A NetJets Challenger 650 hit wake turbulence. The warning never came

On August 8, 2019, a NetJets Challenger 650 (N237QS) was traveling from Dallas-Love Field (KDAL) to Santa Ana-John Wayne International Airport (KSNA). Aboard were two pilots and one flight attendant and seven passengers. The captain informed the flight attendant that it should be a smooth ride. As soon as it was safe to do so, she took drinks to the passengers, took catering orders, and returned to the galley to prepare the food.At 15:47, the business jet was climbing through flight level FL335 to an assigned altitude of FL340. The captain was flying at the time with the autopilot on. The passenger seatbelt sign was switched off. While in the vicinity of Lubbock, Texas, the aircraft encountered extreme turbulence. It shuddered and rolled left and right, then began violently pitching up and down as it rolled. The captain disconnected the autopilot, took control and stabilized the aircraft.The AftermathFood, coffee pots, and broken dishes littered the galley and cabin, and lavatory fluid had spilled onto the cabin interior. The flight attendant hit the ceiling, then hit the floor, and passengers were thrown around and yelling. There was broken glass everywhere, and the flight attendant tried to stand up without cutting herself. Once stabilized, the flight crew assessed the situation, believing it to be wake turbulence, and advised ATC what had happened. They requested a climb to FL360 in an attempt to find smoother air.The flight attendant managed to get to the passengers who requested a bag of ice, so she got some ice and a napkin from the galley and told the flight crew that there were injuries in the back. She made an assessment of the injured passengers. One passenger had hit her head and had a baseball-sized contusion but was not bleeding; the other had an injured leg and was in tears.The captain went into the cabin to check on things and speak with the passengers. He decided to divert to the nearest airport. Passengers helped the flight attendant pick up broken dishes, food, and glass to prepare for landing. They diverted to Lubbock (KLBB) where a normal approach and landing were made. The company was advised of what had taken place. Emergency services met the aircraft; two passengers were taken to hospital; one sustained a tibia fracture that required surgery. The flight attendant cleaned the aircraft the best she could, and they waited for a replacement aircraft for the remaining uninjured passengers to continue their journey. Once the aircraft had left, she was escorted to the hospital by the flight crew and was treated for minor injuries and released.The InvestigationThe flight data recorder (FDR) captured the event, which lasted for 11 seconds. At the time of the encounter, the aircraft was between eight and ten miles in the trail of an Airbus A300 operated by FedEx, which was level at FL340 on the same route as the business jet. The flight crew were not aware of the Airbus in the vicinity until after the wake turbulence. The captain said that there were no PIREPS regarding turbulence or any mention from ATC regarding traffic they were following.According to the FDR, there was a maximum bank of 23.4° right and 12.6° left. The pitch reached a maximum of 8.5° nose up and -2.5° nose down during this time, while the airspeed was stable between 268 and 276 knots. Heading started at 280° and initially went left to 272° before oscillating to 282° (after 2 seconds) and back to 269° (2 seconds later).The NTSB said that the probable cause of the accident was an unexpected wake turbulence encounter while climbing in smooth air to the cruise, resulting in passenger injuries. Contributing to the accident was that the passenger seat belt sign was off.An invisible hazard that can flip a jetWake turbulence is a strong disturbance in the air that forms behind a moving aircraft, caused primarily by pairs of rotating whirlpools of air called wingtip vortices. This type of turbulence is generated by the passage of an aircraft in flight, usually a smaller aircraft behind a large aircraft. This can cause loss of control of an aircraft. The spinning air acts like an invisible tornado, capable of flipping smaller aircraft or causing violent rolls that can exceed a pilot's ability to correct them.It is one of aviation's most serious invisible hazards. Wake turbulence separation is a mandatory spacing rule applied by air traffic controllers or pilots to prevent an aircraft from encountering the wingtip vortices. The FAA says that the best solution is to avoid the wake turbulence hazard.What Operators Need to KnowThere are tools on the ground, like the Wake Vortex Prediction System that pair specific models against real-time conditions. In the air, electronic flight bag software such as ForeFlight's Wake Turbulence Alert does the same job. There's no indication whether those cockpit tools were used and the flight crew never saw it coming. There was no PIREP, traffic call from ATC, or anything else to indicate a FedEx A300 was eight to ten miles ahead on the same route and climbing through the same altitude. While crews are trained to recognize and avoid wake turbulence, they still depend on ATC's help. When that link breaks, the gap can potentially be filled with EFB software. But standardization of using that predictive software needs to be established by the operator and crew.There's a second lesson here for the cabin crew as well. The seatbelt sign carries much more weight on commercial flights than it does in business aviation. Charter and fractional passengers can and often do ignore it once it's off. That makes a flight attendant's own judgment about when to sit down, a safety margin greater than the seatbelt sign itself.