Terminal velocity depends on the object. The terminal velocity of a skydiver is around 200km/h. I'd imagine the terminal velocity of an airliner pointed straight down would be significantly higher even without the engines running. With engines at cruising power they move 700km/h+ in horizontal flight, I'd imagine they could exceed that by a lot in a vertical dive.

NASA has this online Terminal Velocity calculator. For drag coeficient typical values for commercial large planes are in the order of 0.05.

"Terminal Velocity Calculator":

https://www.grc.nasa.gov/www/k-12/airplane/termv.html

Best rough figure I could find for weight of a loaded 737 is around 40 tons, frontal area I'm guesstimating is roughly a 3m diameter cylinder plus ~0.5m average thickness of the 28m wingspan giving 21m^2? For which the calculator gives 262.8m/s or 943.2km/h.

Not far from current preliminary estimates.

"China Plane Was Travelling At Speed Of Sound As It Slammed Into Hill":

https://www.ndtv.com/world-news/china-jets-fatal-dive-took-i...

"The Boeing Co. 737-800 was knifing through the air at more than 640 miles (966 kilometers) per hour, and at times may have exceeded 700 mph, according to data from Flightradar24, a website that tracks planes.

"The preliminary data indicate it was near the speed of sound," said John Hansman, a Massachusetts Institute of Technology astronautics and aeronautics professor who reviewed Bloomberg's calculation of the jet's speed. "It was coming down steep.""

Nose was pointed down according to some security cam footage (unverified).

https://twitter.com/ChinaAvReview/status/1505834279275999236

>terminal velocity?

https://en.wikipedia.org/wiki/Terminal_velocity

If you read the ^^ wikipedia page, you will see the NASA 'Terminal Velocity' url complete with Calculator;

https://www.grc.nasa.gov/WWW/K-12/airplane/termv.html