Belows Wiki's write-up of what happened -
Tom Pryce began his final race weekend, the 1977 South African Grand Prix at Kyalami, by setting fastest time in the Wednesday practice session, held in wet weather. Pryce posted a time of 1 minute 31.57 seconds with the next best, the eventual 1977 World Champion Niki Lauda, a full second slower. The weather dried up prior to the Thursday session, and so he slipped back down the grid to fifteenth place, almost two seconds slower than James Hunt's pole position time.[34]
The Welshman's DN8 made a poor start to the Grand Prix and by the end of the first lap was in last place. Pryce started to climb back up the field during the next couple of laps, overtaking Brett Lunger and team mate Zorzi on lap two, and Alex Ribeiro and Boy Hayje the following lap. By lap 18 Pryce had moved from 22nd to 13th place.[35]
The area just after The Kink is where Pryce and Jansen Van Vuuren collided and the Crowthorne corner where Pryce and Jacques Laffite's cars collided.On lap 21, Zorzi pulled off to the left side of the main straight, just after a brow of a hill and a bridge over the track. The Italian was having problems with his fuel metering unit, and fuel was pumping directly onto the engine, which then caught fire. Zorzi did not immediately get out of his car as he was experiencing trouble in disconnecting the oxygen pipe from his helmet.[36]
The situation caused two marshals from the pit wall on the opposite side of track to intervene. The first marshal to cross the track was a 25-year old panel beater named William (Bill). The second was 19-year old Frederick Jansen Van Vuuren, commonly known as Jansen Van Vuuren, who was carrying a 40 lb fire extinguisher.[36] George Witt, the chief pit marshal for the race, said that the policy of the circuit was that in circumstances involving fires, two marshals must attend and a further two act as back-up in case their extinguishers were not effective enough. Witt also recalled that both Bill and Van Vuuren crossed the track without prior permission.[37] The former made it safely across the track, the latter did not. As the two young men started to run across the track, four cars driven by Hans-Joachim Stuck, Pryce, Jacques Laffite and Gunnar Nilsson were exiting the final corner and coming onto the main straight.
"As we got to the top I suddenly sensed this marshal running across the track from my right, carrying an extinguisher. I took a big chance and I don't know how I got away with it. There was no time, I just reacted on pure instinct."
Hans-Joachim Stuck[38]
Pryce was directly behind Stuck's car along the main straight, Stuck himself sensed Van Vuuren and moved to the right to avoid both marshals, missing Bill by what Tremayne reports to have been a matter of "millimetres". From his position behind Stuck, Pryce could not see Van Vuuren and was unable to react as fast as Stuck. He struck the teenage marshal at approximately 170 mph. Van Vuuren was thrown into the air and landed yards in front of Zorzi and Bill. He died upon impact with Pryce's car.[39] The fire extinguisher smashed into Pryce's head, before striking the Shadow's roll hoop, which threw it over the adjacent grandstand. It came to earth in the car park to the rear of the stand, where it hit a parked car and jammed its door shut.[38]
The impact with the extinguisher had wrenched Pryce's helmet upward, and he had been partially decapitated by the strap. Death was almost certainly instantaneous. Pryce's Shadow DN8, now without a driver or a roll hoop, continued down the main straight towards the first corner, Crowthorne, coasting slowly. The Shadow left the track towards the right, scraping the metal barriers before veering back onto the track after hitting an entrance for emergency vehicles. The Shadow then hit Laffite's Ligier putting the latter out of the race. As for Van Vuuren, the injuries to his body were so severe that he was identified only by exclusion after the race director summoned all of the race marshals and Van Vuuren could not be found.[40]
