A preliminary investigation has found that the captain of a Mozambican Airlines flight that went down in Namibia last month killing 33 people had a "clear intention" to crash the plane.

It says flight recorders showed he manipulated the plane's autopilot in a way which made it clear his aim was for the plane to crash.

Flight recorders showed flight TM470 went down while Captain Herminio dos Santos Fernandes manipulated the Embraer 190's autopilot in a way which "denotes a clear intention" to bring the plane down, said Mozambican Civil Aviation Institute (IACM) head Joao Abreu.

The plane was on a flight from Mozambican capital Maputo to Luanda in Angola.

"The reason for all these actions is unknown and the investigation continues," Mr Abreu said.

The plane went down in torrential rains in the swamps of Namibia's Bwabwata National Park on November 29, killing its six crew and 27 passengers.

Mr Abreu told a news conference that Mr dos Santos Fernandes locked himself inside the cockpit, ignored warning signals and did not allow his co-pilot back in moments before the plane hit the ground.

"During these actions you can hear low and high-intensity alarm signals and repeated beating against the door with demands to come into the cockpit," he was quoted as saying by state news agency AIM.

The altitude was manually changed three times from 38,000 feet to 592 feet - below ground level - and the aircraft's speed was also changed manually, according to the preliminary report.

"The plane fell with the pilot alert and the reasons which may have given rise to this behaviour are unknown. At the time, the co-pilot had left the cockpit and was absent while everything happened."

Of the 33 people killed on the plane, 10 were Mozambicans, nine Angolans, five Portuguese, one French national, one Brazilian and one Chinese.

ABC/AFP