An account in Val-d'Oise, France

On April 18, 2016, at around 5:40 p.m., two witnesses (T1 and T2) at a bus stop observed an object passing across the sky, spinning on itself and glittering. Both witnesses stated that the UAP was square-shaped. The trajectory of the UAP was estimated as being in a straight line by T1 and descending by T2. T1 filmed the observation. The observation lasted about two minutes for T1 and 13 minutes for T2. Both witnesses thought it was debris fallen from an aircraft. No other testimony was collected. The strangeness of this case is low and is mainly caused by the regular rotation of the UAP on its transverse axis (see the investigation report). The witnesses observed two objects. The data provided by the witnesses are numerous and detailed but of limited reliability. The witnesses themselves indeed express difficult observation conditions ("we were badly positioned to be able to judge it", "the sun preventing a clear distinction of the UAP, which was then only visible by the flash of white light it reflected from time to time in our direction"), which may also be the source of discrepancies or contradictions in the descriptions of trajectories. Fortunately, we have a video of object No. 2, on which we base the main part of the analysis, considering that nothing in the statements opposes object 1 having the same explanation. The physical and dynamic characteristics of the UAP observed and filmed (for object 2) by the witnesses are similar to those of light debris, subject to the combined effects of gravity and a rotation of unknown initial cause and/or created by the general wind or its local turbulence (we are in an urban environment) in combination with a favorable shape effect of the object (asymmetry, propeller effect, etc.). The descent of the object itself creates a relative wind that can create or contribute to this rotation, because the witness's drawing indicates a horizontal rotation axis. This same rotation and the induced lift and drag effects in the wind can be the cause of the deviation of 20 to 40° from the wind trajectory. Consequently, GEIPAN classifies the case as "B" as a probable observation of light debris (unknown origin).