Auto Pilot/FG Architecture.
The FG has two loops – essentially two software logic circuits. - the Inner Loop - the Guidance loop The inner loop is the more reactive one and gives short-term commands while the guidance loop has more of a mid-term action/role. The two loops are controlled by two different cards/processors. When in approach, the gain of the inner loop is different from cruise so it is more reactive to any perturbation that could make the A/C deviate from its path. With FMS release 1A when using the RNP AR mode the gain laws have been tightened so the rate of change to guidance commands is enhanced and will mean more rapid adjustment if necessary to maintain desired flight path accuracy.
Airbus Findings
The aircraft was subjected to a lateral gust of wind which was the root cause of the commencement of oscillations. The Auto Pilot inner loop, being more reactive in RNP approach mode, was the cause of the abrupt roll. It was confirmed in simulator tests that these dedicated laws can, in a few cases, generate lateral oscillations when associated with some specific strong lateral gusts. These oscillations did not exceed the RNP AR limits and oscillations remained contained.
Summary
Abrupt roll oscillations may be experienced on an RNP AR approach in gusty conditions. This is most likely associated with the A/P FG architecture. The aircraft should stay within its RNP XX boundaries and the A/P should remain engaged.