In June 1956 a TWA Constellation collided with a United Air Lines DC-7 over the Grand Canyon in Arizona, killing all 128 people on both aircraft. At the time it was the worst ever airline disaster. Struggling with outdated technology and a post-war boom in air travel, overworked air-traffic controllers failed to spot that the planes were on a collision course.

That crash led to the creation of a new body, which became the Federal Aviation Administration (FAA), in charge of running and modernizing the world’s biggest airtransport system. With that system again struggling to keep pace with demand, Donald Trump thinks it is time to privatize America’s air-traffic control service. In June the president outlined a plan to turn air-traffic control into a separate non-profit entity financed by user fees, instead of the present patchwork of taxes and grants. Shorn of its air-traffic responsibility, the FAA would become a safety body.

America’s air-traffic system is vast, consisting of 14,000 controllers working in 476 airport-control towers that handle take-offs and landings, as well as in 21 “en route” centers looking after flights along the nation’s airways. It has a good safety record, but elderly technology limits the number of flights that can be handled. This leads to delays and frustrated flyers. With passenger numbers set to grow from 800m a year to almost 1bn by 2026, the problem will only get worse.

Mr. Trump believes that, no longer mired in a federal bureaucracy, the air-traffic service will become more efficient and better able to invest in technology. Replacing old radar-based methods with accurate satellite navigation and better digital communications is a particular priority. Aircraft using satellite navigation can be safely spaced more closely together, which permits many more planes to be in the air at the same time. Digital systems also provide data links to control centers and to other planes by regularly broadcasting an aircraft’s identification sign, its position and course. This would allow “free routing”, which means pilots can fly directly to a destination, rather than follow established airways, which often zigzag around.

The president’s proposal might even speed a move towards “virtual” control towers in low-rise buildings, which can replace towers physically located at airports. The virtual versions are fed live video from airfield cameras. Proponents argue that they are both safer and around 30% cheaper to operate. Virtual towers can look after more than one airport.

Mr. Trump, though, may struggle to get the proposal through Congress. A similar plan got stuck last year, despite being backed by most airlines and the air-traffic controllers’ union. At least the president can dodge the queues: Air Force One flights get special clearance.

The delays and upset passengers is due to________.