As the wheels went up on Mercedes' charter flight back to the U.K. on Sunday evening, the team's engineers were still trying to figure out how, exactly, they had lost the French Grand Prix. Taking off from the airstrip that runs parallel to the pit straight at Circuit Paul Ricard, the view from the jet window offered a complete picture of the scene of the defeat, yet the detail remained lost in the data.
So closely fought was the race that took place on ground level a few hours earlier, that a single second of "lost" time at Lewis Hamilton's pit stop had decided the outcome. Arguably Max Verstappen would have won regardless of the missing second given the performance he summoned from his Red Bull later in the race, but considering he only looked certain for victory six miles from the end, that second of unaccounted pit stop time at Mercedes could have made all the difference.
But we're getting ahead of ourselves. To fully explain the story behind the missing time, we need to start from the beginning.
After qualifying on pole position, Verstappen lost the lead at the first corner when a tail wind caught him by surprise under braking into Turn 1 and he went deep into the corner. He skipped the apex of Turn 2, allowing Hamilton to pass him and take the lead, and rejoined in second place ahead of Hamilton's teammate Valtteri Bottas in third.
It could have been a decisive error in the race as it gifted Mercedes' strategists the upper hand when it came to the first pit stops -- with Verstappen now needing to keep one eye on Bottas behind as well as Hamilton ahead.
The Mercedes pit wall started to exercise its stronger hand on lap 17 when Bottas was brought in to change tyres. That put pressure on Red Bull to do the same as Bottas' fresh tyres would allow him to lap faster than Verstappen on old tyres -- a strategic play known in F1 as an "undercut". As Mercedes had hoped, Red Bull's strategists countered in the only way they could, bringing Verstappen into the pits one lap later -- at which point he was 3.2s behind race leader Hamilton but still in a position where he could pit and resume the race ahead of Bottas.
Because he stayed out a lap longer than Bottas, Vertsappen lost 1.2s relative to the Finn on the undercut but, surprisingly for Red Bull, Mercedes did not pit Hamilton at the same time.
With the benefit of hindsight, it would have been the right call to bring Hamilton in on lap 18 and it may have won Mercedes the race. If the two race leaders had pitted together on the same lap and the tyre changes on both cars had gone to plan, the two would have exited the pits in the same order as they entered, ensuring Hamilton led at the start of the next phase of the race and controlled the pace.
Of course, if Hamilton had pitted on lap 18, Verstappen would have seen that play out in front of him and Red Bull could have opted to stay out on track. But there would have been very little strategic advantage in doing so as he would only opened himself up to a more severe undercut to both Bottas and Hamilton, potentially leaving him in third place when he returned to the track.
Ultimately, it was a Catch-22 situation for Red Bull.
So why didn't Mercedes pit Hamilton on lap 18 along with Verstappen?
Ultimately, Mercedes' strategy modelling software was telling the pit wall that, thanks to Hamilton's 3.2s lead over Verstappen, it could delay his pit stop until lap 19 and still emerge in front, regardless of what Verstappen did. By staying out to lap 19, Mercedes could ensure Red Bull acted as expected on lap 18 by calling in Verstappen and would then be able to call Hamilton in one lap later and still retain the lead.
At least that's what the strategy model said...
"We thought, when we had just over three seconds to Max, that we were safe from the undercut, and that wasn't the case," Mercedes' head of trackside engineering Andrew Shovlin said as he waited to board his flight home on Sunday evening.
"Even now, we don't fully understand why our models were telling us that would be OK, so clearly there is something we need to go off and understand there.
"If we had brought Lewis in the lap after Valtteri -- whether Max would have followed us in, I don't know -- but if we had done that, it would have put us in a decent position.
"So really we thought we were OK [to stay out until lap 19]. We couldn't have been any further up the road because Lewis had gone flat out in that first stint, but we could have brought Lewis in a lap earlier."
But Verstappen is the type of driver who gets a kick out of making rival strategists, and their software models, look silly.
Whereas Bottas gained 1.2s relative to Verstappen using the undercut between laps 17 and 18, Verstappen gained 1.6s on Hamilton when their split times were compared like-for-like over laps 18 and lap 19.
It was a significant chunk of time to take out of Hamilton's buffer, but it doesn't explain the full 3.2s Mercedes thought it had in hand.
