TOURDUN (32) 17.04.2018

On 17 April 2018 at 21:00, a witness was taking a long-exposure photograph toward the zenith, between Ursa Major and Leo, when he noticed a streak he took to be an ordinary aircraft trail. At the same time, according to his initial account, he noticed the ground intermittently lighting up within his field of view for 2 seconds. Only one account was collected. The investigation demonstrates that he photographed an Airbus A321 operating scheduled Munich–Lisbon flight LH1792/DLH05A (see investigation report). An online star-field calibration tool first identified the photographed stars and the aircraft's passage through them. Overlaying that field on a correctly oriented view of the sky then established the observation azimuth and the axis of the aircraft's path. Comparing this with aircraft tracks over the area identified the aircraft with certainty. However, the sequence and synchronization of its white wingtip anti-collision lights were abnormal. Airbus aircraft ordinarily produce two closely spaced flashes at a frequency of 1 Hz, rather than the four flashes or irregular pattern seen in the photograph. The overall match led investigators to conclude that the lights were malfunctioning. The witness's initial statement that the ground lit intermittently for 2 seconds 'at the same time' differed from his reply to a GEIPAN question: 'My surprise came when I saw the detail in the photo. That was when I became conscious of the impression of light on the ground.' The investigation considers the second version very probably correct. It describes a familiar UAP-reporting effect called priming: an initial unusual observation creates understandable alertness, potentially reaching panic, that can lead a witness to connect other impressions or information with it and increase its strangeness. Those other details might not have been noticed, or at least not caused concern, without the initial event. The illumination's briefness and peripheral rather than direct perception already suggest a local, momentary light source, such as a passing car aiming its headlights toward the witness or a neighbor using a flashlight. The additional account supports this explanation. The case has low evidential strength: - Although a photograph was taken, the original was not supplied. The investigator had to reconstruct angular data from the available material without precise witness measurements of elevation, azimuth or angular distance traveled. - No information exists about the witness's immediate surroundings—exact location, lights, neighbors or roads—that might permit further investigation of the ground illumination, which the source calls the falsely perceived second strange feature. - The initial account described that second feature differently, and more strangely, than the witness later said he had actually been conscious of during the observation. This limited evidential strength does not prevent an explanation, which is compelling from the other available evidence. GEIPAN classifies the case A: an Airbus A321, flight LH1792/DLH05A bound for Lisbon, with strangeness perceived afterward on viewing the photograph.