The UK's Air Accident Investigation Board (AAIB) last month released details of the Dash 8 incident at Exeter Airport (EGTE) in Southwestern England, in September 2010.

According to the report, the two pilots became distracted during an ILS 26 approach to Exeter when a computer problem caused the captain’s primary flight display (PFD) to malfunction. The captain was flying on autopilot and had already armed the flight director for the approach mode as both pilots attempted to resolve the PFD problem. 

At one point, the captain realized the flight director had disengaged the approach mode and he reengaged that function. He eventually decided to allow the first officer to fly the approach, because the weather was good VFR. The horizontal situation indicator (HSI), normally set to the side of the pilot flying, was left set to the captain’s side of the display. Neither pilot believed this was an issue.

Shortly after the first officer assumed command, a GPWS alert “Caution, Terrain,” sounded. Both pilots looked up from their attempts to solve the PFD problem and could see the runway ahead clearly. Shortly after the first alert, another, “Terrain, Terrain, Pull up,” alert sounded. The first officer disengaged the autopilot and pitched the aircraft to five degrees as he brought the power levers to 80 percent, a setting he believed was sufficient to provide adequate terrain clearance.

About this time, ATC also asked the crew to confirm they were descending on the glideslope. The captain realized the altitude select function of the flight director was disengaged, allowing the aircraft to drift down below the selected altitude to about 700 feet agl at eight miles from the runway. The captain told ATC they had experienced an instrument failure.

The crew climbed back to 2,200 feet, returned to capture the glideslope and landed safely without further incident a few minutes later.