R e s e a r c h
Martin SHOUGH & Wim VAN UTRECHT
Our primary source for this case is the summary below published on p. 124 of REPORT ON THE UFO WAVE OF 1947, a study compiled by Ted BLOECHER in 1967 and last updated in 2008 (the updated version of the book is available in PDF and HTML at nicap.org).
Case 849 -- July 28, between Mountain Home and Boise, Idaho: Captain Charles F. Gibian and First Officer Jack Harvey, of United Air Lines Flight Trip 105 (the same as Captain Smith's on July 4) [see Case #1 - WVU], were about to begin their let-down over Mountain Home, preparatory to landing at Boise, 45 miles to the west, when Harvey saw an object ahead of and to the south of their plane, silhouetted against the bright western sky. The time was 8:34 p.m. MST. Harvey thought at first the object was a plane, and turned his attention to the instrument panel to reduce power for the let-down. When he looked back, the object, which at first appeared to have "considerable substance," was seen rapidly diminishing in size. Captain Gibian reported that Harvey had called his attention to the object "as he would have done if it had been an airplane." The co-pilot had asked, "Is that plane going east or west?" When the object began to diminish rapidly in size, it appeared to be going toward the northwest at very high speed. Gibian described it as "going like hell when it disappeared." Both men watched it vanish from view. Gibian said the object appeared to be at 9,000 feet as their airliner began descending from an 8,000-foot altitude. It also appeared to be "weaving," as if "it were going through choppy air." Gibian said that the air at that time was somewhat bumpy. He was convinced the object was no airplane. Although he couldn't estimate its distance, he said that if it was "40 miles or so distant from the airliner, it was as big as an airplane." Both pilots agreed that they had never seen such a thing before, and they expressed their concern over possible military experiments being carried out in commercial air lanes.
At least two newspapers dedicated a short article to the case. Below is the most detailed one of the two, published in the Vallejo News Chronicle of July 29, 1947.
Comment - At 08:34 p.m. the Sun had already set (altitude was then -4° 16'; azimuth 301° 8'). The unknown object was spotted 20 minutes after sunset, so the sky in the west would still have been quite bright.
The strongest element in favour of a mirage is Capt. GIBIAN's comment that the "object" appeared to be "weaving", as if "going through choppy air". Also in good agreement with the mirage theory is the captain's statement that the object appeared to have "considerable substance", yet was seen to "rapidly diminish in size". The Mountain Home area is surrounded by mountains, bringing to mind a mirage of a distant mountain top.
In the Valejo News Chronicle Capt. GIBIAN is further quoted as having said that he is "not convinced this was a disc" (which is how unidentified flying objects were commonly called in those days). He wonders if the phenomenon might not have been "a small fragment of a stratus cloud which dispersed". There is another cloud type, often sharply outlined and closely resembling a "flying disc", that would be better fit the description: altocumulus lenticularis. These clouds form in standing waves created by strong winds blowing over a rugged terrain. As the waves rise, the warm, moist air condenses into water droplets, forming the clouds. As the waves then descend, the moisture evaporates back into water vapor and the clouds disappears, often in just a matter of seconds. So, next to a mirage, we should not rule out an isolated, backlit lenticular cloud that formed in a "bumpy" or undulating airstream on the downwind side of a mountain.
Sunset lenticularis photographed over Merens-l'Hospitalet (French Pyrenees) on January 17, 1977. The photo is part of a gendarmerie report that was prepared for the French official UFO research unit GEIPAN (then GEPAN), operating under the auspices of the country's Centre National d'Etudes Spatiales. [Image found at www.geipan.fr.]
Just for the sake of completeness: the other newspaper, the Walla Walla Union-Bulletin, also of July 29, 1947, mentions on page 8 that "The CAA said no other planes were reported in the area where Givian [sic] Monday night said he sighted the disc".