Southern Air 777 beyond the sound barrier?


New Member
I was tracking a Southern Air 777 flight # SOO275 this afternoon as it crossed the Irish Sea towards northern Britain. According to Planefinder, it was travelling at 725mph/630 kts at FL 35 zer0. I had a look at the speed of sound at various altitudes and it reckons that at 35,000 ft, it's 660mph, about 65mph less than the 777 was tracking at. Is this usual for scheduled flights to exceed the sound barrier?
ADS-B reports groundspeed so with a strong tail wind it may look like they are breaking the sound barrier when actually they are not. They do have to be careful though when coming out of this wind!