The so-called "black boxes" are only carried on airliners but their purpose is to enable the accident investigators to determine what went wrong. There are usually two - the cockpit voice recorder and the flight data recorder.

As Paula has pointed out, all aircraft (except ultralights, and possibly balloons and gliders) are required to carry Emergency Locator Transmitters (ELTs) which activate in the event of a crash. Sometimes a hard landing will set them off, but they still seem to have a very high failure rate.

I once worked in the Intelligence section of a Canadian air force base. I didn't work closely with the SAR Techs but I did have the chance to speak with them from time to time. Their major complaint was having to risk their lives unnecessarily to save someone who'd done something stupid (e.g. fishermen who put out to sea with known medical problems, figuring if their condition got worse, they could always "phone for a lift", as it were).

But I never, ever, heard of one refusing to go to a crash scene if there was any possibility of someone still being alive; and there's always that possibility unless you've ruled it out by direct observation.

As an example, see the transcript of the police debriefing of air rescue paramedic Kristy McAlister following the 1998 Sydney-Hobart Yacht Race disaster.

http://www.equipped.org/sydney-hobart/Volume%2001.htm

McAlister's crew rescued 4 people off a yacht that was sinking in gale force winds. The following day, in much calmer weather, they were patrolling the area when they picked up a distress beacon. (There were dozens of abandoned yachts whose beacons were still active, each of which had to be investigated.) They found the yacht and observed a body lying on deck, and McAlister was preparing to go down and rescue him when they were told not to bother, the person had drowned the day before when the yacht capsized. Later, when she found out that no medical professional had, in fact, examined the man, she was extremely distraught, because she felt that it was her job as a paramedic to be winched down to confirm that the dead man really was dead.

Generally, the easier it is for them to find you, the fewer risks they have to take.