Date: 201901
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: Amateur/Home Built/Experimental
Operator: Personal
Operating under: Part 91
Flight plan: None
Mission: Personal
Flight phase: Landing
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Reporter organization: Personal
Reporter function: Pilot Not Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Human factors: Communication Breakdown; Distraction; Training / Qualification
Second reporter function: Pilot Flying
Anomaly: Ground Event / Encounter Loss Of Aircraft Control; Ground Excursion Runway
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Regained Aircraft Control
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: I am not an instructor; but allowed a friend (with a tailwheel endorsement and current in tailwheel airplanes) to fly the airplane. I sat in the back seat while he sat in the front seat. We had some difficulties with the COM radio which distracted me from providing enough of a briefing on the unique ground handling characteristics of the Sport Cub prior to our landing on XXL at ZZZ. His landing was firm; but acceptable. During rollout; at a very slow speed (approximately 20-25 MPH); he lost directional control and we did a lazy ground loop to the right. We exited the runway to the right onto the grass. There was no damage to the airplane. An airport truck met us and inspected the runway and grass. He later commented that there was no damage to the airport and that he could not even see our tracks in the grass. I had more faith in my friend's ability than I should have.


Second narrative: I was flying the Sport Cub for the first time; with [another pilot] in the back seat of the airplane. I have a tail-dragger endorsement on my license and have been flying a Cessna 180 almost exclusively for the last three years; I am familiar and comfortable with the techniques employed to land a tail-dragger; (as opposed to those used to land a tricycle-gear airplane); in either a three point or main wheel landing.I landed the airplane; main wheels only; as planned; on ZZZ Runway XXL; after one 'bounce' on initial touchdown; a firm landing followed. I would estimate that about 2/3 of the runway remained at the point of landing. Touchdown speed was about 60-65mph; flaps were up. I held the tail off the ground initially; letting the speed bleed off without using brakes; holding the airplane straight with rudder. As speed decreased; the airplane began to turn to the right; and I countered first with rudder and then; as the rate of turning began to increase; I applied left brake. On the airplane I normally fly; the Cessna 180; at lower speeds; gentle differential braking is very effective in bringing the airplane to a gradual stop on landing as speed (and dynamic pressure over the rudder) decreases; I learned quickly that it doesn't work to the same degree on the Sport Cub. Despite progressively firmer application of the left brake I was unable to stop the airplane from turning at least 90 degrees; by this time at about 25 mph; and leaving the runway into the grass by taxiway between [Runway] XXL (our runway) and [Runway] XXR. We taxied into this grass and stopped. I learned later from my instructor that the left brake had locked up; well before our unscheduled turn was completed; there's a lot less weight on the Sport Cub's wheels at 40 mph than there is on the wheels of my Cessna 180; and braking action at those speeds is less effective; as I learned. The airplane was undamaged; and after a short pause; and contacting the Tower to assure them both we and the airplane were ok and able to taxi; we taxied off the grass and back to our parking.I believe I applied the techniques of landing a larger; heavier tail-dragger to attempt to land this very light and 'twitchy' tail-dragger airplane; resulting in a slow speed loss of control during the landing roll-out. Included in this was my reliance in differential braking; less effective on this airplane (less weight on the wheels at those speeds than I typically see in the Cessna 180) as the airplane slowed through 40mph. The result was my loss of control at the end of the landing rollout.


Synopsis: Experimental tailwheel aircraft pilots reported a loss of control during landing resulted in a slow speed ground loop and runway excursion.