Two helicopters collided in mid-air over Rio de Janeiro on Sunday morning, and all six people aboard both aircraft were killed, according to AP News. Brazilian authorities are investigating the crash as an accident. There have been no reports of people killed on the ground.

Rio de Janeiro's airspace is crowded — much more crowded than most cities. Helicopters fly there for business, private transport, and air taxi services. The city's geography makes this worse: mountains and coastline squeeze helicopter traffic into narrow flight paths. Some areas of airspace over Rio don't have strict control. In those zones, pilots navigate by looking out the window and watching for other aircraft. The collision happened in one of these less-controlled areas.

Brazil's aviation safety agency, CENIPA (Centro de Investigação e Prevenção de Acidentes Aeronáuticos), will investigate what caused the crash. The agency will examine radio communications, flight paths, and other details. The identities of the victims and the names of the helicopter operators have not yet been released.

This matters because of history. In 2006, a small business jet and a commercial airliner collided over the Amazon rainforest, killing 154 people. That crash exposed serious problems in how air traffic was managed in Brazil. It prompted changes to the military body that oversees Brazilian airspace, but safety experts have questioned whether those reforms went far enough. This new collision may force Brazil to examine whether helicopters — which are regulated differently and less strictly than large commercial airplanes — are getting the oversight they need in a busy city like Rio.

When two helicopters are on a crash course at low altitude, there is almost no time to prevent impact — seconds separate a near miss from a disaster. What happens next depends on what investigators find. If they discover that helicopter tracking systems weren't good enough, or that the rules for keeping flights apart weren't being followed, those findings could lead to stricter rules for helicopter operations across Brazil.