What happened
On 16 November 2011, a DHC-8-402 Dash 8, registration G-ECOK, was performing a commercial passenger flight from Knock to Manchester. During the ILS approach to Runway 23R, the crew encountered a significant deviation from the localiser centreline. While the aircraft was descending, the crew identified a discrepancy in the flight guidance control panel: the left side was set to 265°, while the right side was set to the correct inbound course of 234°.
As the aircraft intercepted the localiser at approximately 1,300 ft, the flight guidance localiser and glideslope capture modes engaged automatically. Due to the mismatched course settings, the flight director provided conflicting indications. The crew attempted to rectify the situation by manually steering the aircraft to recapture the localiser, but the aircraft continued to deviate. At approximately 80 and 1,400 ft altitude, the localiser deviation increased to full-scale deflection. Following the crew's established procedure to execute a go-around if not correctly established by 1,000 ft, a missed approach was initiated at 800 ft altitude.
The investigation
The AAIB examined flight data from the aircraft's quick access recorder and the operator's safety investigation. The investigation focused on why the flight guidance system responded to the mismatched course settings. Analysis of the flight data showed that the pattern of localiser capture and subsequent deviation was nearly identical during both the initial intercept and a second capture attempt. The investigation also reviewed the operator's simulator exercises, which explored how a mis-set course could cause the flight director to function inefficiently during localiser intercept.
Findings
- The primary cause of the deviation was the discrepancy between the left and right inbound courses selected on the flight guidance control panel.
- The flight guidance system responded to the mismatched settings, leading to inefficient flight director performance.
- The crew maintained sufficient situational awareness to identify the error and execute a timely go-around.
- The operator had previously experienced several similar false localiser capture incidents within its Q400 fleet.