What happened
On 13 March 2011, a Boeing 737-8F2, registration TC-JKF, was operating a scheduled passenger flight from London Stansted to Ankara, Turkey. Following takeoff from Runway 2arm, the aircraft followed the Clacton 8R standard instrument departure (SID). During the initial climb, the aircraft entered a left-hand turn. At approximately 800 ft aal (450 ft agl), the autopilot disengaged, and the aircraft's nose pitched down sharply. The crew experienced "Pull up" and "Don't sink" Ground Proximity Warning System (GPWS) alerts as the aircraft descended to a maximum altitude of approximately 1,050 ft before leveling off.
The investigation
The investigation focused on why the aircraft failed to maintain its climb profile. It was established that the co-pilot, acting as the pilot flying, had selected 800 ft on the Mode Control Panel (MCP) prior to takeoff. This selection was based on a misunderstanding of the departure chart's general information section. The investigation examined the wording used in the UK Aeronautical Information Product (AIP) and compared the Clacton SID instructions with other SIDs used at Stansted, such as the Dover SID. The investigators found that the crew's previous experience with a different phrasing on a Dover SID had reinforced their incorrect interpretation of the Clacton procedure.
Findings
- The crew misinterpreted the vertical profile instructions provided in the general information section of the departure chart.
- The pilots believed the instruction "Initial climb straight ahead to 850 ft" required them to level the aircraft at that altitude and await further ATC clearance.
- The co-pilot selected 800 ft on the MCP because the altitude could only be adjusted in 100 ft increments, and 800 ft was the nearest value not exceeding the perceived 850 ft limit.
- The commander's previous experience with a Dover SID, which used the phrase "No turns below 850 ft," contributed to the error.
- The pilots contacted London Control and provided a frequency change before they were actually instructed to do so by the tower, which was contrary to the SID instructions.
Safety action
Following the incident, NATS harmonized the general information section of published SIDs to ensure consistent wording across the UK AIP.