Incident Overview
During a dual training flight, the certified flight instructor (CFI) retarded the throttle on the left engine to a zero thrust setting. The dual student contacted the airport and received clearance for a straight-in approach.
Approach and Go-Around
During the approach, the CFI did not instruct the student to momentarily clear the left engine. When the go-around was initiated, the dual student applied full power on the right engine, but the airplane did not climb. The CFI delayed in instructing the student to apply full power on the left engine.
Engine Response
When the CFI eventually instructed the student to apply full power on the left engine, the engine did not respond. The airplane was unable to maintain sufficient altitude, prompting the CFI to elect to land in an open field.
Post-Incident Examination
Following the incident, the operator reported that both engines operated normally during examination. However, the left engine spark plugs were found to contain high carbon and soot deposits.