The recent mid-air collision in Brazil of a new regional airliner (fitted out for use as a business jet) and a Boeing 737 has people baffled. How could two brand-new airplanes with advanced avionics, flown by two professional pilots in each plane, collide at 37,000′? The precision of modern avionics may well have contributed to this collision.

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 you sent airplanes up to fly in random point-to-point paths, e.g., from Boston to Denver, they’d be less likely to encounter one another. If you kept the airway system, but introduced some extra logic into the avionics so that planes always flew 1 mile to the right of an airway and + or – 200′ in altitude, they’d be less likely to encounter one another. If you replaced the precise autopilots with imprecise humans, planes would be less likely to encounter one another. If you replaced the high-precision GPS receivers with low-precision VOR receivers, planes would be less likely to encounter one another.