What happened
On 18 November 2009, a Jetstream 4100, registration ZS-OMZ, was performing a scheduled domestic flight from Port Elizabeth to East London. During the takeoff roll at Port Elizabeth Airport, the aircraft reached a speed of approximately 70 knots when it suddenly began veering to the left.
The captain attempted to maintain control by applying full right rudder and full right aileron into the wind, while simultaneously reducing engine torque to 20% on both engines. Despite these efforts, the pilot could not arrest the deviation. The aircraft departed the left side of runway 08, coming to a stop 35 meters beyond the runway edge in level terrain covered in long grass. The crew successfully evacuated all 29 passengers and 3 crew members; notably, there were no injuries and the aircraft sustained no damage.
The investigation
SACAA AIID investigators examined the aircraft's steering system and flight data. The investigation focused on the transition of control from the tiller to the control wheel at 70 knots. Digital Flight Data Recorder (DFDR) analysis confirmed that while all parameters were normal initially, the loss of directional control coincided precisely with this handover.
Technical inspections of the nose wheel steering mechanism revealed that the steering control cable had been damaged during a previous installation. This damage caused the cable to kink, which subsequently led to excessive wear on the gears inside the steering control box. As the gear teeth became worn, the cable began to slip intermittently.
Findings
- The primary cause of the excursion was a damaged steering control cable that had been kinked during a prior installation.
- This kink caused progressive wear on the steering control box gears over several years of operation.
- The resulting wear created excessive free play and caused the cable to slip on the gear, leading to a sudden loss of directional control when the pilot transitioned from the tiller to the control wheel.
- The aircraft was otherwise properly maintained, and the crew was fully qualified with valid licenses and medical certificates.
- Weather conditions at the time of the incident were fine and did not contribute to the event.