On Instruments: How Low Can You Go

But wait. Now comes a clearance to descend to 3,000 feet, followed by a vector, followed by a further descent to 2,000 feet to intercept the final approach course. Did ATC just put you in harm’s way with the descent clearances? Odds are against it, because approach controllers use minimum vectoring altitudes (MVAs) for sequencing air traffic near many airports. You won’t see MVAs depicted on approach charts, but controllers have them superimposed on their screens.

MEAs cover large areas; MVAs have more granularity. They take into account individual, often isolated areas of high terrain and obstacles, and let controllers vector traffic more precisely around a terminal area’s airspace. The MVA within a given sector is designed to provide 1,000 feet of clearance above the highest obstacles in nonmountainous areas, and 2,000 feet above the highest obstacles in mountainous areas. And because these obstacles can be at different altitudes, and sprinkled over the terrain in a haphazard manner, the MVA sectors are designed to allow controllers to thread pilots around them—at altitudes that sometimes are lower than nearby MEAs or minimum obstruction clearance altitudes (MOCAs).

MVAs take into account individual areas of high terrain and obstacles, and let controllers vector traffic more precisely around a terminal area’s airspace.MVAs are mostly found around towered airports that are served by approach control radar services. Aircraft operating in and out of nontowered and remote airports not having approach control often face challenges because they may have to rely on air route traffic control center (ARTCC) radar services for guidance when arriving and departing. That’s nice, but there’s a catch. ATC radars used in en route segments aren’t as accurate as those used by approach control, so arriving aircraft often must be held at higher altitudes for longer distances. This is especially true in mountainous areas, where airplanes transitioning to approach fixes may have to descend rapidly—or in a holding pattern—to reach the altitudes used along final approach courses.

Another scenario involving a lack of MVAs exists where any nearby approach control radars don’t have adequate coverage. Here, reports over common traffic advisory frequencies (CTAFs) may be the only ways to receive traffic advisories, and pilots must provide their own vertical separation from terrain and obstacles by adhering strictly to any published minimum altitudes.

So if MVAs let you fly lower than MEAs or MOCAs, is there any other way or place to safely fly at even lower altitudes? For aircraft departing some major airports with approach control radar services, the answer is yes. Diverse vector areas (DVAs) at busy airports are used to vector aircraft taking off as they leave the departure end of a runway. DVAs let controllers vector you at altitudes lower than MVAs, while still providing vertical clearance from obstacles. However, the FAA’s Instrument Procedures Handbook points out what could be a source of confusion. As with MVAs, ATC knows areas and extents of DVA coverage—but pilots don’t. A pilot using a charted, textual obstacle departure procedure (ODP) for climbout may receive a DVA altitude clearance that’s lower than the one on the ODP chart. What to do in this case? Follow the ATC clearance, as it takes precedence.

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