The 919 Hybrid is equipped with hundreds of sensors. For example, they measure the amount of fuel in the petrol tank, the system's hydraulic pressure, the oil and braking temperatures, the accelerator position, the voltage. Or they record the vehicle behaviour when the brakes are applied or while cornering. The driver is fully transparent, like glass. Nothing escapes the engineers' notice. But they are not allowed to take direct action. One-directional telemetry – the car transmits data to the pit – is the permitted standard. Bi-directional telemetry – engineers having direct influence on the car – is prohibited. That's why the voice radio plays a central role.
Who talks to whom? Only one person talks to the driver so that he is not subject to excessive distraction – his racing engineer. Each vehicle has one, and he sits in the command stand at the pit lane wall. He in turn receives a lot of different information. The engineers keep him up to speed about the car's condition, and about the competitors' behaviour. The strategy engineer discusses suggestions for the respective situation with him, the crew chief wants to know if the tyres should be changed during the next fuel stop, and if yes, which set of tyres should be mounted.
The racing engineer needs to be able to stay exceptionally level-headed. His radio channels are not prioritised, he hears people all talking at the same time. And he must never, ever miss anything the driver says. If he reports that the car's handling is changing or the tyre performance is deteriorating, it means action is needed. Just don't rush anything! When the driver radios that driving with smooth-tread slicks is no longer possible with the rain setting in, the racing engineer has only seconds to assess the situation. If the engineer monitoring the weather satellite images tells him that the rain will stop shortly, and the colleague watching the competitors tells him that the rival is already heading to the pit for a tyre change, it might be beneficial to reassure the driver and have him stay on the track. “Keep the car on track, rain will stop soon, don’t worry, you are doing well, mate.” The sonorous voice of Stephen Mitas, car no. 19's racing engineer, conceals the stress that he is under. He also has to be able to take risks and gamble to gain an advantage over the competition. As a precautionary measure, the racing engineer radios the crew chief and the mechanics: “Prepare yourselves for car no. 19. Full service. Fuelling, tyre and driver change.”
Race Control is also listening in. If a driver radios “Blue flag, blue flag! ” from an LMP1 vehicle – the premier class in the World Endurance Championship – he is complaining about being obstructed while lapping backmarkers. Race Control must then ensure that the the driver to be lapped is actually shown the blue “make-room-flag” and if necessary, punish the driver who ignores the signal. Sometimes the dialogues between driver and racing engineer make it to the television screen to entertain the viewers. Some codes, however, can only be understood by team members, as the rivals are also listening in.