There is increasing circumstantial evidence that the South African Airways (SAA) Rietbok that crashed into the sea near East London in 1967 could have suffered structural failure.

The latest evidence is contained in the book Final Postponement, by the late Judge Cecil Margo, the judge in the controversial Helderberg and Rietbok disaster inquests. The book was published shortly before his death.

At the time of the Rietbok disaster, four Vickers Viscounts crashed. Two of them were found to have suffered structural failure. The other two disappeared into the sea and were never found.

The book reveals that Margo won a court case involving structural failure of a Central African Airways (CAA) Viscount before the Rietbok crash.

However, Margo then found in the Rietbok investigation that "on the overwhelming probabilities" the Rietbok was completely airworthy immediately before it hit the water.

However, shortly after the Rietbok disaster another Viscount, on a flight to Ireland, also disappeared into the sea in similar circumstances.

This was followed by a fourth crash in Australia, but in this case the Viscount disintegrated over land, and it was found that the main spar had failed, causing a wing to come adrift.

In Margo's CAA case, a portion of one of the bolts that had secured the wing in the centre section was found to have failed from fatigue. In his book, Margo said expert opinion was that Vickers "had been negligent in designing the wing bolts to remain as originally installed, without any provision for regular inspection, and without providing cadmium protection against corrosion".

This view was confirmed by a top SAA technician, who said a weakness of the Viscount design was the difficulty in servicing the main spar.

The Rietbok was bought from Cuba and was Fidel Castro's personal aircraft. The technician said certain mechanical work, which was supposed to have been undertaken before SAA took delivery from Vickers, was never done. This included replacing the turbine blades which had been corroded through the use of Russian booster fuel used during take-offs. These blades had to be replaced in SAA's workshops.

Margo's book said the CAA case was the first of the "fatigue" cases, among which was the British Overseas Aircraft Corporation accident in which a Comet broke up over the island of Elba. Another Comet broke up in India, and a third also over Elba. He said several investigations were solved by reference to fatigue failure of primary structure.

Yet in the Rietbok investigation, his inquiry accepted that the Rietbok was "completely airworthy" and concentrated on the possibility that the pilot had suffered a heart attack or stroke.

The Rietbok crash continues to be controversial because of the way the investigation was handled by the apartheid government, and claims that the aircraft was brought down by its agents.