I've always found the off screen killing of Burke very odd for a number of reasons. Apparently the writers didn't know what to do with Burke, but instead of having a final dramatic confrontation with Barnabas, he was killed in a plane crash. Story wise I think this was poor, since it seemed like a very convenient way to (perhaps they thought temporarily?) get rid of Burke. If I'm correct a new writer replaced Malcom Marmorstein around this period, Joe Caldwell, and that may have affected the change. Perhaps another reason was because Dr. Woodard was recently killed off, and they thought it would have been repetitive if Burke was done in the same way. Or, he was too well liked by the audience. Still, I think it would have been better to either keep Burke alive and remain a rival and potential trhreat to Barnabas or kill him off in a dramatic way.
The period following Dr. Woodard's death seemed particularly morbid, aside from the aging of Barnabas the writers seemed to be biding their time for the 1795 storyline.