What happened
On 19 August 2003, a Lockheed L1011-500 Tristar, registration CS-TMP, was performing a positioning flight to Stansted Airport for a pilot command upgrade. During the final stages of an automatic landing on Runway 23, the aircraft's rear fuselage struck the runway.
While the aircraft was established on the ILS approach, the autopilot 'A' disconnected at approximately 350 feet, triggering a 'NO DUAL' warning. The commander attempted to re-engage the autopilot. Around this same time, the Auto Throttle System (ATS) ceased to function. As a result, the aircraft's airspeed dropped to 120 kt—well below the calculated Vref of 145 kt—and the pitch attitude increased to over 15 degrees nose-up. The crew did not intervene, and the aircraft touched down with a three-point attitude, causing the tail to strike the ground.
There were no fatalities and no injuries among the 13 crew members on board.
The investigation
The AAIB examined the flight data recorder (FDR) and the aircraft's technical history. The FDR confirmed that while the aircraft was initially stable, the engine parameters (EPR) remained fixed after the ATS stopped controlling the engines, leading to the loss of airspeed and the increase in pitch.
Engineers investigated the flight control computers and the ATS. While the No 1 Flight Control Computer was eventually replaced and tested without fault, subsequent maintenance revealed that micro-switches on the thrust levers were intermittently mis-set, which could cause an ATS disconnection. The investigation also noted that the aircraft's maintenance troubleshooting was being conducted by various third-party organisations in different geographic locations, which complicated the oversight of repairs.
Findings
- The primary cause of the accident was the crew's failure to detect and react to the diverging airspeed and pitch parameters following the loss of the Auto Throttle System.
- The disconnection of autopilot 'A' and the subsequent attempt to re-engage it may have distracted the crew from monitoring primary flight instruments.
- The Auto Throttle System ceased to function, leading to a significant reduction in airspeed below the required approach speed.
- The aircraft's maintenance coordination was fragmented, as repairs and troubleshooting were carried out by different entities across various countries.
Safety action
- The AAIB issued a safety recommendation to the Portuguese National Civil Aviation Authority to ensure that the operator has a robust system for monitoring and coordinating maintenance troubleshooting, especially when using distant third-party organisations.