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. " />

On my first Atlantic crossing, in a new Seneca II in 1977 with Globe Aero pilot Phil Waldman sleeping in the right seat, I was flying at 11,000 feet and approaching the Irish coast en route to the Paris Air Show. Navigation for little guys in those days was strictly point-and-shoot, and I was more than a little anxious about how far off track I’d be when I got to the other side. That anxiety generated an early call to Shannon radar, knowing there’d probably be no response. I was absolutely amazed when the controller came back almost immediately with, “Radar contact, 196 miles out, two miles south of track.” I didn’t know VHF communication was possible at such distances, and I certainly never expected radar coverage at that range. (Most significantly, until I made that first ferry trip, I had no idea that dead reckoning could really work over an 1,800 nm leg. Lindbergh was right.)

Even if you do have coverage within 100 to 200 nm of each coast, that leaves a huge gap in the middle where ATC can’t maintain an exact position on transatlantic flights. Accordingly, the centers use what’s called “procedural separation,” which utilizes a combination of regular position reports and reasonable performance estimates to come up with an approximate position for all aircraft on the ocean. In a full-radar environment, ATC can sometimes maintain aircraft as close as five miles from one another, but because procedural separation only generates an approximate position, Atlantic ATCs must be far more conservative.

Over the North Atlantic, jets and some turboprops operate on standard, parallel tracks that roughly prescribe great circle routes between marshalling points in Europe and corresponding centers in North America. VLF/Omega, inertial navigation and, more recently, GPS have allowed transatlantic flights to pinpoint their location at all times, but vertical precision has been more elusive. This is because standard altimeters become less and less accurate with increasing altitude. The pressure change between 40,000 and 41,000 feet is only a quarter of that between sea level and 1,000 feet, and for that reason, the old 1,000-foot separation used at lesser heights wasn’t adequate up high. Prior to a few years ago, eastbound altitudes were separated by 4,000 feet: 29,000; 33,000; 37,000; and 41,000 feet. Westbound assignments were 31,000; 35,000; and 39,000 feet. This guaranteed 2,000 feet of separation between airplanes flying in opposite directions.

The established parallel tracks run across the ocean in bands roughly 60 miles apart to meet minimum navigation performance specifications (MNPS), and ATC was in the habit of keeping aircraft on the same track and altitude separated by roughly 30 minutes. On a 480-knot airliner, that’s 240 nm between airplanes—a lot of empty space. ATC was unapologetic and felt the large gap was necessary because of changing winds aloft, performance variations between airplanes, instrument errors and possible pilot variations.




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