Pilot Journal
Sunday, July 1, 2007

What’s RVSM?

A great idea that allows ATC to fit more airplanes into smaller, radar-less airspace

The problem was simple: too many airplanes and too little sky. This flies in the face of traditional wisdom that suggests it’s a very big sky. While that’s unquestionably true above places such as Chad, Antarctica and the Gobi Desert, there are other places where there’s an uncomfortable amount of aluminum vying for roughly the same airspace. " />

Gradually, increasing numbers of airliners and corporate traffic made it obvious that flight levels above the ocean needed to be expanded to accept more airplanes. Tracks were pretty much locked in cement, and controllers weren’t willing to compromise on half-hour separation, so the only reasonable method of accommodating additional traffic was to increase the number of available flight levels.

The state of the art in both traffic avoidance and altimetry has changed dramatically since the days when tracks and altitudes were established. These days, a traffic collision avoidance system (TCAS) is the rule rather than the exception, providing in-cockpit horizontal and vertical warnings of other traffic. Accordingly, the simplest solution was to adopt 1,000-foot separation above 29,000 feet, called reduced vertical separation minimums (RVSM). Most of the rest of the world embraced RVSM for the high-altitude regime around the turn of the century. The United States made the change in January 2005.

As with most things aviation, RVSM certification isn’t free. Both aircraft and pilots need to be approved for RVSM operation. The most obvious improvement is to altimeters. To qualify for the tighter altitude limits, RVSM aircraft need to mount two independent primary pressure altitude reporting systems (coupled to an air data computer), an altitude alert system and an autopilot certified to tighter altitude hold limits. Static source error correction units (SSECs) also need to be in place to assure accuracy.

Despite the errors inherent in high-altitude operation, the new altimeters must be guaranteed accurate to 25 feet at 30,000 feet in contrast to standard altimeters, which can accept a 180-foot error. Pilots rarely hand-fly aircraft above 30,000 feet, and for that reason, autopilot servos must be more refined, capable of holding an aircraft at exactly the proper height.

What's RVSM? All this avionics innovation translates to a significant expense for regular users of RVSM airspace, primarily the commercial airlines, but also for the corporate sector. To qualify for RVSM certification, a typical Cessna Citation or Lear operator pays between $70,000 and $100,000.

Despite the one-time cost, the benefits of RVSM are considerable. If a controller has two airplanes in an overtake situation at the same altitude, for example, he can merely step up the overtaking airplane 2,000 feet for long enough to get the aircraft by the lead traffic, then drop the new leader back down to its former altitude.


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