What happened
On 23 January 2005, an Iberia Boeing B747-300, registration TF-ATI, was performing a scheduled international flight from Madrid-Barajas Airport to Santo Domingo. The aircraft was carrying 318 passengers and 17 crew members. During the takeoff roll on runway 36L, the crew began to experience vibrations at approximately 35 kt, which the crew initially attributed to runway surface irregularities.
As the aircraft accelerated, the vibrations intensified significantly. Upon reaching 80 kt, the shaking became extremely violent, causing the aircraft to jerk sideways and lurch abnormally. The intensity of the vibrations was so great that instrument panels became loose, and the crew noticed a smell of rubber and debris being thrown up from the runway. Due to the loss of control stability and the physical impact on the cockpit, the crew decided to abort the takeoff at approximately 99 kt.
During the heavy braking required to stop the high-weight aircraft, the brakes reached extremely high temperatures, causing the thermal fuses on the main and fuselage landing gear legs to deflate. The aircraft came to a stop on the runway, and the fire brigade was dispatched to inspect the brakes. The passengers were safely disembarking via ground shuttles after the aircraft reached its parking stand.
The investigation
The CIAIAC investigation focused on the origin of the violent oscillations and the physical damage sustained by the aircraft. Investigators examined the Digital Flight Data Recorder (DFDR), which confirmed lateral and longitudinal accelerations reaching the maximum measurable limit of ±1g. The investigation also reviewed the maintenance history of the aircraft and the condition of the nose gear steering system.
Technical analysis of the nose gear steering mechanism revealed that the vibration was caused by a self-sustained oscillation. Upon disassembling the steering system's regulating valve, investigators discovered that an internal centering spring had moved beyond its retainer. While testing at the manufacturer's facility, investigators could not perfectly replicate the vibration, likely because friction caused by internal scratches—possibly created during the actual incident—prevented the exact same malfunction from occurring during bench tests.
Findings
- The primary cause of the aborted takeoff was a self-sustained oscillation in the nose gear steering resulting from a malfunction in the steering system's regulating valve.
- One of the internal centering springs in the regulating valve was found to be displaced beyond its retainer.
- The aircraft crew was properly qualified, experienced, and acted appropriately to abort the takeoff.
- The aircraft's weight and balance were within prescribed limits, and maintenance had been performed according to the established program.
- The intense vibrations caused significant internal damage, including dislodged instrument panels and cracked ceiling support beams in the cockpit and cabin.