* Gleirscher claims Austria's first gold in 50 years
* Champion Loch crumbles on final run to finish fifth
* Mazdzer grabs breakthrough silver for United States (Adds details, quotes)
By Ian Ransom
PYEONGCHANG, South Korea, Feb 11 (Reuters) - Surprise package David Gleirscher claimed Austria's first Olympic luge gold in 50 years at the Pyeongchang Games on Sunday after 2014 champion Felix Loch stumbled dramatically with his third successive title in sight.
The 23-year-old police moved to the top podium position with a composed final run but Loch needed only to complete an error-free effort at the Olympic Sliding Centre to be assured of his golden hat-trick for Germany.
Having been flawless all weekend, he crashed heavily into the wall midway through his run, knocking him completely off medal pace and leaving a disbelieving Gleirscher jumping for joy in the leaders' booth.
The Austrian claimed the title with an aggregate time of three minutes 10.702 seconds, just 0.026 seconds ahead of runner-up Chris Mazdzer who became the United States' first man to claim an individual Olympic luge medal.
Germany's Johannes Ludwig won the bronze, his compatriot Loch finishing a scarcely believable fifth, ending eight years of Olympic dominance, with his bid to match compatriot and mentor Georg Hackl's 1990s hat-trick in ruins.
The bearded Gleirscher has a fine pedigree, with his father a three-times Olympian luger, but he had never made the podium at a World Cup event and only just sneaked on to Austria's team for Pyeongchang.
"This is quite unbelievable," the beaming Austrian told a media conference. "Indeed it has been like a fairytale ... I never made a podium at a World Cup and now here I am at the top of the podium."
On a bone-chilling night at the Olympic Sliding Centre, Loch was smiling after he grabbed the track record in his third run as snow fell in heavy flurries at the venue.
His father Norbert, a former Olympic luger and current head of the German team, clapped his hands and nearly head-butted the ceiling with a jump for joy.
Loch was still grinning in the leaders' booth as his rivals took their turns but he was noticeably surprised when Mazdzer bettered his time with a scintillating 47.534-second run to leap into second.
Mazdzer's camp, consisting of parents, two sisters, his girlfriend and two buddies, duly went ballistic at the finish-line terrace.
The celebrations continued even after he was leapfrogged by Gleirscher in the final run. The American and the Austrian stood side by side as Loch made his mistake and it was bear-hugs all round when their medals were confirmed.
"One very bad run but that's sport," said Loch, who was consoled by IOC President and luge fan Thomas Bach at the venue.
Mazdzer was thrilled with his silver, having been almost at "rock bottom" with his form only weeks before.
"It's 16 years in the making (and) what you dream about as a young child and 20 years later you're finally on the podium," the 29-year-old said. "I don't know how to describe it."
Editing by Clare Fallon and Clare Lovell