What happened
On 11 September 2010, a DHC-8-402 Dash 8, registration G-JECF, was performing a commercial passenger flight approaching Exeter Airport. During the approach, the crew experienced a failure of the number 1 Input Output Processor (IOP 1). This failure caused several cockpit indications to disappear, including speed bugs and minimum descent altitude markers on the primary flight display.
While attempting to resolve this issue, the commander switched the air data computer source selector. This action triggered an ADC reversion, which caused the flight director's altitude select mode to disengage. Unaware that the aircraft was no longer maintaining its selected altitude, the crew continued to descend. The aircraft eventually descended to approximately 700 feet above ground level, triggering a GPWS "caution terrain" alert, followed by a "terrain, pull up" warning. The crew responded by increasing power and climbing, eventually re-establishing the glideslope and landing without further incident.
The investigation
The AAIB investigation focused on the technical cause of the IOP 1 failure and the crew's response to the GPWS warnings. Investigators examined the aircraft's flight data recorder and cockpit voice recorder. Technical analysis of the removed IOP unit revealed that the failure was caused by intermittent electrical contact due to cracked solder on two pins of a transformer within the power supply module.
Furthermore, the investigation looked into the operator's procedures for handling avionics failures and the adequacy of the manufacturer's Quick Reference Handbook (QRH). It was noted that the crew's focus on the hardware failure led to a breakdown in monitoring the aircraft's flight path profile.
Findings
- The primary cause of the altitude excursion was the deactivation of the altitude select mode and the subsequent failure of the crew to notice the change in flight director modes.
- The crew became distracted by the IOP 1 failure, which led to inadequate monitoring of the aircraft's vertical profile.
- The crew did not react to the GPWS warnings in accordance with established procedures, as they believed the visual conditions and familiarity with the area meant there was no immediate risk.
- The initial trigger was an intermittent electrical fault in the IOP power supply module caused by cracked solder.
- There was a lack of appropriate operational guidance for flight crews to manage the specific cockpit effects resulting from such avionics failures.