Sighting in Silly-le-Long, May 31, 2015

Shortly before one in the morning, two witnesses on duty in a van, one driving and the other seated to their right, joined an expressway heading northeast. The night was clear and the Moon was behind them. At low altitude they noticed steady white lights and flashing red lights. They knew the area well and were close to Roissy airport, so they often saw aircraft, but these lights seemed far too low and motionless. As they slowed down, they made out a massive, dark, very flattened V-shaped object on their left that appeared to hover in place. After they passed a bridge, only the driver could still see it, through the windscreen, the side window or the rear-view mirror. The sighting lasted between one and four minutes, until the road dipped and the terrain hid the object. The witnesses reported what they had seen to the gendarmerie and filled in a GEIPAN questionnaire. No other testimony was collected.

The investigation found that a light aircraft was flying at low altitude on a touch-and-go exercise, in the opposite direction to the van. An on-site reconstruction with the witnesses and a 3D simulation, based on the aircraft's GPS data and the van's path, revealed a pivot effect: the line joining the witnesses to the aircraft turns mostly on itself, which can make the aircraft look fixed. The pivot was at the bridge, the same place where the witnesses saw the motionless object. The aircraft's reconstructed position and the object's reported position stayed close in angle, with a gap of up to 20 to 30°, similar to the gap between the two accounts. The witnesses had not distinguished the aircraft from the object, and the pilot, when questioned, had noticed nothing imposing near the flight path.

Significant discrepancies remained, particularly in the lights and above all in angular size: 0.7° for the aircraft, against 14 to 27° according to the witnesses. The case was therefore submitted to the GEIPAN panel of experts. Most of them accepted a strong perceptual illusion, explainable by night driving, viewing through the mirror, fatigue or stress. GEIPAN classified the case as A: a light aircraft seen at night and mistaken for an unknown object.