It was an accident, I think the purpose was to show him as a hero by sacrificing himself to steer the plane away from a residential area rather than ejecting.
It was an accident. It's confirmed at the funeral that he was trying to steer the plane away from the city, which is what he radioed in just before the crash.
"You liked Rashomon." "That's not how I remember it."
Early jet engines were really unreliable and when his started cutting up he didn't want the plane to crash in a populated area, so he headed out to sea with the intent of ejecting and being picked up later. Unfortunately, ejection systems were unreliable too and he got killed either ejecting or trying to get into his raft. Nobody knows because he was never found. Early ejection seats weren't powered by rockets like today but by explosives under the seat and injuries and deaths were common. Ejecting was just a better chance of living than riding the plane in.
As others have pointed out, it wasn't an accident but rather a selfless act of heroism. There are two takeaways from this scene and the end of the movie:
1. Meechum is a soldier, and understands that putting his life on the line is an act of duty. Instead of ditching the plane over a populated area, he chooses to glide the plane out to sea, significantly decreasing his chances of survivability. Remember, the setting is 1962, not 2016. There are no GPS locators.
2. Despite his heavy-handed, authoritarian rule of the household, Meechum was training them to be strong so they could adapt to a life without him. Like any soldier, Meechum knew there was a strong possibility that he someday would not return from a mission.
Both points help complete the character study of Meechum. There was some reasoning behind what appeared to be just restless rage.
Meechum/Santini was an abusive *beep* but also a man who would sacrifice anything for his family and country.
He is both a hero and a villain, which is what make sthe character so complex, and the story so compelling.
He can eject and have a high chance of surviving, but the plane may crash into the town and kill innocent peopel, or he can stay in the plane, and likely burn to death (or die when the plane explodes). Even though he is a cruel person, he also loves others (in his own way) enough to die to save them,