Basketball: Doomed Kobe Bryant flight too low to be picked up by radar

Audio from the helicopter crash that killed NBA superstar Kobe Bryant and his 13-year-old daughter suggests the pilot was flying too low to be detected by radar.

Bryant and eight others died on Monday (NZ time), when his private helicopter plummeted into hilly terrain in the Los Angeles suburb of Calabasas. 

A conversation between pilot Ara Zobayan and air traffic controllers indicates the chopper was flying under special visual flight rules (VFR).

Those rules allow helicopters to fly in conditions where visibility is an issue, which was the case on Monday, when heavy fog hit the Los Angeles area. 

Just before the crash, an air traffic controller told the pilot he was "still too low a level for flight following", stressing the chopper couldn't be picked up by radar.

ESPN reports the pilot informed air traffic controllers he was going to climb above the layer of clouds.

Unlike airplanes, helicopters are allowed to fly in poor visibility, but Los Angeles Police Department spokesman Josh Rubenstein said the dense fog had grounded the department's own choppers.

"The weather situation did not meet our minimum standards for flying," Rubenstein said.

A preliminary report into the crash is expected within 10 days.