The Japanese Ministry of Defense have posted their official report on the crash:
https://www.mod.go.jp/asdf/news/houdou/H31/20190610.pdfI'm attaching two documents; the first is a raw Google translation of the report's text (created using Google Translate's 'translate document' function):
download/file.php?id=30637The second document is my semi-manual English interpretation of the report - I do not speak / read Japanese, but what I've done is used Google translate on paragraphs, individual sentences, phrases and even words or characters to get a better understanding of what the actual meaning is behind the Japanese text, then I've re-written it in English as closely as possible while maintaining half-decent grammar and edited it back into the original PDF, including with the graphics on the last 2 pages:
download/file.php?id=30639In short, the F-35A had just killed 2 targets during air-to-air training and had radioed "21 (his aircraft code), 2 kills". A US military aircraft (type not disclosed) was flying nearby at 37,000ft however, so air traffic control orders the F-35A to descend to increase separation. The pilot replies with "Yes. Roger that" but is now in a slight left turn and has a serious descent rate (around 820ft/s). About 20 seconds later, ground control asks him to further separate by performing a left turn, to which the pilot changes heading by about 100 degrees and replies calmly with a "Yes, Knock it off". At this point he's at about 15,500ft and still descending. For the next 15 seconds the jet is descending at about 1000ft/s (factoring in his horizontal velocity he would have been travelling at near Mach 1), up until radar / data link contact is lost at <1000ft from the surface and the plane hits the water moments later.
Because the pilot was awake and replied with "Yes, knock it off" in a calm manner after that left turn (and didn't communicate anything else), the Japanese MoD believes that it was spatial disorientation and not G-LOC or a problem with the jet's engine, controls or electrical systems in general. That said, they will be educating their pilots on G-LOC and performing special inspections on the jets just in case (a false instrument reading might possibly resulted in the spatial disorientation).