A heartfelt "Good luck" wish.
Airplane pilot slang, dating back before WWI, but still in use.
Severeal things were considered unlucky, like having one's picture taken in the cockpit, or to be wished 'Good luck' before takeoff. As pilots are strong believers in Murphy's law
(things never go as you want them to), this would most likely lead to disaster. Therefore, the natural way of wishing good luck on somebody, would be to call the worst possible thing down on their heads.
It is similar to the Luftwaffe's "hals und beinbruch" ("Break neck and legs") or the German Navy's "Mast- und Stengebruch" ("Break masts and yards"), or when actor's wish each other "Break a leg
" before they go on stage.
! Bandits at four o'clock, Angels 12! Crash and burn!" (Translation: "Prepare for combat. Enemy aircraft sighted on starbord side, slightly from behind, at 12.000 feet. Good luck!")
"So you got yourself a mahogany Spitfire? Doing bumph? Well, crash and burn, old chap!" ("So you got a ground job at a desk, filling out papers? (presumably away from the front line) Good luck, mate!")
"You finally got a date with her? Well, Crash and burn!"