What happened
On the evening of 18 January 2007, an ATR42-300, registration EI-SLD, was performing a cargo flight from London Stansted Airport. Shortly after departing from Runway 23, the crew experienced a persistent yawing and rolling motion, which the crew described as being similar to a Dutch roll. During the climb, the co-pilot attempted to engage the yaw damper, but the system failed to engage, and an "AFCS INVALID" message appeared on the flight deck.
The aircraft' and crew's ability to manage the flight was significantly hindered by the varying severity of the oscillations. As the aircraft was vectored for a return to Stansted, the motion continued to affect the flight path. During the final approach, the aircraft descended below the glideslope, triggering several EGPWS warnings, including altitude and flap configuration alerts. The crew eventually landed the aircraft using a reduced-flap setting. There were no injuries and no damage to the aircraft.
The investigation
The AAIB examined the aircraft's rudder control system, including the Releasable Centring Unit (RCU), the yaw servo, and the flight data recorders. While the mechanical components of the rudder system were found to be serviceable, investigators noted that the FDR (Flight Data Recorder) had been experiencing intermittent faults for some time.
Analysis of the FDR data revealed significant rudder travel and lateral control inputs. The investigation also reviewed the cockpit voice recordings and the crew's actions. The investigation focused on whether the oscillations were caused by mechanical failure or pilot input, and whether the crew's response to the situation contributed to the unstable approach.
Findings
The investigation found that the aircraft's motion was exacerbated by pilot input, as the crew struggled to maintain control during the oscillations. The lack of an effective yaw damper, combined with the crew's difficulty in managing the aircraft's stability, led to an unstable approach. Furthermore, the investigation noted that the crew's focus on the technical malfunctions contributed to the degradation of the approach profile.