What happened
On 10 December 2006, a DHC-8-402, registration G-JECG, was operating a commercial passenger flight from Edinburgh to Belfast. During the climb to 16,000 feet, the aircraft encountered heavy precipitation and icing. Shortly after reaching cruising altitude, the crew received an 'ALT MISMATCH' alert on the Primary Flight Displays. This was followed by a rapid decay in altitude and airspeed indications on the commander's display, which were replaced by red failure indications. The autopilot disconnected, and the crew was forced to descend to 8,000 feet to clear the icing layer. During the descent, the co-pilot's altitude indication also began to fluctuate and eventually showed a deceleration rate inconsistent with the descent. The crew eventually stabilized the aircraft, and the flight concluded safely at Belfast with no fatalities and no injuries to the 71 passengers or 4 crew members.
The investigation
The AAIB investigation focused on the discrepancy between the crew's recollection of the cockpit warning panel and the recorded flight data. While the crew believed they had checked the pitot heat switches, the Flight Data Recorder (FDR) indicated that the standby pitot/static probe heat switch had remained in the OFF position from the start of the flight until approximately three minutes after the aircraft leveled at 8,000 feet. The investigation also examined the cockpit environment, noting that the crew had been distracted by incorrect engine display indications and the pressure of an aircraft on final approach at the runway. A simulator trial was conducted to determine if the pitot heat caution lights could be easily missed; the trial suggested that the position of the Central Warning Panel (CWP) and the use of night 'dim' lighting could make such cautions less conspicuous to the crew.
Findings
- The primary cause of the instrument failures was that the pitot/static probe heat switches were left in the OFF position.
- The crew's use of non-standard checklist procedures, specifically performing memory items rather than a formal check, contributed to the omission.
- Distractions during the taxi phase, including the need to rectify incorrect engine condition lever settings, likely led to the missed switch selection.
- High workload during the climb through heavy icing conditions contributed to the crew failing to notice the instrument discrepancies until they became critical.
- The physical location and reduced conspicuity of the warning lights on the CWP made the error harder to detect during the flight.