Andrew Scheffer, 18, apparently veered off course and hit near the top of the mountain about 40 miles south of Billings, authorities said.
When he met up with rescuers around 11:30 a.m., Scheffer was suffering from hypothermia.
"He ended up hiking quite a ways in his shorts and tennis shoes, in waist-high snow. He was very cold and cut up by the time we found a place to land and could hike into him," said Jon Trapp, assistant coordinator of Carbon County Search and Rescue.
He was taken to St. Vincent Healthcare in Billings, where he was in critical condition Wednesday afternoon, hospital spokeswoman Jeanelle Slade said.
Scheffer had stayed with the 2006 Piper through the night, Trapp said. With overnight temperatures dropping close to zero, Scheffer wrapped himself in an orange tarp to keep warm and also wore a jacket and wool cap.
The student had contacted his flight instructor via cell phone at about 8:30 a.m. to report he survived the crash with a dislocated shoulder and other minor injuries, said Mike Fergus, a spokesman for the FAA's Northwest Mountain Region in Seattle.
Scheffer started to hike out after he was spotted by rescue planes, around the time he reached the flight instructor.
With cell phone service in the area spotty, Scheffer made several other calls and sent text messages before his phone went dead, Trapp said. Rescuers on the ground had searched for him through the night in an area about 8 miles from the crash site, Trapp said. The downed aircraft was spotted from the air at about 9 a.m.
Dan Hargrove, director of aviation for Rocky Mountain College, described Scheffer as an experienced pilot with 150 hours of flying time. He said the solo flight - attempted as part of the flying program's syllabus - was not Scheffer's first.
"He's a private pilot and an instrument pilot - he's a real pilot," Hargrove said.
About 100 students are enrolled in Rocky's 10-year-old aviation program, which offers degrees in aeronautical science and aviation management. The Billings-based program has a fleet of eight Piper and Beechcraft planes.
