The 1994 San Marino Grand Prix at Imola, Italy, was a race marred by tragedy.
On 30 April, 31-year-old Austrian novice driver Roland Ratzenberger was killed in a high-speed crash during a qualifying session.
The following day Brazilian Ayrton Senna died during the race itself when his car spun off the track at the Tamburello curve.
The Williams-Renault star - considered one of the finest Formula One drivers of his generation - was mourned by racing fans around the world.
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The Death of Ayrton Senna: How it Happened
1. Accident Mystery
The causes of Ayrton Senna's fatal crash in the San Marino Grand Prix on 1 May, 1994 will probably never be known.
Italian prosecutors blamed steering failure, but the Williams team’s two design chiefs were acquitted at the original trial and a subsequent appeal.
This is what was discovered from the car’s on-board computer.
2. Fateful Decision
The crash happened on the second lap after a re-start. Senna took a tighter line at Tamburello to ease the car’s travel over bumps that upset it on the first lap.
But at 191mph, he was going faster than the previous lap.
Combined with the altered trajectory, that more than doubled the cornering forces on the car from 1.5G to 3.27G.
3. Problems Start
The car’s rear tyres began to slide as it hit the first of two sets of bumps.
This happened, Williams say, because the airflow under the car was disrupted, combined with the higher G-forces.
The cornering speed of F1 cars is strongly dependant on aerodynamics. Any disruption dramatically reduces grip, and can cause a loss of control.
4. Senna Reacts
Just under 0.15secs after the car started to slide, the throttle reduced from fully open to 40% and the force going through the steering reduced.
Williams say this was Senna lifting off and trying to correct the slide.
The reaction time elapsed from the moment the car hit the bump is what would be expected of a driver of Senna’s ability.
5. The Crucial Moment
Just 0.04secs after Senna responded to the initial slide, the car hit the second bump.
This resulted in a sudden loss of front-end grip which, in combination with the rear tyres sliding, meant the car suddenly turned right.
This is crucial in undermining the steering-failure theory, as in that case the car would have continued straight on.
6. Senna Senses Trouble
Just 0.18secs after the car hit the second bump, Senna lifted off the accelerator as he began to try to slow it down from 191mph.
By now, he almost certainly knew he was not going to be able to keep the car on the track and so was determined to reduce his speed as much as possible before the car ran off the circuit onto rough ground.
7. Time Running Out
Four-hundredths of a second later, with the car already heading alarmingly quickly towards the outside of the track, the Williams’ Renault engine was no longer accelerating after its throttle dampeners had done their work.
8. Braking Hard
It took 0.27secs more for the brakes to start to slow the car – a delay accounted for by Senna’s reactions and pressure build-up in the system.
The car decelerates at more than 4G while on the track, losing 54mph before impact.
The braking discounts one outlandish theory – that Senna passed out briefly because he was holding his breath.
9. Out of Time
Just 1.9secs after the start of the chain of events that caused the crash, Senna ran out of time and the Williams smashed into the concrete wall on the outside of the Tamburello corner at 137mph.
The right front wheel was knocked back towards the cockpit and a suspension arm pierced Senna’s helmet visor, inflicting the fatal injury.
10. Did the Steering Fail?
The prosecutor argued that a steering-column modification was not carried out with enough care and that the new piece fatigued and broke.
Williams agree there was a partial stress fracture, but say their data prove the column (inset) was working and that it broke on impact.
The judge said the prosecution failed to prove its case.
Source: BBC Sports