What altitudes or altitude types are used in IFR enroute airspace and how are they selected?

• A. MEA and MOCA are used only with ATC assignments; terrain is not a factor.
• B. MOCA is the only altitude type used on IFR routes.
• C. MEA, MOCA, and ATC-assigned altitudes are used; selection depends on terrain, airspace structure, and navigation coverage.
• D. ATC assigns altitudes without regard to terrain.

Answer: (C)

In IFR enroute planning you work with three kinds of altitudes: published MEA, MOCA, and ATC-assigned altitudes. The one you fly on a given segment is chosen based on the terrain and obstacles, how the airspace is laid out (including NAVAID coverage and airspace structure), and what navigation signals you’re relying on.

MEA, or Minimum Enroute Altitude, is published to guarantee obstacle clearance along the route and to ensure adequate navigation signal reception for the route’s defined navigation aids. MOCA, the Minimum Obstruction Clearance Altitude, also guarantees obstacle clearance, but it specifically guarantees VOR/DME reception within 100 miles of the navigational aid used; beyond 100 miles you can’t rely on that VOR signal, though obstacle clearance still applies. ATC-assigned altitudes are provided by air traffic control to manage traffic flow, weather avoidance, or other routing needs; you must comply with these even if they differ from published MEA or MOCA, as long as they maintain safe separation and terrain clearance.

So, the best answer reflects that all three types can be used, and the choice among them depends on terrain, airspace structure, and navigation coverage.