARTCC ("Center") Services
- Issues route clearances and re-routes
- Provides vectors when needed (asks for intentions when planning your descent into an approach control's airspace)
- Issues NOTAMs en route
- Provides current weather, SIGMETs, AIRMETs as required
- Center control on airways begins at 1,200 ft AGL