What happened
On 14 January 2017, a Boeing 737-8AS, registration EI-EBW, was descending toward Manchester Airport when it encountered a significant increase in headwind. The aircraft was operating at high altitude within a northerly jetstream, where the margin between minimum and maximum operating speeds is naturally reduced. As the aircraft descended through FL367, the windspeed rose rapidly, causing the airspeed to increase from 0.78 Mach to a maximum of 0.818 Mach, triggering an overspeed condition.
In response to the rapidly increasing speed trend, the commander disengaged the autopilot and applied manual control inputs to arrest the acceleration. This manual intervention involved a significant nose-up pitch input on the control column. The resulting abrupt maneuver caused two cabin crew members in the rear galley to lose their footing; one of them sustained a serious injury consisting of a broken ankle. All 89 passengers remained seated and were uninjured.
The investigation
The AAIB examined the aircraft's flight data and the mechanical operation of the flight control system. Investigators found that the commander's manual input on the control column reached 42.76 lbs, which was more than double the 21 lb threshold required to disconnect the autopilot. The investigation also reviewed the aircraft's software, noting that a recent update (P8.0) changed how the autopilot responds to force overrides, meaning the autopilot would disconnect rather than transitioning to a control wheel steering mode.
Furthermore, the investigation looked into the aerodynamic sensitivities of high-altitude flight. At high altitudes, aircraft are more sensitive to control inputs, and the margin between the maximum operating Mach number and the stall speed is narrower. The investigators also reviewed the pilot's training records and the manufacturer's flight crew operating manuals regarding overspeed procedures.
Findings
- The primary cause of the cabin crew injury was the significant manual control inputs applied to the aircraft during the overspeed event, which led to an abrupt pitch change.
- An increasing headwind associated with a jetstream caused the aircraft to exceed its maximum operating speed.
- The reduced speed margins and increased handling sensitivities characteristic of high-altitude operations acted as contributing factors.
- The pilot experienced a 'startle effect' due to the rapid rate of airspeed increase and the magnitude of the speed trend indication.