By Byron Kaye PERTH, Australia (Reuters) - Australia vowed on Wednesday to keep searching for a missing Malaysian plane despite no sign of wreckage after almost seven weeks, and as bad weather again grounded aircraft and an undersea drone neared the end of its first full mission. Australian Prime Minister Tony Abbott admitted the search strategy may change if seabed scans taken by a U.S. Navy drone failed to turn up a trace of Malaysian Airlines flight MH370, which vanished on March 8 with 239 people on board. "The only way we can get to the bottom of this is to keep searching the probable impact zone until we find something or until we have searched it as thoroughly as human ingenuity allows at this time." The Bluefin-21 drone, a key component in the search after the detection of audio signals or "pings" believed to be from the plane's black box flight recorder, is due to end its first full mission, possibly on Wednesday. In a sign of the families' growing desperation for answers, a group purporting to be relatives of the missing flight's passengers published a letter to Malaysian Defence Minister Hishammuddin Hussein, urging the government to investigate old media reports that the plane landed in Kandahar, Afghanistan.