Crash investigators probe why engine of Sydney light plane failed mid-flight

Crash investigators are working to determine why the engine of a light plane failed, forcing it to make an emergency landing on a sports field on Sydney's northern beaches yesterday.

A young flying instructor with the Sydney Flying Academy and his co-pilot remain in hospital after crash-landing on a cricket oval in Cromer about 4.30pm yesterday.

"It started to cut out, the next thing I know I'm getting told 'Run, run,'" Cromer Cricket Club member David Wilson told 9News.

"I was just screaming 'Run, run,' because all I could see was the plane hitting that shed roof," fellow club member Greg Rollins told 9News.

Mr Rollins said he and five others rushed to aid of the instructor and his passenger, dragging them out of the plane.

"One guy had blood all over his face. The other guy had a broken leg," he said.

A customer of the NSW flying school turned up at the hangar today to fly the same plane, unaware it had been damaged in the crash.

He had attempted to book the plane on Friday but was told it was undergoing maintenance.

"I don't know if it was maintenance for anything specific," the pilot told 9News.com.au.

The Civil Aviation Safety Authority's investigation into the crash will focus on what caused the engine to fail.

9News understands it will be checking if the maintenance on the aircraft was done properly.

Recreational Aviation Australia will look at whether environmental, mechanical or human factors played a part.