Small Aircraft
Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Multiengine
Flight Crew Last 90 Days 21; Flight Crew Total 16964; Flight Crew Type 505
Distraction; Situational Awareness
Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Loss Of Aircraft Control; Ground Excursion Runway
General None Reported / Taken
Human Factors
I'm submitting this form for a mishap because I feel that this was a 'human factors' event.  I had not flown any ag aircraft for about 90 days; and had been instructing a multi-engine student in an airplane he owns for about 2 weeks prior to the mishap.  I feel there were 3 separate distractions; each of which contributed to a breach of discipline resulting in a critical checklist deviation which led to landing without the tailwheel lock engaged.  The free-swivelling tailwheel severely compromised my ability to control the airplane once I selected beta thrust to decelerate the airplane on landing roll; the result of which was a departure from the runway and a ground-loop.The first human factor was a lack of recency of experience.  I flew this aircraft exclusively all of 2019; accumulating over 500 hours. As such; I did not do a refresher of ops procedures before going to pick the airplane up from its annual inspection.  Additionally; I have extensive flight experience in Air Tractor aircraft that DO NOT have a separate tailwheel lock control in the cockpit; the tailwheel locks whenever the stick is aft of a 'nose down' position.  I may have reverted to habit patterns from those other aircraft and 'dropped' the tailwheel lock check from my normal flows.  Lastly; the intense effort I'd been expending instructing in a dissimilar aircraft also played a part in this critical breakdown in discipline.  The need to check the tailwheel lock had been de-emphasized.Second was the fact that the right cockpit door latch vibrated to a barely-latched condition during the takeoff.  I noticed this during the 15 minute flight to my home airstrip; and it occupied my attention to the point of distraction.  I was concerned that the door might open in flight before landing; which would have certainly resulted in its separation from the airplane.  I focused on this concern; and overlooked my normal 'safety checks' in preparation for the landing.Another distraction in this process was the fact that the maintenance performed prior to departure had included new brake discs and brake pads.  These components normally require 2 to 3 landings to 'break in' properly; and reduced braking effectiveness can be expected.  I elected to land at our home airport; with a narrower runway because of my familiarity with the runway and the excellent weather.  I was aware of the 'soft' brakes; and I checked the firmness of the pedal and ensured that pressure would be available while on short final.  This concern; and the mental preparedness for the softer brakes; also intruded on my normal processes and played a part in my omission of the critical 'tailwheel lock - check engaged' before landing checklist step.Following this mishap; I noticed that there was not an accessible checklist in the cockpit of this airplane.  I will produce a copy of the relative necessary checklists from the AFM and post them in the cockpit in full view of the pilot; for routine use.  I preach checklist discipline and use with great fervor; and my error is a testament not only to my own fallibility; but to the overriding need to follow my own advice.
AT-602 pilot reported failure to lock the tailwheel led to a loss of directional control on landing roll and a runway excursion.