A preliminary investigation has revealed that a Mozambican Airlines captain, whose plane went down in Namibia in November, had a clear intention of crashing the aircraft.

Captain Herminio dos Santos Fernandes manipulated the autopilot in a way which "denotes a clear intention" to bring the plane down, said Mozambican Civil Aviation Institute (IACM) head Joao Abreu on Saturday.

The plane was flying from the Mozambican capital Maputo to Luanda in Angloa in torrential rains and crashed in the swamps of Namibia's Bwabwata National Park on November 29, killing 27 passengers and its six crew.

Dos Santos Fernandes locked himself in the cockpit, ignored alarm signals and refused to allow his co-pilot back into the flight deck until moments before the crash.

"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," Abreu was quoted as saying by state news agency AIM.

Dos Santos Fernandes also manually changed the aircraft's altitude three times from 11,582 metres to 180 metres.

Airbrake parameters showed the spoilers, and aerodynamic resistance plates on the wings, were deployed and held in that position until the end of the recordings, which proved the throttle was manually controlled, according to Abreu.

Banned airline

"The plane fell with the pilot alert and the reasons which may have given rise to this behaviour are unknown," said Abreu.

Dos Santos Fernandes had logged 9,053 flight hours, his licence was renewed in 2012 and he underwent a medical exam last September.

The passengers were from Mozambique, Angola, Portugal, Brazil, France and China.

This accident is the deadliest for Mozambique since a plane carrying then-president Samora Machel went down in 1986 in South Africa following an African leaders' summit and killed an estimated 34 people.

The European Union banned Mozambican Airlines (LAM) and all air carriers certified in Mozambique from flying in its airspace in 2011 due to major concerns over safety.

The investigation into the airline's most recent fatal crash is ongoing and includes a team of experts from Botswana, Angola, Mozambique, Brazil, China, the US and Namibia.