What happened
On May 27, 2008, a Boeing 737/300F, registration EC-KDJ, operated by Agroar and owned by Flyant Cargo, was preparing for a cargo flight from Madeira (LPMA) to Lisbon (LPPT). Following a heavy downpour, the runway surface was wet. As the crew aligned with runway 05 and initiated the takeoff roll, the pilot advanced the thrust levers to 55% N1 and engaged the Autothrottle (A/T) using the TO/GA function.
Immediately upon the activation of the Autothrottle, the engines failed to accelerate symmetrically. The number 1 engine accelerated to approximately 88% N1, while the number 2 engine only reached 67% N1. This imbalance caused the aircraft to veer sharply to the right. Despite the pilot's attempts to regain control through nose wheel steering, differential braking, and reducing engine power, the aircraft continued to slide toward the right edge of the runway. During the excursion, the outboard left main tire struck and destroyed a runway edge light and sustained a deep gouge. The aircraft eventually came to a stop on the right safety shoulder, where the crew was able to regain directional control.
The investigation
The GPIAAF investigation focused on the performance of the Autothrollle system and the physical conditions of the runway. Analysis of the Flight Data Recorder (FDR) confirmed that while the manual advancement of the thrust levers was symmetrical, the subsequent engagement of the Autothrottle caused the significant power imbalance between the two engines.
The investigation also examined the runway surface conditions. While the aircraft did not reach the speeds required for dynamic or reversed rubber hydroplaning, investigators identified the occurrence of viscous hydroplaning. This was caused by a thin film of mud and water—a result of recent rain mixing with dust—on the smooth, ungrooved concrete threshold of the runway. The lack of macrotexture at the runway threshold prevented the tires from penetrating the fluid film, facilitating the skid.
Findings
- The primary cause of the incident was the asymmetrical activity of the Autothrottle system, which caused the number 1 engine to accelerate significantly more than the number 2 engine.
- The presence of a thin film of mud and water on the runway surface, created by recent rain and dust, led to viscous hydroplaning.
- The runway threshold area lacked grooving, which contributed to the loss of directional control on the slippery surface.
- The crew was properly certified and the aircraft was in a valid state of maintenance.
- The aircraft sustained no structural damage, though the left main tire required replacement and a runway light was destroyed.
- There were no injuries to the two crew members on board.