An IFR route is chosen from a menu the airspace publishes, and it comes fenced with altitudes: floors that guarantee reception and terrain clearance, caps that guard the airspace above, crossing restrictions at fixes. This lesson learns the menu, the fences, and the arithmetic that decides which flight level actually gets flown over the high ground on a cold day.
Picking the airway
Routings are identified from the en-route charts together with the national AIP, which carries the regulatory fine print the chart compresses: route availability and restrictions, level allocations and the conditions attached to segments. The routes themselves come in named kinds:
Conventional ATS routes — defined by ground aids, VOR to VOR, flown by tuning what the chart names.
RNAV routes — defined by waypoints for area-navigation equipment, freed from overflying the aids.
RNP routes — RNAV with performance monitoring and alerting required of the aircraft: the navigation must not only be accurate but know when it is not.
Conditional routes (CDRs) — routes that exist only sometimes: CDR 1 is permanently plannable (closures published daily), CDR 2 is plannable only as the daily airspace-use plan allows, and CDR 3 is never plannable, flown only on ATC instruction.
Direct routes (DCT) — chart-independent point-to-point segments, where the airspace permits filing them.
Reading the en-route chart
Courses and distances are read, not measured: the chart prints the magnetic track of each segment and the leg distances, with totals between the bigger fixes, so the IFR plan copies rather than protracts. Waypoints defined by aids are read the same way: the chart gives each fix as a radial or bearing and a DME distance from the parent aid, and the planner lifts exactly those numbers into the plan and the navigation kit.
The altitude alphabet
Every en-route chart altitude answers one specific question, and the exam asks them as a family:
MEA — minimum en-route altitude: the lowest altitude giving both acceptable navigation coverage and the required obstacle clearance along the segment.
MOCA — minimum obstacle clearance altitude: the obstacle requirement alone; navigation reception is not assured along the whole segment.
MSA — minimum sector altitude: 1,000 ft above obstacles within 25 NM of a navigation facility or aerodrome reference, published per sector, for contingency use.
MORA — route variant: clearance within 10 NM either side of the route (and around its ends) of 1,000 ft, or 2,000 ft where terrain or obstacles exceed 5,000 ft.
Grid MORA — the same clearances computed per latitude-longitude grid cell, printed in every rectangle of the chart.
MAA — maximum authorised altitude: the segment's ceiling, set by the airspace above or by navigation-signal limits.
MCA — minimum crossing altitude: the altitude a fix must be crossed at or above, typically before a segment with a higher MEA.
MHA — minimum holding altitude: the lowest altitude for the published hold at a fix.
All of them are extracted, not derived: each figure is printed against its segment, sector, grid cell or fix on the chart, and the planner's job is to read the right one for the right piece of route.