Cessna 208 pilot experienced a flap system asymmetry; resulting in a significant roll. Flaps were retracted and a no-flap landing was accomplished.

Narrative

Flight test. Test condition was to successively accelerate while retracting flaps at specified airspeeds. All operations were within the normal flight envelope for the airplane. Test sequence started with flaps 30 (full down) at 1.1 Vso; then accelerate to 1.3 Vso; and then to Vfe (125 KIAS for flaps 30). Flaps were then retracted to flaps 20 and the aircraft accelerated to Vfe (150 KIAS for flaps 20). The next step in the test was to retract flaps to 10. As soon as the flap control was slightly moved; we heard a load bang and the aircraft rolled hard to the left. Right aileron control was immediately applied to stop the roll and return to level flight. The flap control was left at the bottom of the flaps 10 detent (no further movement of the flaps) and we assessed the situation. The individuals in the cabin observed the right flap was still extended to the flaps 20 position but the left flap inboard edge appeared to be retracted while the outboard edge appeared to still be in the flaps 20 position. After confirming control and that the flap motor was operable; we decided to attempt to retract the flaps to eliminate the roll condition. Flaps were retracted in small steps and confirmed that the right flap was retracting normally and the outboard portion of the left flap appeared to be retracting towards the up position. The iterative process continued until both flaps were in the flaps up position. At this point control was confirmed to be normal and a flaps up landing was satisfactorily accomplished.