A320 flight crew reported unexpected severe turbulence; resulting in passenger injuries.

Attributes
ACN	1586018
Time
Date	201810
Local Time Of Day	1201-1800
Place
Locale Reference	ZAU.ARTCC
State Reference	IL
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	A320
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Person 1
Function	First Officer Pilot Flying
Qualification	Flight Crew Instrument Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 160 Flight Crew Total 6112 Flight Crew Type 595
Person 2
Function	Pilot Not Flying Captain
Qualification	Flight Crew Multiengine Flight Crew Instrument Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 270 Flight Crew Total 14800 Flight Crew Type 1731
Events
Anomaly	Deviation - Speed All Types Flight Deck / Cabin / Aircraft Event Illness Inflight Event / Encounter Weather / Turbulence

Narrative:

We were operating aircraft X to ZZZ. We were in cruise at 21;000 in clear air; with smooth flight conditions and no restrictions to visibility. The seatbelt sign was off. With no indications or reports of turbulence at our altitude; we suddenly encountered moderate to severe turbulence. The aircraft oversped by 3-5 knots and the autopilot disconnected. I was the pilot flying and immediately took control of the aircraft. The turbulence action plan was immediately initiated. The captain turned on the fasten seatbelt sign and immediately made a PA for the flight attendants to be seated immediately. We then requested a descent to 18;000 and the ride improved as we passed through 20;000. The captain then had the flight attendants check in where we were notified of two possible passenger injuries. One was to a woman in the lav who hit her head on the door but stated that she was fine. A male passenger reported hitting his knee on a galley cart. He requested ice and said he was fine. The captain initiated contact with dispatch and medlink and requested that paramedics meet the airplane upon arrival in ZZZ. The rest of the flight was uneventful. On taxi in; the lead flight attendant called to inform us that another male passenger was complaining of back pain. We arrived at the gate; shut down; and the paramedics were there to offer assistance to the injured passengers.

Original NASA ASRS Text

Title: A320 flight crew reported unexpected severe turbulence; resulting in passenger injuries.

Narrative: We were operating Aircraft X to ZZZ. We were in cruise at 21;000 in clear air; with smooth flight conditions and no restrictions to visibility. The seatbelt sign was off. With no indications or reports of turbulence at our altitude; we suddenly encountered Moderate to Severe turbulence. The aircraft oversped by 3-5 knots and the autopilot disconnected. I was the pilot flying and immediately took control of the aircraft. The turbulence action plan was immediately initiated. The Captain turned on the fasten seatbelt sign and immediately made a PA for the flight attendants to be seated immediately. We then requested a descent to 18;000 and the ride improved as we passed through 20;000. The Captain then had the flight attendants check in where we were notified of two possible passenger injuries. One was to a woman in the Lav who hit her head on the door but stated that she was fine. A male passenger reported hitting his knee on a galley cart. He requested ice and said he was fine. The Captain initiated contact with dispatch and Medlink and requested that paramedics meet the airplane upon arrival in ZZZ. The rest of the flight was uneventful. On taxi in; the lead Flight Attendant called to inform us that another male passenger was complaining of back pain. We arrived at the gate; shut down; and the paramedics were there to offer assistance to the injured passengers.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.