IT COULD have been one of the worst aviation disasters in history. Last August only a last-minute intervention stopped an Air Canada aeroplane from landing on top of four planes that were lined up on a taxiway at San Francisco’s airport. The incoming craft, which had mistaken the taxiway for a runway, pulled out of the landing at about 400 feet and narrowly missed the aircraft on the ground. Flyers spooked by such stories should concentrate, perhaps, on the fact that the accident did not happen. Indeed they are more likely to choke to death on a pretzel or drown in the bath than die in a plane crash. With a rate of just one fatal accident for every 16m passengers flown, 2017 was the safest year for air travel. How has it become so safe? And will it remain so in the future?

In the early 1920s aviators would fly with little navigational aid. Lost pilots would simply descend to read the signs on railway stations. Some relied on hand-signals, flags or giant concrete arrows on the ground to show them where to land. In 1935, with pilots now able to communicate through cockpit radios, the American government backed the first air-traffic control facility to regulate the skies over Newark in New Jersey. The invention of radar, just before the second world war, brought even more changes, enabling air-traffic controllers to determine the position of aircraft at all times.

The next raft of safety measures was rather more recent. Since 1993 and 2000, in America and Europe respectively, it has been mandatory for aircraft carrying 19 or more passengers to have an onboard Traffic Collision Avoidance System (TCAS). This communicates with the transponders of nearby aircraft to determine their bearing, speed, distance and relative altitude. On spotting an aeroplane that is getting too close, the TCAS issues a warning to both pilots and recommends an evasive manoeuvre, such as climbing or descending, in co-ordination with the other aircraft’s system. The system, however, is prohibitively expensive for small aircraft and cannot be installed without a redesign. Enter Automatic Dependent Surveillance-Broadcast (ADS-B), a more affordable alternative that uses a satellite-based global positioning system to spot flying objects. Information about the aircraft’s precise location is dispatched to the nearest air-traffic control and also to nearby aircraft.

The danger posed by other aircraft is now, arguably, trumped by that posed by drones. A recent report by America’s Federal Aviation Administration (FAA) estimated that remote-controlled civilian drones buzz past other sorts of aircraft about 250 times every month. Although light, they are capable of harming an airliner’s engine, wing or windscreen. The latest such incident occurred in November when a drone in Buenos Aires hit an airliner carrying 121 passengers, causing minor damage to its fuselage. Drones fitted with low-power ADS-B would be better able to avoid aeroplanes and each other. Other challenges remain. Although FAA regulations prevent drones from flying near airports, some operators ignore these rules and others are simply unaware of them. The British government has considered passing a bill that would regulate the use of all drones weighing 250g or more. With more than 3.1m shipped last year in America alone, the need to devise smarter technology and better rules to keep drones away from aeroplanes, and from each other, is growing urgent.