What happened
On September 30, 2005, a Boeing 737-300 operating from Turin, Italy, to Frankfurt/Main experienced a severe flight disturbance during its approach to runway 25L. While following a Boeing 747-400 on a parallel approach to runway 25R, the aircraft underwent a sudden rolling motion. The aircraft initially rolled 23 degrees to the right, which was corrected by the autopilot. This was immediately followed by a much more intense roll to the left, reaching a maximum bank angle of 62 degrees.
At the time of the disturbance, the aircraft was at an altitude of 2,700 feet, approximately 9.8 nautical miles from the runway threshold. The crew had to disconnect the autopilot to manually recover the aircraft to a normal flight attitude. Following the recovery, the crew continued the approach slightly above the glide path and completed a safe landing. There were no injuries and no damage to the aircraft or third parties.
The investigation
The BFU investigation utilized flight recorder data, radar plots, and meteorological information to reconstruct the encounter. To determine if the aircraft had encountered wake turbulence, the BFU worked with the German Aerospace Center (DLR) to perform a complex simulation.
Using the "P2P Probabilistic Two-Phase WV transport & decay model," investigators analyzed the flight paths of both the Boeint 737-300 and the preceding Boeing 747-400. The analysis accounted for the mass of the preceding heavy aircraft, the velocity of the wake formation, and the decay of the vortices over time. The investigation also incorporated wind data, noting significant wind fluctuations at altitudes below 1,300 feet, which are characteristic of encountering wake turbulence.