When it came to Hamilton's own pit stop on lap 19, his stationary time was actually 0.1s faster than Vertsappen, but his total time from entering the pit lane to exiting was 0.6s slower. That discrepancy between stationary time and pit lane time was likely down to how late Hamilton was on the brakes entering his pit box and how much wheelspin he had leaving it, but when added to the 1.6s Verstappen found on track, it still only accounts for 2.2s of the 3.2s advantage Hamilton seemed to have before Verstappen pitted.
Clearly the remaining second went missing somewhere as Verstappen arrived at the apex of Turn 1 on the start of lap 20 ahead of Hamilton and took the lead.
"We still need to go through because we haven't quite understood how we lost the position," Shovlin added. "We can only account for about 2.5s of the 3.0s, so there is something that we need to look at in fine detail, how we were undercut from so far back, because we didn't expect that."
It explains why the team's chief strategist, James Vowles, took to the pit-to-car radio after the race to tell Hamilton: "That one was on us".
Speaking to the media after the race, team principal Toto Wolff also acknowledged the error.
"Our car performance was good, I think we had the quicker car probably," he said. "We lost the race at the [first] pit stop, thinking that we had enough protection against the undercut, which we didn't.
"We had a solid three-second gap to protect against the undercut and that wasn't enough as it looks and from there on we were on the back foot actually."
Once Vertsappen had regained the lead, he was able to control the race -- but it still required some top-level driving and some bold calls from the Red Bull pit wall to secure victory.
Hamilton pushed Verstappen hard after the first pit stops in an attempt to regain the lead on track, which took life out of the tyres on both cars. As more and more radio calls from the drivers expressed concerns over the state of the front left tyres in particular, it became clear a two-stop strategy could be the fastest way to the chequered flag.
A similar situation arose at the Spanish Grand Prix earlier this year when Verstappen was leading the race and Hamilton made a pit stop from second to take on fresh tyres and hunt the Red Bull down to take victory.
That option was also available to Mercedes in France, but the key difference was the presence of Sergio Perez in fourth place. Perez ran longer into the race on his original set of Pirellis than the top three drivers and therefore still had relatively fresh tyres once Hamilton and Verstappen started considering a second stop.
Perez's position on track behind the top three meant the Mercedes drivers would have to overtake him if they wanted to have a shot at making a second pit stop work, and the relative youth of Perez's tyres meant that would be tricky.
"The fight between the three cars was intense at the front and I think you then basically had to opt to continue with the one stop or the two," Wolff said. "The two stop was a danger for us because Perez was in the way, and we got it wrong today."
For Verstappen, of course, there was no such worry as Perez would be asked to move out of his way, making it possible for Verstappen to make a second pit stop, fall to fourth place and still hunt down Hamilton before the end of the race.
"At that point it felt like Mercedes were pushing very hard, and we just didn't want to be in the same position as Barcelona," Red Bull team boss Christian Horner said after the race.
"It's always a difficult thing to pit from the lead with 21 laps to go, but that's what we did, whilst in the meantime leaving Checo on a one stop, and it worked out.
"Obviously Max had the pace to catch and pass the three cars ahead -- it was crucial to get past Bottas quickly -- and then it was great to get Lewis [for the lead].
"It was a little bit of payback for Barcelona earlier in the year with a lap and a half to go."
It was clear that the victory meant an awful lot to Verstappen and Red Bull.
"I think that we came here knowing that this would be one of Mercedes' strong circuits," Horner said.
"I mean, they've led every lap here prior to this race bar one, and I think all of that was with Lewis.
"So it's fantastic that we've had a big score here this weekend, and it's the win Max should have had two weeks ago [in Baku]
"But you can see how close it is, there's nothing between the two cars.
"So we've just got to keep pushing, keep looking for performance, and keep pushing all the way through.
"There's such a long way to go in this championship, we can't take anything for granted."
To add to Red Bull's satisfaction, this defeat hurt all the more for Mercedes because it had been in a winning position before the first pit stops.
"It is annoying because I think we could have won the race and had two cars on the podium and we are in a championship where we can't afford to let these opportunities go by like they did today," Shovlin said.
"But they weren't easy decisions at the time they came up and fundamentally we have got a good a race car - we are lacking a bit in qualifying - but in a normal race we are able to put them under pressure.
"But this team at its best can beat Red Bull. We saw that the opportunities missed were the difference.
"We know it's going to be difficult and we think that they are the favourites, they clearly have a very good package, but if we perform at our best we can beat them and win the championship."