[pgc hunter]

Looking at Flightradar24, eastbound jets are doing in-excess of 1200km/h ground speed at cruising altitude of 30-40000ft all across Vic and NSW. Yesrterday, I was viewing the Boeing 747-400 of QF 64 from Johannesberg to Sydney doing about 1,240km/h above Adelaide and Mildura/ NW Victoria and SW NSW... that is well over Mach 1 at SEA LEVEL, let alone at FL 390 where the jet was crusing. IN zero wind, Mach 1 at 35,000ft equals about 1,058km.h. This aircraft was travelling inexcess of Mach 1.15 ground speed at cruising altitude, fast enough for a sonic boom to reach the ground, but thanks to the tailwinds, the airflow over the wings was no more than 900km/h, fooling the aircraft into an airspeed of 900km/h, yet doing in excess of 1200kmh groundspeed....but because the wings were doing 900km/h relative to the windspeed, there was no sonic boom to be heard.

Assuming perfectly still conditions at the same altitudes...Mach 1 (the "sound barrier" threshold) equals 1078km/h at 33,000ft, or 1220 km/h at sea level.

The typical groundspeed of commercial jets is around mach 0.85, or about 900km/h.

The jetstream is so strong, that east-bound commercial airliners are literally travelling at supersonic ground-speed. In still conditions aloft, 1200km/h would equal Mach 1.12.