What happened
On May 23, 2019, an Airbus Helicopters EC 135 T2+ operating as a Helicopter Emergency Medical Services (HEMS) aircraft was engaged in a rescue mission near St. Anton am Arlberg, Tyrol. After an initial landing on a forest track, the crew decided to reposition the aircraft closer to the patient's location to shorten the transport distance. To prevent the aircraft from sinking into the soft ground, the engines were left running during the second landing.
While the medical crew was loading the patient through the left sliding door, the aircraft began to slide. As the physician attempted to alert the pilot via radio, they observed the pilot experiencing a medical episode, characterized by physical convulsions and a loss of control. Consequently, the helicopter underwent an uncontrolled rotation of approximately 270 degrees around its vertical axis. During this movement, the patient was displaced from the aircraft, and the flight medic sustained minor injuries. The aircraft eventually came to a halt after the Fenestron struck an embankment and collided with a snow wall.
The investigation
The SUB investigation focused on the pilot's medical history and the environmental conditions. Investigators examined the possibility of flicker vertigo, noting that the rotor blade flicker frequency for this aircraft type (26.3 Hz) falls outside the critical 4–20 Hz range. Furthermore, sunlight was entering the cockpit from behind rather than frontally, making flicker vertigo unlikely. The investigation also reviewed the pilot's medical records, noting a significant head injury sustained in a domestic accident in 2017, which involved an open traumatic brain injury.
Findings
- The primary cause of the loss of control was a medical emergency involving the pilot, specifically a tonic-clonic seizure.
- The seizure was likely triggered by the pilot's previous history of traumatic brain injury.
- The aircraft sustained damage to the Fenestron and its housing (tail bumper and structure).
- The engines were running during the loading process to maintain stability on soft ground, which contributed to the aircraft's ability to slide during the event.