Important Altitudes in IFR Flights

General Overview

• IFR flights typically receive altitude assignments from ATC for clearance or enroute instructions
• Various types of minimum altitudes must be known for planning, emergencies, and situational awareness
• These minimum altitudes help ATC in assigning altitudes and approving flight plans
• Depicted on IFR Enroute Low Charts

Key Minimum Altitudes

MOCA (Minimum Obstruction Clearance Altitude)

• Ensures obstruction clearance along the route
• Guarantees 1000 feet clearance in non-mountainous terrain and 2000 feet in mountainous terrain
• Assurance of navigational coverage within 22 miles of VOR stations if flying at or above MOCA
• Designated by an asterisk (*) next to the altitude

MEA (Minimum Enroute Altitude)

• Ensures both obstacle clearance and navigational signal coverage along the route
• Higher than MOCA and allows for continuous navigation and obstacle clearance

GNSS MEA (Minimum Enroute Altitude with GPS)

• Allows flight at lower altitudes using GPS navigation
• Signals come from satellites, providing consistent coverage regardless of terrain
• Depicted in blue on charts

MRA (Minimum Reception Altitude)

• Lowest altitude at which an aircraft can receive signals from off-route nav aids
• Flagged with an ‘R’ on intersection points

MCA (Minimum Crossing Altitude)

• Specifies altitude that must be reached before crossing a fix
• Ensures correct altitude during climbs between altitude segments
• Flagged with an ‘X’ on fixes

OROCA (Off-Route Obstruction Clearance Altitude)

• Provides obstacle clearance for whole chart quadrant
• Typically higher than MEA due to terrain variability within quadrant
• Not used for routine navigation but for emergencies and situational awareness

MVA (Minimum Vectoring Altitude)

• Lowest altitude ATC can assign for terrain clearance during vectoring
• Often lower than MOCA and similar altitudes, focused on vectoring concerns

MAA (Maximum Authorized Altitude)

• Highest altitude that ensures no conflicting navigation signals and contains top of airspace limits
• Used to avoid dual VOR signals or airspace issues

Practical Situations

• ATC usually assigns altitudes but pilots must know these minimums, especially during lost communication scenarios
• In emergencies or special conditions (like icing), pilots can request to fly at different altitudes (e.g., MOCA with GPS)
• Situational awareness regarding OROCA and MVAs can aid in better decision-making during unforeseen circumstances

Example Areas

Central Wyoming (Victor 298)

• MOCA: 10,300 feet
• MEA: 11,000 feet

Iowa (Victor 138)

• MOCA and GNSS MEA: Same altitude, allows for GPS use at lower levels

New Mexico

• Various OROCA examples
• ATC uses more finely divided areas for MVAs

Chesapeake Bay (Victor 170)

• Example of MAA usage