What happened
On 24 January 1999, a Boeing 737-30 and (registration G-IGOH) was operating a public transport flight from London Stansted to Edinburgh. While cruising at Flight Level 260 amidst moderate turbulence, the flight crew noticed a continuous clicking or ticking sound. Shortly thereafter, the crew found they could no longer adjust the altitude via the Mode Control Panel (MCP).
As the crew transitioned to a manual descent, severe arcing became visible on the front windscreens. This was accompanied by a loss of heading and airspeed indications on the MCP. To ensure safety, the crew initiated a high-speed descent to descend below the cloud layer and declared a PAN to Air Traffic Control. The aircraft eventually performed a manual approach using raw ILS data and landed safely at Edinburgh with no injuries to the 5 crew members or 45 passengers.
The investigation
Investigators initially examined the MCP, its associated connectors, and nearby cabling but found no immediate defects. However, upon removing the instrument panel coaming, investigators located a power cable integrated with the MCP floodlight panel that had been chafed through. This damage occurred near an in-line connector and suggested the wire had been rubbing against the MCP.
Maintenance records noted that the insulation on these specific wires appeared less robust than the standard wiring used elsewhere on the aircraft. While some minor polishing was found on the MCP where contact had occurred, no direct arcing damage was visible on the unit itself. To ensure airworthiness, the damaged wiring and connector were replaced, and the MCP was swapped with a known functional unit before the aircraft returned to service.