Small Aircraft; Low Wing; 1 Eng; Fixed Gear
Flight Crew Flight Instructor; Flight Crew Commercial
Training / Qualification; Communication Breakdown; Human-Machine Interface
Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Loss Of Aircraft Control; Ground Excursion Runway
Flight Crew Became Reoriented; Flight Crew Regained Aircraft Control
Procedure; Aircraft; Human Factors
Was instructing in owner's aircraft. He wanted to practice takeoff and landings in the pattern at ZZZ. We did three three pointers. They were acceptable. Not greasers but nothing fundamentally wrong. We then did two wheel landings. Again; acceptable but not greasers but no warning flags. It was getting hot and thermal activity was affecting the landings so we decided to do one more and then call it quits. He did an acceptable wheel landing and at first everything was OK. Straight on the runway and wheels down. He then put the tail down - prematurely in my judgement as we still had a lot of speed (not excessive). The plane immediately yawed to the right. I intervened and applied full opposite rudder (I think he did as well) but it did not stop the yaw. This aircraft has no brakes on the instructor side so unable to apply brakes. The pilot did not use brakes to control the yaw. We ended up in the grass next to the runway. The pilot was pretty shaken up so I taxied back onto the runway and to the tie down. We determined there was no damage to the aircraft. I recommended to the pilot to have the tail wheel inspected in case there was some issue with it.I was surprised that we had the yaw to the right after what appeared to be an acceptable wheel landing. There were no wind gusts or other factors that I was aware of. It was either a result of lowering the tail too quickly (gyroscopic precession) or there is an issue with the tail wheel mechanism.
Instructor pilot reported student having control difficulties during landing resulted in a runway excursion with no damage or injuries.