I recently watched the film Dunkirk (I highly recommend watching it).
Without wanting to give away too much information about the film, a Spitfire pilot is flying over part of The Channel. They lose their engine and state the sea looks calm and opt to ditch over bailing out.
Why would a Spitfire pilot choose to do this? Is ditching in the sea really safer than parachuting out?
Bailing out the cockpit during that era was neither an easy nor always a successful task. The pilot had to either roll the plane, open the canopy, and release themselves to the void, or if rolling wasn't possible, walk on the wing and jump.
That's inherently unsafe. Don't forget that you are not exiting a healthy plane. There might be smoke, debris, fire, propellers spinning, the tail that might cross your way out.
Read what happened to Hans-Joachim Marseille when he tried to bail out. Quote from Wikipedia:
the left side of his chest striking the vertical stabiliser of his fighter, either killing him instantly or rendering him unconscious to the point that he could not deploy his parachute.
Now, regarding ditching versus bailing out, I don't have any source to substantiate the chances of survival, because in the end, what matters is to get out of there alive. But I guess it depends on many factors, one of them being the sea state as they mention in the film.
Another one is the type of damage the airframe has sustained. It might be better to risk a "gentle" contact with the water if there is no "urgent reason" to leave the plane and you can ditch reasonably soon, than to exit the cockpit and increase the chance of being injured by anything that I've mentioned in the first paragraph.
Finally, it turns out that the choice heavily depends on the situation. I haven't seen the movie so besides the "sea state" I don't know the details. So provided that you are above a water mass capable of supporting the ditching idea, the factors mostly influencing the decision are:
In case of Spitfire, ditching in sea is not safer than bailing out. Spitfire XIV & XIX pilot's notes specifically states:
71 Ditching
(i) Whenever possible, the aircraft should be abandoned by parachute rather than ditched, since the ditching qualities are known to be very poor.
I don't think the advice would be any different in case of earlier models.
In such a case, the pilot's decision depends on his mental state under extreme stress. Maybe he was afraid of getting caught in the suspension lines of the chute, or hypothermia, which was quite possible if there is no timely rescue.
Or maybe he simply felt safer flying the aircraft than bailing out. It has to be noted that the pilots were not given any practical training in bailing out, which was hazardous at the best of times (even now with ejection seats). The description of a Spitfire pilot about bailing out goes like this:
... you loosened your straps, jettison the canopy and held the aircraft level, but trimmed forward and you let go the stick, which bunted the aircraft and you shot out and then the aircraft would drop away and as you dropped, you could pull your parachute...
No wonder the pilot preferred to be in the cockpit. This particular pilot forgot to jettison the canopy and his chute got caught in it, then escaped somehow and forgot to pull his chute before finally getting on ground in one piece. In that state, the pilot might have decided to take his chances rather than follow the notes.
Airplanes tend to float after ditching due to the air trapped in fuel tanks, wings and fuselage spaces, acting as a life raft which is very valuable. A ditched plane is also much more visible than a single pilot floating in the water, making rescue more likely.
The English Channel is also very cold throughout the year, being able to stay out of it by climbing on a wing will limit exposure and increase survival time dramatically. Spitfire pilots didn't have rafts, so this was an important consideration.
For baling out, height is the main criteria. If at a low height above the water there is a chance you will strike the water without the parachute being fully open, depending on the trajectory of the aircraft and the time it takes to get out of the cockpit.
Assuming there is enough height, I would have thought that ditching is more dangerous, because the impact forces could be high enough to knock the pilot unconscious against the gun-sight just ahead of his head, who will then drown when the aircraft sinks. Spitfires and Hurricanes were not known for floating for very long on the surface.
The procedure for ditching is not straightforward, but the whole aim is to minimize impact forces because these are a square of the speed (i.e. you double the speed you quadruple the forces felt), so its important to fly at the lowest controllable airspeed into wind (to reduce groundspeed) where possible but along the swell/waves so you don't crash headlong into a wave on touchdown. Try also to make the tail-plane the first thing that touches the water because the deceleration won't be as fierce as hitting wings or nose first. Also if its the nose or the wings first, on the Spitfire you could "submarine" (i.e. the flow of water over the wings would take the aircraft down before its even stopped). Undercarriage also must be up to prevent it making the aircraft flip or submarine on touchdown. Canopy open so it doesn't get damaged during the impact and then impossible to open quickly.
For whatever reason, in WW2 they were often feet underwater before the aircraft stopped moving so while you needed to be strapped in to survive the impact, you then had to unstrap pretty quickly and make your way to the surface.
I haven't seen the film, but based on the large number of WW2 biographies I have read, I think that in reality, given it is your own life on the line, and that you most likely haven't parachuted before, its entirely your own personal preference based on your own personal understanding and fears.
My personal view is that if you are doing much over 60mph in a small aircraft at touchdown in a ditching then the outcome is probably not going to be pretty, even 70mph may well make a big difference and 80 or 90 would most probably kill you. I'm not saying you can't survive higher speeds, but if you do then you are very lucky. You only have to read accident reports to understand that if you don't get the speed right, e.g. ditch downwind, or not at the minimum speed, you will be dead. This information only heightens my respect to the people who flew in wartime.
In this very specific case, he was close enough to safe shores that the glide path provided significant benefit. Ditching would allow him to get significantly closer to safety than jumping.
So looking at the short term, jumping may have been safer, but looking at the long term, ditching was seen as safer even once you account for the fact that ditching itself may be less safe than jumping.
No pilot decision is made in a vacuum, however, and as pointed out by many others there were many reasons for this decision that wouldn't necessarily apply generally or in other situations.
