  We present SDO/AIA observations of an eruptive X-class flare of July 12,
2012, and compare its evolution with the predictions of a 3D numerical
simulation. We focus on the dynamics of flare loops that are seen to undergo
slipping reconnection during the flare. In the AIA 131A observations, lower
parts of 10 MK flare loops exhibit an apparent motion with velocities of
several tens of km/s along the developing flare ribbons. In the early stages of
the flare, flare ribbons consist of compact, localized bright transition-region
emission from the footpoints of the flare loops. A DEM analysis shows that the
flare loops have temperatures up to the formation of Fe XXIV. A series of very
long, S-shaped loops erupt, leading to a CME observed by STEREO. The observed
dynamics are compared with the evolution of magnetic structures in the
"standard solar flare model in 3D". This model matches the observations well,
reproducing both the apparently slipping flare loops, S-shaped erupting loops,
and the evolution of flare ribbons. All of these processes are explained via 3D
reconnection mechanisms resulting from the expansion of a torus-unstable flux
rope. The AIA observations and the numerical model are complemented by radio
observations showing a noise storm in the metric range. Dm-drifting pulsation
structures occurring during the eruption indicate plasmoid ejection and
enhancement of reconnection rate. The bursty nature of radio emission shows
that the slipping reconnection is still intermittent, although it is observed
to persist for more than an hour.
