The best rule is tell the truth, the whole truth and nothing but the truth. BUT only if asked.
If asked only answer the question.
Mind you, may years ago I watched a senior Yorkshire policeman do just that. Every, and I mean every, question was a closed question to which the only plain answer was yes or no. That was exactly the answer that the journalist got and the further she tried to drag out other answers the firmer the yes and no became.
He did not even give 'I'm sorry I can't answer that question' phrase. A joy to watch but not the best
PR for the police.
On any crash tell the journos what is blindingly obvious or what will be revealed in a short while.
The east anglican regional PRO told a good tale. He briefed the TV journos at a field location in Norfolk about breaking news of the Tornado crash at Newcastle. The news anchor then read back his entire verbatim report and ended with 'Is there anything you can tell us about the circumstances of the crash Colonel.' Stiched up!