What happened
On May 29, 2001, an Aer Arann ATR 42-300, registration EI-CBK, was performing a scheduled passenger flight from Dublin to Cork. While descending through 10,000 feet, the aircraft encountered light turbulence and subsequently experienced intense, severe vibrations throughout the airframe, particularly concentrated in the tail section. The flight crew, which included a training captain, immediately took manual control and disengaged the autopilot to manage the instability.
As the aircraft approached Cork Airport, the crew noted that the vibrations were most severe when using pitch trim. The intensity of the shaking was significant enough that the crew briefly considered an emergency landing at a different location to avoid flying over a densely populated urban area. However, by extending the flaps to 30 degrees and reducing airspeed, the crew successfully stabilized the aircraft. The plane landed safely at Cork Airport with no injuries to the 45 passengers or 3 crew members.
The investigation
The AAIU investigation focused on the structural integrity of the aircraft's wing-to-fuselage fairing. Following the landing, inspectors discovered that a specific composite panel, known as Panel 291BL, had become detached along its trailing edge. Further examination revealed significant cracking within this panel. The investigation also reviewed the aircraft's maintenance history and the manufacturing processes of the composite materials used in the panel's construction.
Findings
Investigators determined that the primary cause of the vibration was the displacement of the trailing edge of Panel 291BL. This displacement occurred because the panel had lost its structural bending stiffness. This degradation was driven by cyclic fatigue stresses caused by aerodynamic loads and the natural flexing of the wing during flight.
Technical analysis revealed that the panel's construction—specifically using only three layers of Kevlar for the inner skin—made it susceptible to a phenomenon called "telegraphing." This occurs when the thin skin sags between the honeycomb supports during manufacturing, creating microscopic buckles. Under the repetitive compressive loads of flight, these buckles worsened, leading to fiber fatigue and the eventual formation of a crack in the outer skin. This structural failure allowed the panel's edge to lift, creating turbulent airflows over the tailplane that induced the severe vibrations.