What happened
On 23 December 2008, an AgustaWestland AW139, registration G-CHCV, was conducting a commercial passenger flight from North Denes Heliport to a North Sea drilling platform. While flying in instrument meteorological conditions, the crew received a 'vne miscompare' message, followed by the loss of several critical parameters on the commander's primary flight display, including altitude, airspeed, and vertical speed.
The situation escalated when the crew received a warning indicating a fire in the baggage compartment. Although there was no visible smoke or smell of burning, the crew declared a Mayday and decided to return to North Denes. During the return flight, the aircraft experienced further intermittent failures, including the loss of engine parameters and various other system cautions. The crew eventually landed safely at the heliport, and a subsequent inspection confirmed no fire had occurred.
The investigation
The AAIB investigation focused on the cause of the cascading system warnings and the loss of flight information. Investigators examined the aircraft's Modular Avionics Units (MAUs) and analyzed data from the flight recorder. Laboratory testing conducted by Honeywell involved subjecting a CSIO2 module to water spray to replicate the incident conditions. These tests successfully reproduced the loss of engine analogue parameters and the appearance of spurious baggage fire warnings.
Further examination of the CSIO2 module revealed that it was contaminated with dirt and debris, with visible corrosion on the pins of several circuit card components. The investigation also identified that the cooling system for the MAU cabinets in the 'short nose' version of the aircraft used unfiltered, non-conditioned air drawn from the fuselage underside.
Findings
- The primary cause of the spurious warnings and loss of indications was corrosion on the CSIO2 module components.
- This corrosion was caused by moisture and salt-laden air being drawn into the MAU cabinet via unfiltered cooling air ducts.
- The contaminated components became conductive when wet, leading to short circuits that corrupted the data processed by the module.
- The high level of integration within the Primus EPIC avionics system meant that a single module failure could trigger a cascade of seemingly unrelated warnings across multiple aircraft systems.
Safety action
Following the incident, AgustaWestland issued a Service Bulletin to inspect MAU modules for corrosion and to modify the cooling air ducts to move the outlet away from the MAU 2 cabinet. The AAIB has made a Safety Recommendation to the European Aviation Safety Agency to mandate these modifications on all short-nose versions of the AgustaWestland AW139.