Incident Summary
On September 6, 2015, at EPBY airport, a helicopter EC 135 P2+ experienced caution messages during engine start. After engine start and avionics power-up, the pilot performed an autopilot test, which resulted in the display of YAW SAS (yaw stabilization system) and GYRO (sensor or gyroscope) cautions. The pilot then toggled all engaged SAS modes off and on using the cyclic stick button. The GYRO caution reappeared. The pilot subsequently shut down the helicopter and reported the event to the duty engineer at the Technical Coordination Center.
Maintenance and Findings
Analysis of the fault indicated a malfunction or lack of power supply to the YAW FOG (fiber optic gyroscope) in the yaw stabilization system. In accordance with the operator's procedures, MEL category C item 22-1.8, valid until September 16, 2015, was applied. The helicopter was cleared for flight with a restriction to VFR (Visual Flight Rules) operations only.
On September 12, 2015, the helicopter was ferried to SOT Warsaw-Babice under order No. 8/OL/HXE/2015. Maintenance personnel at SOT replaced the faulty gyroscope (serial number 00/00481) with a serviceable unit (serial number 00/003837) per work order No. WO/HXE/040/2015. On September 14, 2015, an autopilot check and helicopter test were performed without discrepancies. The MEL restriction was removed, and the helicopter was returned to unrestricted service.
Cause
The display of YAW SAS and GYRO cautions was caused by a fault of the YAW FOG fiber optic gyroscope in the yaw stabilization system.