When we first meet Tom Hardy's character Farrier in the film, he's already in the air, and the big words on the screen handily announce that his story lasts an hour. In that hour, we see him engage in several dogfights, pull crazy stunts, and mercilessly hit the throttle, all while chalking his remaining fuel on his instrument panel. It's very, very intense. Butt-clenchingly intense. We're right there in the cockpit with him, feeling the pressure as that fuel starts to run out. But...you know where we're going with this. He should have had plenty of time to do his thing.
The main fighter used by the Royal Air Force at Dunkirk was the Supermarine Spitfire Mk1. That's what RAF pilots flew at Dunkirk, and what Farrier flies in the film. They were fast, agile planes with a fuel tank that could hold 85 gallons, giving them a 395-mile combat range. Now, here's a different number: 55 miles. That was the distance ships had to travel across the English Channel between England and Dunkirk. By air, it's even shorter. In the hour showing Farrier's story arc, he starts at 50 gallons in the tank and has to pop his reserve tank by the time he reaches Dunkirk.
Even accounting for the dogfights and switchbacks to chase the bomber going after the British destroyer, that hour's flight time should have gotten him to Dunkirk and back across the channel before teatime, considering the fact that Spitfires could fly upwards of 360 mph. Oh, and unless they were refitted for reconnaissance missions, there was no reserve tank on the Spitfire Mk1. But hey, that powerless glide he did at the end was totally legit.
