Phillip Robertson produced a UFO newsletter in the early 90s called "Independent Aerial Phenomena Research" - IAPR
1. Sunset at 50 degrees N latitude occurred at 307 degrees azimuth on 540629. Reported phenomena were located at an approximate azimuth of 317 degrees or only 10 degrees north of the point of sunset. [ According to Howard's original sketch, the phenomena were located to the south of the setting sun. This would place the phenomena at an azimuth of, perhaps, 300-305 degrees. In any event, it is obvious from the sketches that the phenomena were located very near the position of the setting sun. ]
2. Thayer attempts to explain this case in terms of a superior mirage, with the terrain below the observing aircraft being the source for the observed phenomena. This hypothesis suffers from a serious difficulty in that mirages can be viewed only within approximately one degree above or below the observer's horizon. It is known that the observing aircraft was flying at an altitude of 19,000 feet, at which altitude the distance to the observer's horizon is approximately 169 miles. For a depression angle of one degree, there would have to be a mountain chain having a constant elevation of approximately 5000 feet at the computed distance to the horizon. Consultation of various charts and maps does not reveal any peaks above 3700 feet in the general direction of the observed phenomena and within 169 miles of the observing aircraft. There certainly is no mountain chain with a near-constant elevation of 5000 feet anywhere in the sighting area. This fact rules out Thayer's superior mirage hypothesis.
3. The possibility of the observed phenomena having been products of an inferior mirage appears to be slight. As stated in "1", above, the phenomena were located within a very few degrees of the setting sun. From this fact it is difficult to see how the sky could have been dark grey or black, in that the sky sectors refracted would necessarily have been within one degree of the position of the observed phenomena. This fact is no doubt the reason Thayer opted for an explanation in terms of a superior mirage.
4. Chapman Pincher hypothesized that the observed phenomena were a mirage of the observing aircraft itself. This hypothesis involves the assumption that a mirage can not only act as a mirror but as a mirror that casts its reflections toward the sun! Such an assumption certainly is not consistent with standard meterological knowledge! How did the shiny surface of the observing aircraft produce the very dark images described? This hypothesis of mirror-like reflections from an inversion layer has very little, if anything, to recommend it. I would suggest that Pincher probably "lifted" his "explanation" from Menzel's FLYING SAUCERS, a book that, in its own way, is as bad as anything any "true believer" ever turned out. [ For Chapman Pincher's hypothesis, see FATE, November 1954 issue, pp. 21-22 ].
5. Thayer hypothesizes that the disappearance of the phenomena might be explainable in terms of the observing aircraft beginning descent, with the resulting change in viewing angle causing the mirage to no longer be visible to the observers. This hypothesis is refuted by Howard's original statement: "The objects immediately began to grow indistinct until only one was visible. This grew smaller and finally disappeared (0123 G.M.T.) still at the same bearing to us. I reported to the fighter which direction to head for AND THEN COMMENCED DESCENT. [ my emphasis ]..."
6. One possible source for refraction by mirage phenomena remains, another aircraft flying a parallel course to the observing plane. In the "Condon Report', Thayer makes the comment that phenomena of the type reported by Howard are unique. Had Thayer taken the time to review either the BLUE BOOK files or the literature in this field, he would have found that a very similar sighting was reported October 25, 1963, near Mitchell, Missouri [Jacques and Janine Vallée, CHALLENGE to SCIENCE: THE UFO ENIGMA, Henry Regnery Company hardcover edition, 1966, pp. 180-181; BLUE BOOK files, Case #8613, R49.] In the Mitchell case, it appears that refraction phenomena may have produced a distorted image of a refueiing operation taking place approximately 120 miles from the observing aircraft. Since no one took the time to really investigate the Howard sighting at the time of its first being reported, we are not able to establish whether another aircraft was or was not on a parallel course to that pursued by Howard. If there was such an aircraft and if a continuous refracting layer was located to the northwest of the observing aircraft and if this refracting layer extended for almost 100 miles and if this refracting layer was within one degree of the observers' horizon, then it is conceivable that Howard and the other witnesses aboard the BOAC aircraft did see a refraction of another airplane. A large number of "ifs" are involved in this hypothesis and exactly none of them can be or has been established or refuted at this late date.
7. It is worth the effort to attempt to obtain temperature and humidity distributions for the areas involved in the BOAC sighting. The probability of getting these data is, however, very low.
8. The version of Howard's sighting presented in Timothy Good's ABOVE TOP SECRET: THE WORLDWIDE UFO COVER-UP [ pp. 189-191 ] contains a number of items which are at variance with Howard's initial report and suggested is the possibility that some features of the sighting have been "improved" to rule out the mirage possibility. In the newer version of the sighting, Howard states that the phenomena were initially below a cloud layer located at approximately 8000 feet and they subsequently ascended to an altitude approximating that of the BOAC plane. If the phenomena actually did ascend in the manner described in the newer version, mirage effects could be ruled out of consideration. Unfortunately, Howard's original words were: "I noticed on our port beam a number of dark objects at approximately the same altitude as our aircraft." There is not even a hint here that the objects were ever observed to ascend. I believe that we must take Howard's original statement as being the more valid one - at least until someone demonstrates a reason for giving preference to the newer version of the sighting.
In conclusion: The behavior of the reported phenomena suggests some type of refracted images but most types of mirage phenomena must be ruled out of consideration. The one remaining possibility in terms of refracted images of another aircraft travelling parallel to the BOAC plane cannot be confirmed or refuted. There remain so many complications in the way of accepting the mirage hypothesis that this sighting should still be considered unexplained, with the caveat that mirage/refractive phenomena are still a possibility.
I hope that the comments I have been able to provide on the above cases are of interest and value to you in your research. Reports of UFO sightings have to be viewed from all angles and communication between researchers is essential.