This lesson explores the different types of IFR altitudes charted on an Enroute Low Altitude Chart.
The types of altitudes most commonly found on the low altitude chart are the Minimum Enroute Altitude (MEA), Minimum Obstacle Clearance Altitude (MOCA), Minimum Crossing Altitude, Minimum Reception Altitude, Maximum Authorized Altitude, and Off Route Obstruction Clearance Altitude (OROCA).
The MEA is depicted above the route segment it is valid for and is show in thousands of feet. When flying at MEA you are guaranteed obstacle clearance of 1000ft in non-mountainous terrain or 2000 ft in mountainous terrain. You are also guaranteed the ability to receive all navigation aids required to fly the depicted route.
MOCAs are charted much the same was as an MEA, only they are preceded by an asterisk. A MOCA will guarantee the same obstacle clearance as a MEA, however it only guarantees navigation signal coverage within 22NM of the nearest NAVAID that defines the route. That means if your NAVAIDS are 60 miles apart, there is an 18 mile navigational signal gap in the middle of the route if flying the MOCA.
Minimum Crossing Altitudes are shown using a flag with an X in the middle and the text “MCA” with altitudes and routes below. The minimum crossing altitude is the minimum altitude at which you can cross a fix, and is usually associated with a change in MEA at the fix.
Minimum Reception Altitudes are shown the same was as an MCA, however they have an “R” in the flag to denote reception. A minimum reception altitude is specified any time there is an intersection or fix which requires a certain altitude so you can receive all of the NAVAIDS that define the fix.
OROCAs are basically the same as the Maximum Elevation Figure (MEF) found on a VFR sectional. The main difference is that they will give you IFR clearances of 1000ft or 2000ft depending on the terrain. OROCAs are to be used when planning RNAV or GPS flights which do not stay on defined airways. Flying at the OROCA guarantees obstacle clearance within the quadrangle, however it does not guarantee any navigational reception or communications reception.
I hope this lesson helps!