Police are appealing for witnesses to a collision on the A15, Sleaford Road, Waddington near the aircraft viewing area, which happened about 6.30pm on Thursday, January 15.
Enquiries into how the collision occurred are still ongoing, but it is believed a man in his 50s who was riding a blue Honda motor cycle had fallen from his motor cycle on the A15 and was then in collision with a red Ford Mondeo.
It is believed the injured man had turned on to the A15 from the aircraft viewing area at RAF Waddington.
The man was taken to Lincoln County Hospital with multiple fractures and is in serious condition.
Anyone who was in the area and saw the collision or saw the blue Honda motor cycle or the red Mondeo before the collision is asked to contact police on the non-emergency number 101 or the witness hotline 01522 558855.
PC Gareth King is the officer dealing with the incident.