Pilot witness in Haute-Savoie, France

On October 2, 1954, an experienced pilot (about 10,000 flight hours, specialist in flying in the Alps), at the controls of a Nord 1203, is flying on heading 250°, at 220 km/h and at an altitude of 2000 m. At 4:20 p.m., he observes on his right a "device" moving on the same heading at an altitude of about 4000 m, located at a distance he estimates at about 10 km. Its trajectory follows an upward curve. Its shape is that of a disk tilted on its axis by 30 to 40°, whose rear edge shows a phosphorescent glow. Its diameter is estimated at 3-4 m. It disappears at the horizon after 70 seconds. The pilot makes a brief report of his observation. No other testimony is collected. The sun (azimuth 245°, elevation 18°) is in line with the flight. The aircraft travels about 4,300 m during the observation period. The object is seen at an elevation of about 11° in an azimuth close to 340° (no astronomical confusion identified in this area, see sky chart). It follows an upward trajectory then disappears at the horizon, which indicates that the witness fails to describe the descending part. The witness interprets what he sees as a disk tilted 30-40° "on its axis," but which axis is it? One can imagine a disk whose front is lower than the back (or vice versa), or whose left edge is lower than the right (or vice versa). In the latter case, the object would have the shape of an ellipse. Does the witness have objective reasons to describe a "tilted disk," or is it a simple interpretation? Furthermore, the angular size of the object is very small, since an object of 3-4 m at 10 km distance is seen at an angle of 1' to 1.4' (0.15 to 0.2 mm at arm's length), which is the resolution limit of the eye (under strong contrast and luminance). The pilot, flying on heading 250, is almost facing the sun (azimuth 244°, elevation 18°): a reflection of the sun on a small object much closer (balloon) or even a reflection of the sun on the canopy cannot be ruled out. GEIPAN classifies this case as C due to lack of information and cross-correlation.