What happened
On 23 December 2008, a DHC-8-402 Dash 8, registration G-JECI, was performing a scheduled passenger flight from Southampton to Edinburgh. During the final approach to Runway 24, the aircraft descended below its cleared altitude, reaching within 800 feet of local terrain approximately 5 nautical miles from the runway threshold.
The deviation was initially unnoticed by Air Traffic Control (ATC) due to high workload, though a ground movement controller eventually observed the aircraft appearing significantly below the glidepath. The flight crew, attempting to rectify the vertical profile, manually disconnected the autopilot and struggled to regain the correct path. The aircraft eventually landed without further incident, though the approach was notably low.
The investigation
The AAIB investigation examined flight data from the Quick Access Recorder and interviewed the crew and ATC personnel. The investigation focused on why the aircraft failed to maintain the intended altitude and why the crew did not identify the deviation earlier.
Investigators found that the crew had selected a new altitude of 2,100 feet but had not properly armed the altitude select mode. Consequently, the aircraft continued to descend at a constant vertical speed rather than leveling off. The investigation also looked into the flight director (FD) interface, noting that the crew's reliance on the FD bars led them to believe the flight path was correct, even though the aircraft was below the ILS glideslope.
Findings
- The primary cause of the altitude deviation was the failure to properly arm the altitude select mode on the flight guidance control panel after selecting a new target altitude.
- The crew did not identify the vertical deviation because they were monitoring the flight director bars rather than verifying the aircraft's position against raw ILS data.
- The approach was characterized by excessive airspeed and a lack of adherence to standard operating procedures regarding altitude checks.
- ATC controllers did not immediately notice the altitude excursion because the approach controller was busy with other traffic.
Safety action
- It is recommended that Bombardier Aerospace enable the automatic arming of the altitude select mode upon the selection of a new altitude and vertical mode for Dash 8-400 series aircraft.
- It is recommended that the operator amend its standard operating procedures to require an altitude check during final approach, even when the approach is conducted visually.