What happened
On January 21, 2011, a Eurocopter EC-135 T2 helicopter, registration PH-ULP, was performing a medical transport flight at Rotterdam The Hague Airport. During the initial takeoff phase, the pilot noticed a sudden downward movement while hovering. Believing the issue was related to insufficient collective friction, the pilot increased the setting. However, as the aircraft climbed to approximately 100 feet, it began experiencing repeated, rapid downward surges.
Recognizing the instability, the pilot declared a 'pan pan' emergency via radio. The pilot initially intended to land on a taxiway parallel to runway 24, but after observing emergency vehicles moving toward the runway, opted to land directly on runway 24 instead. The pilot executed a 270-degree turn and performed a running landing. Following the touchdown, the crew carried out a 'Double Engine Emergency Shutdown' and evacuated the aircraft. There were no injuries and no damage to the aircraft.
The investigation
Technical specialists from the maintenance organization and the manufacturer examined the aircraft under the supervision of the Dutch Safety Board. The investigation focused on the hydraulic-electric servo units, which are part of the Stability Augmentation System (SAS) used to manage the main rotor via Electro Hydraulic Actuators (EHA).
Investigators found that while the aircraft's maintenance logs showed all inspections were up to date, one specific servo was malfunctioning. The investigation also reviewed the pilot's decision-making process and the existing emergency checklists provided in the Aircraft Flight Manual (AFM).
Findings
- The uncontrolled downward movements were caused by uncontrolled input signals in the longitudinal function, generated by a defective hydraulic-electric servo interacting with the stability augmentation system.
- The pilot did not initiate the specific emergency procedure to disable the SAS because the aircraft's movements did not clearly manifest as longitudinal or lateral oscillations, leading the pilot to believe no specific checklist applied.
Safety action
- The manufacturer has updated its training programs to better help pilots distinguish between control oscillations and simple vibrations during theory and simulator sessions.
- The manufacturer issued a Service Information Notice (SIN) instructing pilots to disable the stability augmentation system if abnormal aircraft movements or uncontrolled control inputs occur.
- To resolve discrepancies, the manufacturer committed to updating the Aircraft Flight Manual (AFM) so that the emergency checklist aligns with the instructions provided in the SIN.