What happened
On September 25, 2014, a Boeing 737-76N, registration PR-GOR, operated by VRG Linhas Aéreas, was descending through approximately FL200 when the aircraft's indicated airspeed approached the Maximum Operating Speed (VMO). To prevent an overspeed condition, the pilot manually applied pitch commands using the Control Wheel Steering (CWS).
This sudden maneuver caused a significant variation in vertical acceleration, ranging from +2.43g to +1.02g in less and than one second. Because the seatbelt sign was not illuminated, two flight attendants who were standing in the aft galley were thrown against the service trolleys. The aircraft subsequently landed at Tancredo Neves International Airport (SBCF) in Confins, MG, without further incident. While the aircraft sustained no damage, one flight attendant suffered serious injuries, including a spinal fracture, and another sustained minor injuries.
The investigation
CENIPA's investigation focused on the sequence of events leading to the vertical acceleration change. Investigators examined the flight parameters, noting that the aircraft's speed had increased due to variations in wind and atmospheric pressure while maintaining the descent profile. The investigation also reviewed the flight crew's use of the CWS and the programming of the descent mode.
Technical analysis revealed that the pilot's decision to program a specific speed for fuel economy contradicted manufacturer recommendations. Furthermore, the investigation found that the recovery technique used to avoid the overspeed was not in accordance with the manufacturer's manual or the airline's standard operating procedures, which suggest using the Speed Brake or V/S mode, or applying smooth manual inputs if the autopilot must be disconnected.
Findings
- Abrupt control inputs: The primary contributing factor was the sudden and heavy application of pitch commands in both directions via the CWS, which triggered the vertical acceleration change.
- Inadequate flight parameter evaluation: The crew's decision to use a specific speed setting to optimize fuel consumption was deemed an improper evaluation of operational parameters.
- Pilot attention: A low level of situational awareness in the cockpit may have contributed to the event, as the crew was surprised by the speed increase.
- Safety precautions: The seatbelt sign was not activated, leaving the cabin crew vulnerable to the physical effects of the maneuver.