1994 death of Brazilian race car driver
On 1 May 1994, Brazilian Formula One driver Ayrton Senna was killed after his car crashed into a concrete barrier while he was leading the 1994 San Marino Grand Prix at the Imola Circuit in Italy. The Supreme Court of Cassation of Italy ruled that mechanical failure was the cause of the crash, as post-crash analysis found that Senna's steering column had snapped around the time that his car was about to round the Tamburello corner.
Senna's death was the capstone to one of the darkest weekends in Formula One history. The previous day, Austrian driver Roland Ratzenberger had died when his car crashed during qualifying. Several other collisions took place that weekend, including a serious one involving Rubens Barrichello. Ratzenberger and Senna's crashes were the first fatal accidents to occur during a Formula One race meeting since Riccardo Paletti died at the 1982 Canadian Grand Prix.
Senna's death, as well as the other events of the race weekend, helped trigger significant reforms prioritizing driver safety in Formula One. The Formula One drivers' union, the Grand Prix Drivers' Association, was re-established in the wake of Senna's death. Formula One did not suffer a fatal accident for another twenty years, until Jules Bianchi sustained fatal injuries at the 2014 Japanese Grand Prix.
Background.
Ayrton Senna joined McLaren-Honda in 1988 and won three World Drivers' Championships with the team from 1988 to 1993. However, he had spent the last two years of his career in uncompetitive machinery, as the combination of Williams-Renault began dominating the sport.
Although Senna's fellow drivers considered him the best driver in the sport, Williams outpowered him with the help of its technological advantages. Renault boasted the best engine on the grid. Williams also employed a talented group of chassis designers, including veteran engineer Patrick Head and ascending aerodynamicist Adrian Newey. However, the most controversial aspect of Williams' package was its set of innovative electronic driver aids, including active suspension, traction control, and anti-lock brakes. In 1992 and 1993, Williams' Nigel Mansell and Alain Prost stormed to the Drivers' Championship, each winning eight of the first ten races.
Although most teams in Formula One scrambled to copy Williams' electronic advantage following its 1992 successes, including Senna's McLaren, Williams maintained its advantage in 1993. To equalise competition, the sport's top regulator, Max Mosley, attempted to ban active suspension midway through the 1993 season. However, he was unable to force through such an abrupt change. Williams was the only team that specifically optimised its chassis for active suspension, and it was pointed out that without active suspension, Williams would have to sit out at least three races while Head and Newey redesigned the car. Mosley had to settle for banning the electronics at the end of the season.
Senna joined Williams in 1994, driver aids or no driver aids. Although the Williams FW16 was still very fast (Williams won the Constructors' Championship in 1994), Senna was disappointed with its poor handling. He frequently called his old rival Prost, now in retirement, to complain about the car. The car was touchy because its design philosophy was still oriented towards active suspension. In addition, its set-up window was very narrow, leaving minimal room for error. Newey later said that the car was "aerodynamically unstable," explaining that "having had active suspension for two years, when we then lost it we had more trouble re-adapting to passive suspension than other people who hadn't been on it for very long." In addition, Senna was unhappy that the cockpit was not a good fit for his body. 
Williams did not have enough testing time to perfect the car before the start of the season. In fact, the car was not even ready in time for the traditional pre-season test at the Estoril Circuit, where Senna actually drove a modified version of the previous year's Williams FW15C (the FW15D). Senna expressed deep misgivings about the FW15D following the test:
I have a very negative feeling about driving the car and driving it on the limit and so on. Therefore I didn't have a single run or a single lap that I felt comfortable or reasonably confident. I am uncomfortable in the car. It all feels wrong. We changed the seat and the wheel, but even so I was already asking for more room. Going back to when we raced at Estoril last September, it feels much more difficult. Some of that is down to the lack of electronic change. Also, the car has its own characteristics which I'm not fully confident in yet. It makes you a lot more tense and that stresses you.
Williams rolled out the FW16 at the Jerez test on 25 February 1994, but it was not an improvement.
Senna got off to one of the worst starts of his career, retiring from the first two races. In the season opener at Interlagos, Senna took pole and initially got out to a large lead, but fell behind Benetton's Michael Schumacher after a poor pit stop. Senna pushed the car hard to catch up, but on lap 55 he lost control and spun off at the Subida dos Boxes corner. At the Pacific Grand Prix, Senna took pole again, but the car continued to perform poorly in slow-speed corners. Senna spun off track after being hit from behind by Mika Hakkinen's McLaren. Senna's teammate Damon Hill also spun off several laps later, but was fortunate to recover and made his way back to second place before retiring with a transmission failure.
Entering round 3 at Imola, Senna trailed Schumacher by 20 points. Williams readied upgrades to hopefully correct the car's handling issues.
1994 San Marino Grand Prix weekend.
The weekend of the San Marino Grand Prix was marred by a variety of dangerous racing incidents and two deaths, including Senna's. It is widely considered one of the darkest weekends in Formula One history.
Friday practice and qualifying.
