On July 11, 2001, approximately 1830 Pacific daylight time, a Boeing 757-232 transport category airplane, N644DL, operated by Delta Airlines as Flight 1669, experienced an abrupt maneuver during descent over eastern California en route to San Francisco, California. Delta Airlines, Inc., was operating the airplane as a scheduled domestic passenger flight under the provisions of 14 CFR Part 121. The airline transport pilot licensed captain, first officer, 4 flight attendants, and 92 passengers were not injured; however, 1 flight attendant sustained minor injuries, and 1 passenger sustained serious injuries. The airplane was not damaged. The nonstop flight originated from Cincinnati, Ohio, at an undetermined time. Day visual meteorological conditions prevailed, and an instrument flight rules flight plan had been filed. The National Transportation Safety Board was notified of the accident on June 5, 2002. Captain's Statement The captain was the pilot flying (PF), while the first officer was acting as the pilot not flying (PNF). The captain reported that the flight was normal until reaching the airspace above the Sierra Nevada Mountains. The captain illuminated the fasten seat belt light in the cabin. He made a passenger announcement directing passengers to return to their seats, and to check the security of their seatbelts "because of turbulence." Later, while still flying in turbulence, a flight attendant notified the captain that a passenger had fallen and injured her ankle. First Officer's Statement The first officer indicated that they were given an initial descent clearance from flight level (FL) 390 (39,000 feet) to FL330. The captain entered the assigned altitude into the autopilot system and began the descent. The PNF began programming the flight management system (FMS) for the approach into San Francisco. The PNF looked up from the FMS to see that they were descending through FL330, and queried the captain. The captain "immediately pulled the aircraft out of the descent and leveled the aircraft off at FL330." According to the first officer, the maximum deviation was 200 feet below the assigned altitude. He described the maneuver as "abrupt," and the flight crew immediately received a call from the flight attendants in the rear of the airplane inquiring what had happened. The captain reported they had hit turbulence. The first officer reported that the fasten seatbelt sign was illuminated prior to the occurrence. Flight Attendants' Statements According to the on-board leader (OBL), during the cruise portions of the flight, she recalled no discussion of turbulence with the flight crew, and reported it was "pretty smooth during cruise." She had just left the cockpit and was in the forward galley when the flight crew turned on the fasten seat belt light. She assumed the flight crew was signaling the initial approach. It was then that she felt the "ground came up" and she "buckled down," but she caught herself before falling. She was then informed that a passenger was injured and briefed the captain, who explained the event as "unexpected rough air or an air pocket." The remaining flight attendants reported a smooth flight prior to the event. The fasten seat belt sign illuminated a few minutes prior to the event; however, they could not remember an accompanying announcement from the flight crew. All of the attendants reported being pushed downward, and one hit her knee. A few of the attendants assisted the injured passenger, and stated that the captain reported that the event was "turbulence" related. Passenger Statement The passenger reported hearing an announcement from the pilot indicating they were 30 minutes from San Francisco, and it was a good time to get up before the flight began its descent. The passenger when into the lavatory, and was washing her hands when she was "thrown into the sink," and "held hard and flat against it by gravity." She remembered something hitting her left foot but "blacked out." When she came to, she noticed her foot "dangling and misshaped." The flight attendants assisted the passenger to a seat and arranged for paramedics to meet the flight upon landing. Medical examination of the passenger's ankle revealed it was fractured. Captain's Follow-up Statement The captain provided a supplement statement, in which he reported making a few heading changes to afford a view of Yosemite National Park. The flight encountered "some light turbulence," and he directed that the fasten seat belt sign be turned on and made an announcement to the passengers. Air traffic control directed the flight to descend from FL350 to FL310. Since the first officer was busy programming the FMS, the captain initiated the descent utilizing the vertical speed feature of the autopilot (1,000 feet per minute). Just after leveling off at FL310, the flight experienced a "ripple" of turbulence. The first officer, whose head was down, looked up and exclaimed that the flight was supposed to be at FL350. The flight then experienced a second "turbulence bump," and both flight crewmembers grabbed the control yokes. The first officer asked the captain what he was doing. The captain assumed that the first officer was concerned with the altitude. The captain asked the first officer to confirm the altitude clearance with air traffic control, which he did. While on the ground, the captain and first officer discussed the matter in the cockpit. The first officer believed that the captain caused the "bump" by moving the control yoke. The captain added that the autopilot never kicked off, and there was no altitude warning alert. As reported previously, the Safety Board was not informed of the event until almost 1 year after the event. Flight data recordings, weather briefings, air traffic control information, and cockpit voice recordings pertaining to the accident flight no longer existed.