What happened
On August 4, 2013, an ATR 72-202, registration EC-GQF, operated by Naysa, was performing a commercial passenger flight from La Palma to Tenerife South Airport (GCTS). While descending through approximately 8,100 ft, roughly 30 NM from the destination, the aircraft encountered a sudden and unexpected vertical disturbance.
Flight data recorder analysis shows that within a four-second window, the aircraft experienced a rapid change in vertical acceleration, fluctuating from -0.27g to +2.28g. This resulted in a sudden drop of approximately 134 ft. During this event, the autopilot disconnected and a "MASTER WARNING" signal was activated. The sudden movement caused two flight attendants in the aisle to fall, resulting in one serious injury (a fractured tibia and fibula) and one minor injury (a contusion).
Despite the disturbance, the flight crew maintained control of the aircraft, which showed no structural damage. The crew completed the flight to Tenerife South and landed safely. Following the event, the crew requested medical services and, as a precaution, the aircraft underwent a maintenance inspection.
The investigation
The CIAIAC examined the flight data recorder (FDR) and cockpit voice recorder (CVR) to reconstruct the sequence of events. The investigation focused on the vertical acceleration values and the aircraft's load factor. Investigators also reviewed meteorological reports from the Agencia Estatal de Meteorología, including sounding data from Guimar, to determine if any predictable weather phenomena were present.
Findings
- The aircraft encountered clear air turbulence (CAT) that was not predictable through standard meteorological forecasts or radar.
- While atmospheric soundings indicated potential moderate turbulence at much lower altitudes (between 1,200 ft and 5,000 ft), there were no visible convective phenomena or significant weather warnings at the flight's altitude at the time of the event.
- The load factor experienced during the event, when combined with the actual landing weight, indicated that the aircraft was operating within the normal flight envelope and did not exceed the thresholds for mandatory post-turbulence inspections.
- The sudden descent was an isolated atmospheric event with no identifiable external cause or preceding weather indicators.