On the Friday before the race, Senna's protégé, Jordan's Rubens Barrichello, clipped a curb and crashed heavily at at the Variante Bassa chicane. Senna got out of his Williams car and went to the scene of the collision. Barrichello's tongue blocked his airway and emergency work performed by FIA doctor Sid Watkins saved his life. Barrichello regained consciousness and found Senna looking over him. After learning Barrichello had survived, Senna returned to his car. He set the fastest time in qualifying, picking up provisional pole with a time half a second faster than Schumacher's.
After the session concluded, Senna left his car and went to the Williams motor home to review the car with race engineer David Brown. He was so absorbed in checking for problems that he kept his pre-arranged press interviewers waiting for an hour. Following the interviews, Senna continued his work with Brown for another two hours.
Once he arrived back at his hotel in Castel San Pietro, Senna reportedly telephoned his girlfriend Adriane Galisteu and broke down in tears while recounting Barrichello's crash earlier that day.
Saturday qualifying.
On Saturday morning, Senna tried out the car again and agreed with teammate Damon Hill that the car had improved. Barrichello was released from the hospital and told Senna that he would fly back to England and watch the race on television.
In the afternoon, the second qualifying session began. 18 minutes into the session, Simtek's Roland Ratzenberger struck the concrete wall on the outside of the Villeneuve curve at . He had previously damaged his front wing when he went off track, and at the Villeneuve corner, part of the wing broke off completely, sending the car off course. He suffered a basilar skull fracture and a ruptured aorta, and was pronounced dead on arrival at the hospital.
Qualifying was red-flagged following Ratzenberger's accident. Williams and Benetton agreed to sit out when qualifying resumed out of respect for Ratzenberger. In any event Senna's Friday time was fast enough to secure the pole position.
Senna saw the replays of the collision and rushed into the pitlane to get inside a course car. When he arrived, with Ratzenberger taken into an ambulance, Senna inspected the damaged Simtek. He then attended the circuit's medical centre where he learnt from neurosurgeon Sid Watkins that Ratzenberger had died from injuries sustained in the accident. When the two left the centre together, Watkins told Senna that he did not have to race ever again and suggested that he withdraw from the race and go fishing with him. Senna responded by telling Watkins he could not stop racing and then went back to the Williams garage, where he summoned Patrick Head and Frank Williams, telling them of the situation and deciding to withdraw for the remainder of the qualifying session.
Reportedly, Senna retired to his motor home where he broke down in tears and collapsed onto the floor. This had concerned Williams, who asked Betise Assumpção, Senna's PR chief to arrange a meeting to discuss Senna's emotional state. Senna decided not to attend the post-qualifying press conference, leading the FIA to discuss but decide not to take disciplinary action against him. On the following day, however, race stewards called Senna out of his motor home to discuss his having commandeered a course car to visit Ratzenberger's crash site. A row ensued and Senna stormed off in disgust. The stewards decided to take no action.
Sunday warm-ups and drivers' briefing.
On Sunday morning, Senna was the fastest in the warm-up session by nine-tenths of a second. 
Senna was scheduled to film an in-car lap of the Imola Circuit for French television channel TF1, where his now-retired rival Alain Prost was working as a pundit. They ate breakfast together, and Senna encouraged Prost to help out with a proposed revival of the Grand Prix Drivers' Association (GPDA), the old Formula One drivers' trade union. They agreed to meet before the Monaco Grand Prix with further details, but this was not to be. Senna confided to Prost that "I'm not optimistic at all about this race." After the breakfast, Senna filmed the lap for television and personally greeted "our dear friend Alain" on camera, to Prost's surprise.
At the drivers' briefing, Senna attended along with Gerhard Berger. Since he was unwilling to speak out due to the earlier row with race officials that had left him still fraught with emotions, Senna asked Berger to raise his concerns about the pace car's presence during the formation lap, which had no role other than to promote the then latest Porsche 911. At the San Marino Grand Prix, this pace car was thus made to leave the grid in advance of the Formula One cars, instead of together.
Senna then met with fellow drivers to discuss the re-establishment of the GPDA in an attempt to increase safety in Formula One. As the most senior driver, Senna offered to take the role of leader, starting with the next race event in Monaco. Niki Lauda suggested that Senna lead the group because of his strong personality and standing in the sport, relative to the other drivers.
In preparation for the race, Senna placed a furled Austrian flag in the cockpit of his car. It was later inferred that Senna intended to raise the flag in memory of Ratzenberger (who was Austrian), if Senna won the race.
Sunday start.
At the start of the race, Pedro Lamy and JJ Lehto were involved in a serious collision, spraying debris into the crowd and injuring bystanders. Track officials deployed the safety car, an Opel Vectra driven by Formula Three driver Max Angelelli, to slow down the field and allow the removal of debris. The competitors proceeded behind the safety car for five laps.
The Vectra was based on a family sedan so not particularly fast; Senna pulled alongside the Vectra to gesture to its driver to speed up because his tyres were getting cold and were losing grip. A subsequent investigation showed that the Vectra was inadequate for the role of safety car, because its brakes overheated and it therefore had to be driven slowly, lest it cause another crash itself. One of the theories explaining the accident suggests that this slow running caused the cars to suffer a drop in tyre temperature and pressure. Before the start of the sixth lap, David Brown told Senna via pit-to-car radio that the safety car was pulling off, and Senna acknowledged the message.
Senna's crash.
On lap 6, the race resumed and Senna immediately set a quick pace with