A pilot and passenger were arrested early Thursday on suspicion of illegal transportation of narcotics after their small plane made an emergency landing on state Route 76 in Oceanside, police said. Officers found the plane — a single-engine Piper Cherokee — on state Route 76 near Canyon Drive shortly before 1:45 a.m. after the pilot had to make an emergency landing, police said. The pilot, who was not injured, said the plane had experienced engine failure, police said. The landing site is about a mile west of the Oceanside Municipal Airport. When police searched the plane, they discovered “a large quantity” of narcotics. The amount and type of drugs found was not released. Police said they arrested the pilot and passenger, both of whom are Oceanside residents. Their gender and ages were not immediately available. The Federal Aviation Administration is investigating the circumstances of the emergency landing. According to FAA records, the plane is registered to a San Diego limited liability company.