What happened
On December 2, 2001, an Aer Arann ATR 42-300, registration EI-CPT, was performing a visual approach to Donegal International Airport. The flight had departed from Dublin Airport with a crew of three and 32 passengers. While descending toward Runway 21, the aircraft passed approximately 300 feet above the peak of Mt. Errigal.
Shortly after clearing the mountain, the aircraft encountered sudden, severe turbulence lasting approximately 10 seconds. The intensity of the event was significant, with flight data recorder analysis revealing G-loads ranging from +4G to -2G. The violent motion caused the aircraft to buffet heavily, leading to physical injuries within the cabin. One passenger sustained serious injuries, including a suspected fractured humerus, while a cabin attendant and another passenger suffered minor injuries. Despite the turbulence, the crew successfully landed the aircraft at Donegal International Airport.
The investigation
Investigators examined the flight path, meteorological conditions, and the physical impact on the aircraft. The investigation established that the aircraft was flying in a position that placed it directly in the lee of the mountain peak. While the weather was clear with high visibility, a strong transverse wind of 40 knots was blowing across the summit.
Technical inspections of EI-CPT revealed that the turbulence was powerful enough to cause structural damage to interior components, including a dented cockpit doorframe and a damaged passenger seat unit. Analysis of the flight data confirmed that the wing loading had exceeded certified limits during the encounter. The investigation also noted that during the turbulence, the first officer inadvertently activated the cockpit annunciator lights by striking the overhead panel.