The single-engine Piper PA-32 came down not far from the stands, which were filled with people attending the NHRA Drag Racing finals.
Fortunately, no one on the ground was injured, but the plane took out about five cars and three RVs.
"That was shocking to me to see a plane just cut a whole truck right in half, from the roof to the door," said witness Tristan Szalonek.
Pomona police said the pilot and three passengers "suffered minor to moderate non-life-threatening injuries" and were brought to a nearby hospital.
The crash happened just after 11 a.m. NHRA temporarily stopped the event but then resumed around 1 p.m. after consulting with police and the fire department.
Firefighters were already on hand for the drag-racing event and were able to quickly respond.
"When the units were on scene and saw the plane crash was happening in real time they initiated a call," said Los Angeles County fire engineer Jonathan Torres. "Having been on scene (they) were readily available to render aid."
Authorities say the plane had left from Apple Valley and was heading for Brackett Field Airport, located just west of the Fairplex track.
The Federal Aviation Administration and National Transportation Safety Board are investigating the cause of the crash.