A recent thread in a pilot forum highlights a recurring point of confusion for instrument-rated pilots, including one currently preparing for a CFII checkride: the relationship between IFR cruising altitudes (the odd-thousand/even-thousand-plus-500-foot rule under 14 CFR 91.179) and the various minimum IFR altitudes published for airways, including MEA (Minimum Enroute Altitude), MOCA (Minimum Obstruction Clearance Altitude), and MCA (Minimum Crossing Altitude). The poster's scenario—flying eastbound with an MEA of 8,000 and a MOCA of 6,000, wondering whether 7,000 is the lowest usable altitude—is a textbook example of the kind of applied regulatory knowledge that trips up even experienced instrument pilots who haven't recently reviewed the nuances of 91.177 and 91.179 together.
The correct answer, which the community will likely converge on, is that both rules apply simultaneously and the pilot must satisfy the more restrictive one. Minimum IFR altitudes (14 CFR 91.177) establish the floor for terrain and obstruction clearance and are non-negotiable regardless of cruising altitude conventions—an aircraft can never legally descend below the MEA on a direct point-to-point segment, or below the MOCA when within 22 nautical miles of the VOR defining that segment. Separately, 91.179 dictates that altitude selection in cruise flight must conform to the hemispheric/magnetic-course-based odd/even scheme (plus 500 feet under VFR-on-top, though not relevant here). In the poster's example, since the MEA is 8,000, no altitude below 8,000 is legal at all, MOCA notwithstanding—MOCA only becomes usable once the aircraft is within 22 NM of the VOR, and even then, the pilot still must select an altitude that both meets or exceeds the 6,000 MOCA and complies with the cruising altitude rule. So while 7,000 would satisfy the odd-altitude and MOCA requirements once within range of the station, it would still violate 91.177 anywhere along the airway where the MEA of 8,000 applies. The pilot's instinct that minimum IFR altitudes are essentially a "floor" that interacts with, rather than replaces, cruising altitude rules is correct—an assigned or selected cruising altitude must be at or above the applicable minimum IFR altitude for that segment, and must also conform to the appropriate odd/even convention unless ATC assigns something different.
This distinction matters significantly for pilots operating in the National Airspace System, particularly those flying single-pilot IFR in piston or turboprop equipment where terrain, MEA gaps, and off-airway routing are common. Controllers routinely assign altitudes that satisfy both minimum vectoring altitudes (MVA) and separation requirements without pilots needing to independently verify MEA/MOCA compliance in radar environments, but pilots must still understand these floors for non-radar segments, loss-of-communication procedures under 91.185, and off-airway or direct GPS routing where they are responsible for their own obstacle clearance. Business aviation and Part 135 operators flying into non-radar or mountainous terrain—Alaska, the Rockies, remote international routes—depend heavily on correct application of MEA, MOCA, and MCA logic, especially during engine-out or emergency descent scenarios where minimum altitudes may need to be balanced against terrain avoidance software (EGPWS/TAWS) guidance.
For CFII candidates specifically, this is a favorite oral exam topic because it tests whether an instructor-applicant can teach the difference between regulatory minimums (obstacle clearance) and operational conventions (traffic separation via altitude flow). Examiners often probe scenarios exactly like the one described—assigning a cruising altitude below MEA but above MOCA, or asking what happens off airways where no published MEA exists (requiring the 91.177(b) rule: 1,000 feet above the highest obstacle within a horizontal distance of 4 NM in non-mountainous terrain, or 2,000 feet in designated mountainous areas). The exercise reflects a broader trend in instrument and CFII training: even highly experienced pilots benefit from periodically revisiting foundational IFR regulations, since day-to-day flying in radar-dominated airspace with GPS direct routing can erode fluency with legacy airway-based altitude logic that remains essential for checkrides, non-radar operations, and safety-of-flight decision-making when automation or ATC services are degraded.