Date: 202106
Local time of day: 0001-0600
Location: IAD.Airport, DC
Weather elements / visibility: Turbulence
Aircraft: Embraer Jet Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZDC
ATC / advisory: Center ZDC
Second aircraft: Heavy Transport
Aircraft component: Autopilot
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Wake Vortex Encounter; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; General Maintenance Action
Contributing factors: Procedure; Environment - Non Weather Related; Weather
Primary problem: Ambiguous

Narrative: We had begun a descent from our cruising altitude of FL280. Passing FL270 the aircraft violently rolled to the left and then to the right. The pilot flying used TCS (Touch Control Steering) to override the autopilot and maintain control of the aircraft. We where thrown around in our seats from the turbulence. AUTOPILOT FAIL" message was displayed on the EICAS. The forces of the turbulence had disengaged the autopilot. We ran the QRH checklist for "AUTOPILOT FAIL" -which did not mention to re-engage the autopilot so we took the conservative approach and the rest of the flight was hand flown. We reported the severe turbulence to ATC and asked if there was any traffic nearby that may have caused the turbulence -they advised that there was a heavy Airbus 15 miles ahead. We could not be sure as to whether the turbulence was caused by clear air turbulence or wake turbulence. There was no SIGMETS for the area and the weather was clear.  I checked in with the Flight Attendant and he said that both he and the passengers where okay. I advised that he remain seated for the rest of the flight. The fasten seat belt sign was on prior to the event and was turned on at top of descent. I sent an ACARS message to Maintenance about the "AUTOPILOT FAIL" message and they responded with that it would require a maintenance write up. The rest of the flight continued uneventful. Upon arrival I wrote up 'Encountered Severe' turbulence and also a second write up for 'AUTOPILOT FAIL'. As we where waiting for the flight data recorder download to be completed by Maintenance a message populated on the EICAS. This was an additional write up 'SPOILER FAIL'. The spoilers where indicating "OPEN" on the EICAS. Speed brakes where set to "CLOSED". We inspected the spoilers outside and they where both in the deployed position. The right hand side spoiler seemed to be deployed a little less than he left hand spoiler. Even though we did not see any indications suggesting this in flight. It's a possibility that the spoilers briefly deployed in flight and possibly caused the turbulence however I feel that this is unlikely to cause the severe turbulence that we encountered. We swapped into a new tail after this 3rd write up. Check the flight data recorder to see if there was a momentary uncommanded spoiler deployment."


Second narrative: We started our descent from FL280 at ATC's direction.  We were descending at 290 KIAS as per ATC instruction.  Passing FL270; the aircraft uncontrollably rolled left and then right.  I used the TCS (Touch Control Steering) button to override the autopilot.  At that time; autopilot also kicked off and gave an 'Autopilot Fail' master warning message on the EICAS.  The PIC; pilot not flying; ran the QRH.  We did not consider the hardover protection to be an issue.  We flew the remainder of the flight without use of autopilot out of caution; conservative considerations. We reported the severe turbulence to ATC.  We checked EICAS and MFD system indications.  All appeared normal.  We checked with the Flight Attendant; who stated that the turbulence was unexpected; but all was normal after the turbulence.  ATC advised that there was a heavy Airbus 15 miles ahead.  Weather conditions were very clear at the time with no visible moisture and winds were relatively calm for that altitude.  We sent an ACARs message to the company to relay the situation.  Upon landing; the First Officer did a walk around and did not notice any significant details to report.  After landing and while sitting in the aircraft while waiting for it to be released for the follow-on leg; the aircraft gave a 'SPOILER FAIL' advisory on the EICAS.  Upon exterior inspection after the message; the spoilers were raised/partially extended.  The right side was lower than the left side.  Pictures were taken of the spoilers; spoiler control handle in the cockpit; and EICAS. Suggest checking aircraft system data after download to verify severe turbulence or other system anomalies.


Synopsis: EMB flight crew reported encountering severe turbulence in descent; unknown whether turbulence was related to wake of preceding aircraft or clear air turbulence.