Future Now
The IFTF Blog
Accidents and computer accuracy
I follow Peter Neumann's RISKS List, and always find interesting (and usually scary) stuff in it. Today it pointed to a post by engineer and pilot Philip Greenspun about the role that increasing accuracy in avionics may have played in a recent mid-air collision in Brazil:
Airplanes under instrument flight rules fly from one navigation beacon to another along published standard routes. In the old days, with radio navigation receivers and pilots flying by hand, a plane wouldn’t fly its clearance exactly. The airways include a tolerance for error of +/- 4 miles. If you’re 4 miles to the right of course, in other words, you’re still legal and safe from hitting mountains or other obstacles. Altitude was similarly sloppy. If you reached for a drink of coffee or to look at a chart, you might drift up or down 200′. Air traffic control wouldn’t get upset.
How does it work now that the computer age has finally reached aviation? The GPS receiver computes an exact great circle route from navaid to navaid. All GPS receivers run from the same database of latitude/longitude coordinates, so they all have the same idea of where the Manchester, New Hampshire VOR is, for example. The autopilot in the plane will hold the airplane to within about 30′ of the centerline of the airway and to perhaps 20′ in altitude. If two planes in opposite directions are mistakenly cleared to fly on the same airway at the same altitude, a collision now becomes inevitable.
Almost any other system would be safer.
If correct-- and apparently some other navigation systems deliberately introduce a measure of fuzziness in order to avoid problems like these-- this is another good example of how terms like "accuracy" are interpreted and enacted by computers and people, and what happens when different interpretations (pardon the term) collide. We speak of computer and human "memory," but these are actually very different things. Likewise, it may be that the degree of inaccuracy in old avionics systems served as a buffer that helped prevent mid-air collisions.