What happened
While the provided document serves as a technical appendix to an investigation rather than a specific accident narrative, it details the physical mechanics of how a transport-category airplane reacts to sudden vertical atmospheric currents. The analysis examines how upward and downward air movements alter the aircraft's angle of attack and subsequent stability.
The investigation
The investigation examined the relationship between aircraft balance, stability, and the forces exerted by atmospheric disturbances. By using a theoretical model of a 60,000 kg aircraft, the study established how vertical currents affect the center of gravity (CG) and the aerodynamic center (AC). The investigation focused on the transition from steady flight to the dynamic oscillations—such as phugoid or Dutch roll—that occur when the balanced state is disturbed.
Specifically, the study modeled the impact of a vertical current with a velocity of 10 m/s. It established that such a current does not merely affect the aircraft at its center of gravity but creates significant angular accelerations. These accelerations cause the local load factor to vary significantly depending on the distance from the CG.
Findings
- Load Factor Fluctuations: An upward vertical current increases the load factor at the CG, but the resulting pitch-down correction causes a much higher local load factor in the aft section of the cabin. In the modeled scenario, the load factor at a point 15 meters aft of the CG rose to 2.08G.
- Negative Load Factors: Downward currents can reduce the load factor so drastically that it becomes negative. In the studied model, the load factor at the aft section dropped to -0.08G, a condition sufficient to lift unrestrained passengers and objects toward the cabin ceiling.
- Distance Sensitivity: The impact of turbulence-induced disturbances is most extreme in the sections of the aircraft furthest from the CG. In modern aircraft with wing-mounted engines, the aft section is identified as the most critical area for experiencing these high-magnitude disturbances.
- Operational Vulnerability: Aircraft are more susceptible to these disturbances when flying at high true airspeeds, high air density (low altitudes), or when the aircraft is light due to fuel burn.