1.   But it did not say that an ice buildup could make an aileron pop up, leading to a sudden roll.

2.   Charles Pereira, an NTSB investigator, said that on five other occasions ATR aircraft had encountered icing problems serious enough to cause sudden rolls during flight.

3.   Investigators, however, suspect that wear and tear on the rudder standby power control units on both aircraft could have contributed to the sudden rolls.

4.   Many of the most recent events involved unexplained sudden rolls on USAir flights.

5.   The NTSB, meanwhile, also urged the FAA to require additional training for flight crews in dealing with sudden rolls caused by unexpected rudder movement on the airliners.

6.   The National Transportation Safety Board also urged additional training for flight crews in dealing with sudden rolls caused by unexpected rudder movement on the airliners.