Air crash investigators will spend at least two days at the scene of a light plane accident that has left a student pilot and an instructor fighting for life in Melbourne hospitals.
The ultralight Bristell two-seater crashed into a paddock at Clyde North about 10.40am on Thursday, with one person, believed to be a man in his 30s, flown to Royal Melbourne Hospital in a critical condition.
A second man, thought to be in his 20s, was freed from the training plane after more than an hour and taken to The Alfred Hospital with facial, chest and spinal injuries in a serious but stable condition.
A hospital spokeswoman said his condition had been upgraded to critical on Thursday afternoon.
Recreational Aviation Australia will be investigating the crash as the governing body for ultralight aircraft and pilots.
"We will have two investigators on scene this afternoon and all day tomorrow," Mr Linke told AAP on Thursday.
It was too early to comment on the cause of the crash but investigators would look at environmental, human and mechanical factors to determine the cause, he said.
Witness Kane Loveday who was driving in the area, "said the engine didn't sound anywhere near right".
"It sounded like a lack of power basically and before I could see anything it was below the tree line," he told the Seven Network from the scene.
Civil Aviation Safety Authority spokesman Peter Gibson says the ultralight was on a training flight when it crashed.
"It was on a dual training flight with an instructor and a student on board," Mr Gibson said.
The plane broadcast a mayday message before crashing into a paddock on Hardys Road, the CASA spokesman said.
The aircraft was registered to the Learn to Fly Melbourne school based at Moorabbin Airport.