Instructor pilot reported a loss of directional control on landing roll and; despite the instructor'...

Narrative:

Aircraft X landed in a three-point attitude; straight; on center line; on speed. Student had gone around on his first attempt rather than try to recover from a minor bounce. Go-around was gracefully executed. Student was calm and in control. After aircraft touched down on the second attempt; stick was held full aft and roll-out seemed normal. Aircraft then swerved to the right; seemed to correct back left; then swerved hard right. The left wingtip contacted the ground as the aircraft continued into a snowbank at the edge of the runway. The right gear leg contacted the snow bank and the aircraft nosed over onto its back. Neither pilot was injured; both exited the aircraft through the door; and the aircraft was shut down; fuel shut off.I did not detect the student applying excessive control inputs; I did not feel him fighting me on the controls; but my efforts to straighten the aircraft had no effect on the ensuing ground loop. Power was not applied nor could either of us recall going for the brakes. There were no tire marks on the runway; nor were there any flat spots on the tires. There were no apparent failures noted in the tail wheel assembly.this was my second instructional flight that day in the same aircraft. We had light east winds right down the runway (AWOS had reported wind 060 at 4kts- we were landing on runway xy). No issues or problems were encountered on the first flight. Both students are relatively low-time tail wheel pilots so I was actively involved with all maneuvers and close to if not on the controls at all times. I have [many] hours of tail wheel time; fly in a lot of wind; and have never experienced a ground loop before. I have no idea why we lost control and I could not keep us on the runway.

Original NASA ASRS Text

Title: Instructor Pilot reported a loss of directional control on landing roll and; despite the Instructor's efforts; the aircraft departed the runway and flipped onto its back.

Narrative: Aircraft X landed in a three-point attitude; straight; on center line; on speed. Student had gone around on his first attempt rather than try to recover from a minor bounce. Go-around was gracefully executed. Student was calm and in control. After aircraft touched down on the second attempt; stick was held full aft and roll-out seemed normal. Aircraft then swerved to the right; seemed to correct back left; then swerved hard right. The left wingtip contacted the ground as the aircraft continued into a snowbank at the edge of the runway. The right gear leg contacted the snow bank and the aircraft nosed over onto its back. Neither pilot was injured; both exited the aircraft through the door; and the aircraft was shut down; fuel shut off.I did not detect the student applying excessive control inputs; I did not feel him fighting me on the controls; but my efforts to straighten the aircraft had no effect on the ensuing ground loop. Power was not applied nor could either of us recall going for the brakes. There were no tire marks on the runway; nor were there any flat spots on the tires. There were no apparent failures noted in the tail wheel assembly.This was my second instructional flight that day in the same aircraft. We had light east winds right down the runway (AWOS had reported wind 060 at 4kts- we were landing on runway XY). No issues or problems were encountered on the first flight. Both students are relatively low-time tail wheel pilots so I was actively involved with all maneuvers and close to if not on the controls at all times. I have [many] hours of tail wheel time; fly in a lot of wind; and have never experienced a ground loop before. I have no idea why we lost control and I could not keep us on the runway.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1426247
Time
Date	201702
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	PA-18/19 Super Cub
Operating Under FAR Part	Part 91
Flight Phase	Landing
Route In Use	Visual Approach
Person 1
Function	Instructor Pilot Not Flying
Qualification	Flight Crew Commercial Flight Crew Flight Instructor Flight Crew Instrument
Experience	Flight Crew Last 90 Days 30 Flight Crew Total 8400 Flight Crew Type 80
Events
Anomaly	Ground Event / Encounter Loss Of Aircraft Control Ground Event / Encounter Ground Strike - Aircraft Ground Excursion Runway

Instructor pilot reported a loss of directional control on landing roll and; despite the instructor's efforts; the aircraft departed the runway and flipped onto its back.