Being Belgian, deeply interested in the UFO phenomenon and and also in lots of related sciences, I must only say this: if maybe not all black
triangles are not human-made, the ones that have been flying over Belgium in (actually not really the 80's) end of 1989 until spring 1991 are
Several documents are available publicly on the net. Just search for the Lambrechts report, the name of the military who was in charge of the
Lots of number and measurments coincide perfectly with hundreds (if not thousands) of witness reports, also by state agents, policemen, scientists,
fighter pilots,... Those numbers, such as altitude changes (and thus acceleration, measured to be over 40g sometimes, more even), speed changes, so
on, all of this without a sound, and again all of this being official and publicly available make of the so-called belgian wave of the best case of
airship observation only I've been given to study.
Think what you want, but no nation would have been so stupid as to test secret weapons over a country where the average population is around 350
inhabitants per square kilometer!!!
The end of the story is that the belgian authorities and military have officially
admitted that no known aircraft was capable of what was
observed (which is a little less than understatement!), that all other options (mass illusions, meteorologic phenomenons, so on) have been ruled out
easily, and that in the end, no one knows what it was, how it came there, how it went away (Belgium is a place that airborne and ground radars can
very well observe, remember that we have SHAPE 50kms south of brussels, and the NATO HQ in Brussels itself).
Just as an anecdotic souvenir, the Belgian Air Force even agreed to have 2 planes ready for takeoff 24hrs a day during several months, just in case!
The matter was investigated very seriously.
My final word on that initial question? Maybe some black triangles are earthly aircrafts, but some are definitely