What happened
On 3 November 1999, an Agusta A109A II, registration G-TBGL, was conducting a private VFR flight from a location near Lyneham toward Halfpenny Green. While passing west of Gloucestershire Airfield, the pilot observed a heading failure indication on the Horizontal Situation Indicator (HSI). At that time, the autopilot was not engaged, so the pilot did not initially consider the issue critical.
As the aircraft reached the Malvern area, the pilot attempted to engage the autopilot and HeadingHold functions. However, the helicopter failed to maintain its heading, prompting the pilot to disengage the system and conclude the H/SI was unserviceable. Shortly after, near Droitwich, the Stability Augmentation System (SAS) 1 warning light illuminated, and the SAS automatically disconnected. This was followed by an Inverter 1 warning light, indicating a failure in the first inverter. As the pilot reduced airspeed, the Inverter 2 warning light also activated.
During this period, a burning smell became noticeable, and a passenger observed smoke emerging from beneath the instrument panel. The pilot promptly switched the electrics master and battery switches to the OFF position. The aircraft was brought down for a precautionary landing in a stubble field west of Droitwich. Following the landing, the smoke dispersed and the smell of burning subsided. There were no injuries and no damage to the aircraft.
The investigation
Following the incident, the helicopter was flown to a maintenance facility with the autopilot and SAS systems disabled to undergo inspection. Investigators examined the electrical systems and identified that the insulation on several conductors within a wiring loom connected to the Autopilot Control Unit had been chafed. This damage was caused by the loom being in continuous contact with a duct located behind the instrument panel.
Findings
- The primary cause of the electrical malfunctions was the chafing of the insulation on the Autopilot Control Unit wiring loom.
- The wiring loom had been positioned against a duct behind the instrument panel, leading to conductor degradation.
- The electrical failures triggered a sequence of warnings, including the SAS disconnection and inverter failures, which ultimately produced smoke and a burning odor.