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INSTRUMENT PROCEDURES HANDBOOK

OFF ROUTE OBSTRUCTION CLEARANCE ALTITUDE

An off-route obstruction clearance altitude (OROCA) is an off-route altitude that provides obstruction clearance with a 1,000-foot buffer in nonmountainous terrain areas and a 2,000-foot buffer in designated mountainous areas within the U.S. This altitude may not provide signal coverage from ground-based navigational aids, air traffic control radar, or communications coverage. OROCAs are intended primarily as a pilot tool for emergencies and situational awareness. OROCAs depicted on NACO en route charts do not provide you with an acceptable altitude for terrain and obstruction clearance for the purposes of off-route, random RNAV direct flights in either controlled or uncontrolled airspace. OROCAs are not subject to the same scrutiny as MEAs, MVAs, MOCAs, and other minimum IFR altitudes. Since they do not undergo the same obstruction evaluation, airport airspace analysis procedures, or flight inspection, they cannot provide the same level of confidence as the other minimum IFR altitudes.

If you depart an airport VFR intending to or needing to obtain an IFR clearance en route, you must be aware of the position of your aircraft relative to terrain and obstructions. When accepting a clearance below the MEA, MIA, MVA, or the OROCA, you are responsible for your own terrain/obstruction clearance until reaching the MEA, MIA, or MVA. If you are unable to visually maintain terrain/obstruction clearance, you should advise the controller and state your intentions. [Figure 3-34]

For all random RNAV flights, there needs to be at least one waypoint in each ARTCC area through which you intend to fly. One of the biggest problems in creating an RNAV direct route is determining if the route goes through special use airspace. For most direct routes, the chances of going through prohibited, restricted, or special use airspace are good. In the U.S., all direct routes should be planned to avoid prohibited or restricted airspace by at least 3 NM. If a bend in a direct route is required to avoid special use airspace, the turning point needs to be part of the flight plan. Two of the most prominent long range navigation systems today include FMS with integrated GPS and stand-alone GPS. The following example is a simplified overview showing how the RNAV systems might be used to fly a random RNAV route:

In Figure 3-35 on page 3-30, you are northeast of Tuba City VORTAC at FL 200 using RNAV (showing both GPS and FMS), RNAV direct on a southwesterly heading to Lindbergh Regional Airport in Winslow. As you monitor your position and cross check your avionics against the high altitude en route chart, you receive a company message instructing you to divert to Las Vegas, requiring a change in your flight plan as highlighted on the depicted chart excerpt.

During your cockpit review of the high and low altitude en route charts, you determine that your best course of action is to fly direct to the MIRAJ waypoint, 28 DME northeast of the Las Vegas VORTAC on the 045° radial. This places you 193 NM out on a 259° magnetic course inbound, and may help you avoid diverting north, allowing you to bypass the more distant originating and intermediate fixes feeding into Las Vegas. You request an RNAV random route clearance direct MIRAJ to expedite your flight. Denver Center comes back with the following amended flight plan and initial clearance into Las Vegas:

“Marathon five sixty four, turn right heading two six zero, descend and maintain one six thousand, cleared present position direct MIRAJ.” The latitude and longitude coordinates of your present position on the high altitude chart are N36 19.10, and W110 40.24 as you change your course. Notice your GPS moving map (upper left) and the FMS control display unit (below the GPS), and FMS map mode navigation displays (to the right of the GPS) as you reroute your flight to Las Vegas. For situational awareness, you note that your altitude is well above any of the OROCAs on your direct route as you arrive in the Las Vegas area using the low altitude chart.