What happened
In the early hours of 24 October 2011, a violent storm featuring a microburst struck Faro Airport in Portugal, with wind speeds reaching nearly 85 knots. During this event, a parked Boeing 73 7-8AS, registration EI-DPA, jumped its chocks and shifted approximately 5 meters.
Later that day, the aircraft was scheduled for a flight to Beauvais, France. During the takeoff roll, the flight crew noticed the aircraft drifting toward the right centerline and observed significant aileron deflection. As the aircraft climbed, the crew experienced severe control difficulties, noting that the aircraft required approximately eight units of left rudder trim just to centralize the control column. Recognizing the potential for an unmanageable situation in the event of an engine failure, the crew decided to return to Faro. The aircraft landed safely approximately 28 minutes after departure.
The investigation
An investigation by the AAIU established that the aircraft's rudder system had sustained significant structural damage during the overnight storm. Upon inspection of the vertical stabilizer, engineers found that the support brackets for both the Main and Standby Power Control Units (PCUs) had suffered cracking, fracturing, and displacement. Specifically, four fasteners on the Main PCU bracket had sheared, and the Standby PCU bracket had partially pulled away from its flange.
Furthermore, the investigation revealed that the rudder had over-traveled due to the force of the wind, causing the leading edge of the rudder to strike the hinge mechanism. This impact perforated the rudder skin. The damage was not visible from the ground and was not detected during the pre-flight inspections or the initial ramp check.
Findings
- The primary cause of the incident was undetected structural damage to the rudder system, caused by extreme wind gusts while the aircraft was stationary.
- The wind speeds during the microburst significantly exceeded the 65-knot threshold specified in airworthiness standards (FAR 25.415).
- The physical damage to the PCU tailstocks caused the rudder's neutral position to be offset by roughly 8 degrees to the right.
- The damage was hidden from view during standard ground inspections because it was located behind the blade seals.
- There was no requirement in the existing maintenance manual to perform a comprehensive flight control system check following an extreme meteorological event.