Date: 202104
Local time of day: 1201-1800
Flight conditions: VMC
Weather elements / visibility: 10
Light: Daylight
Aircraft: Amateur/Home Built/Experimental
Operator: Personal
Operating under: Part 91
Flight plan: None
Mission: Personal
Flight phase: Takeoff / Launch
Aircraft component: Parking Brake
Component problem: Improperly Operated
Reporter organization: Personal
Reporter function: Single Pilot
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Human-Machine Interface; Time Pressure; Troubleshooting
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Ground Strike - Aircraft; Ground Event / Encounter Loss Of Aircraft Control
Detector: Person Flight Crew
When detected: In-flight
Result: Aircraft Aircraft Damaged; Flight Crew Rejected Takeoff; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: I was parked on the gravel bar with the tail of the plane oriented significantly downhill. I had the parking brake set but also used a good size piece of driftwood to chock the left main tire. When it was time to leave; I released the parking brake while still outside the aircraft; which is the only practical way to release it since the lever is situated deep within the footwell. However; when I removed the log (so as not to hit it with my tailwheel when taxiing) the plane started to roll. Therefore I reset the parking brake; deciding to release it from inside the cockpit after starting the engine to provide enough thrust from rolling backward while I reached within the footwell to release the parking brake lever. I struggled slightly but was able to reach the parking brake lever and satisfied it was released I increased power and the plane began to move forward. Taxiing was sluggish but this was expected due to the low-inflation bush tires combined with two other factors: the 'digging' or plowing of the tailwheel and the soft gravel/sand. The tail wheels of all aircraft were digging that day; causing extra power to be needed for ground operations. We deal with this situation every time we land and takeoff from gravel bars and had dealt with it the previous 10 takeoffs and landings earlier this day. So at the time I didn't notice or feel anything unexpected during taxi.As I lined up for takeoff; I took notice of the same small bush I always use on this gravel bar as an abort point if my tail isn't up and flying as well as a familiar tree I use as my abort point if I am not airborne by the time I reach it. I applied full power and well before the small bush abort-point; my tail was already up and flying. This was unusual as it normally takes much longer for the tail to come up but I mistakenly saw this as a good thing so I continued the takeoff roll. A second or two later; I was suspicious that I wasn't accelerating normally so began preparing to abort if not airborne by my second abort point. I momentarily diverted my eyes from outside the cockpit to check my airspeed and this check confirmed my suspicions - I was not accelerating normally. As I began to pull the throttle off; I hit a small rut and momentarily became airborne a few inches so I hesitated a fraction of a second to abort the takeoff. However; I settled back to the surface and instantly; before I had a chance to pull the throttle to idle; it felt as if the tires 'grabbed' and the resulting inertia put the plane on its nose and the remaining momentum caused it to flip over onto its back. I was able to exit the aircraft quickly and within seconds; several people were there to assist me. Multiple people then assisted me with carefully turning the plan back over and we were all very surprised at the lack of damage. Other than the propeller; some scrapes and scratches to the paint and a crack in the top of the [type] windscreen; there were no other obvious signs of damage. We pushed the plane out of the way and secured it and I was given a ride with one of my friends to ZZZ1 where we spent the night as planned.Unable to figure out what happened I began recreating the events that led up to the incident and became suspicious that even though I was able to taxi; maybe I had not fully released the parking brake. Returning to the plane the next day; I opened the door and several of us looked inside and saw the position of the parking brake lever. It was barely to the halfway position. I'm not familiar enough with the internal valving to know for sure; but the halfway-released parking brake lever seemed to confirm my theory. The chain of events leading up to this incident were extremely interesting and like any error chain; there were numerous opportunities to break it; beginning from the moment I landed on the gravel bar and picked my parking spot.  I only parked my plane in the same manner because everyone else was parked that way. However; I could have simply chosen to park farther away from the bank on more level ground; thus avoiding the need to use the parking brake to begin with. The next opportunity came when I first removed the log and the plane began rolling backwards. I could have simply asked any one of 10 people to help me keep the plane from rolling while I released the parking brake (from outside the cockpit as I always have in the past) thus enabling me to get in and start the engine. The next opportunity to break the error chain came when I accepted the sluggish taxi as 'normal' thus exhibiting confirmation bias. Rather than being more suspicious I was actually being complacent. Yet another chance to break the error chain was when my tailwheel came up unusually quickly. Had I recognized this situation as abnormal (rather than welcoming the unusual 'bonus' of the tail coming up so quickly) I could've accomplished an easy; low speed abort and checked everything to see what was amiss. Finally; the last chance I had to avoid this outcome was to abort as soon as I became suspicious of my lack of acceleration rather than focusing on my predetermined abort point. Once I went beyond that last opportunity to abort; I believe my fate was pretty much sealed. Contributing factors (in my opinion) were an unusually large 'audience' of fellow pilots watching our group of planes takeoff. This probably reveals the biggest failing on my part that day. Ego. Not wanting to screw up in front of a bunch of fellow pilots. Looking back; a simple aborted takeoff would never have been seen as a screw up but the exact opposite. Everyone on that gravel bar would've been extremely supportive of the prudent decision to abort. For ANY reason whatsoever. Maybe another contributing factor could be the design/location of the parking brake lever of the [Homebuilt type]. Though I hesitate to include this as a factor as the location of the parking brake lever had never been an issue in the past. But this unusual scenario may have shined some light on a potentially troubling aspect of the location of the parking brake lever deep within the footwell as opposed to someplace in plain sight and easy to manipulate and confirm whether it is set or released. All in all; this was a scenario fraught with poor aeronautical decision making and will inform my attitude towards flying for the rest of my time spent piloting aircraft; regardless of which type of aircraft I am flying. Lessons learned.




Synopsis: Pilot reported a loss of control during attempted takeoff resulting a ground encounter with damage to aircraft.