What happened
On 5 April 2018, an EC135 P2+ helicopter, registration G-POLA, was conducting a scheduled maintenance flight near Morpeth, Northumberland. The purpose of the flight was to perform an N2 adjustment to clear a deferred defect following a recent engine change.
During the procedure, the pilot and engineer attempted to adjust the engine speed to the required parameters. However, as the adjustment was made, the N2 speed began to fluctuate uncontrollably. The engine speed rose past the target, triggering an overspeed warning and causing the main rotor speed (NR) to increase to its maximum continuous limit. In an attempt to correct the setting, the engineer adjusted the control in the opposite direction, which caused the N2 to drop sharply to 98%, subsequently driving the rotor speed down to its minimum continuous limit.
This instability caused the aircraft to enter a moderate descent with increasing airspeed and triggered FADEC failure cautions. The pilot managed to stabilize the aircraft by adjusting the collective and pitch attitude, eventually returning the helicopter to Newcastle Airport safely. There were no fatalities and no injuries to the crew or passenger.
The investigation
Investigators examined the N2 adjuster mechanism to determine why the engine speed could not be maintained. They discovered that the mechanical stop ring, which is designed to limit the rotation of the adjuster spindle, was missing from the unit. Without this ring, the spindle could rotate freely through approximately 330 degrees, meaning the engineer had no way to know the neutral position or prevent over-adjustment.
Testing by the manufacturer confirmed that while the adjuster worked correctly when paired with a stop ring, the absence of the ring allowed for the uncontrolled N2 variations observed during the flight. The investigation also noted that the pilot, while experienced, had not received specific training for this particular maintenance flight test, and the crew had not specifically briefed the potential hazards of the procedure.