B737-800
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Wake Vortex Encounter
Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Regained Aircraft Control; Flight Crew Took Evasive Action
Procedure; Environment - Non Weather Related
On arrival into PHX; 6 miles in trail of an A321. Started getting violent rapid side loading; banking; and minor airspeed fluctuations. Normally; we go in and then out of wake turbulence before we have a chance to react; but this was sustained. I describe it as violent but not obscene; for example the bank changes; side loading and airspeed fluctuations were uncontrollable; but limited to +/-7 degrees of bank; and +/- 4 knots on the airspeed. No way to tell side loading. As we reentered it for the third time; we told ATC we needed a half mile offset. ATC complied; and then asked us if we could rejoin the arrival. When they asked; we hit another bit of wake turbulence and said no. They gave us vectors until final; where we were 4 and a half miles in trail again. We flew one dot high and then shifted to a normal glideslope and landed uneventfully. Captain remarked that he had not felt wake turbulence like that in years and felt like we were following a 757.Close trail vectoring; and zero crosswind component. Winds were calm at altitude.It seems that spacing is going to a mileage restriction only; instead of a timing and distance based one. However; [with] airliners flying at 210-250 KIAS on the arrival; 6 miles can be as little as 1 minute; depending on tailwinds. Recommend going back to a combination of time and distance based restrictions for in trail spacing.
Reporter recommended returning to time and distance based in-trail techniques instead of distance only.
B737-800 First Officer reported encountering wake turbulence in trail of an A321 on arrival into PHX.