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While this is an incredible travel speed, I wouldn't consider it "hitting supersonic" speeds based on ground speed. I read the article wondering about the safety of passenger aircraft at 800MPH, but it seems to not apply.
I'm not a pilot, though.
Yeah I was curious about that too.
The plane can't really slow down to compensate in guessing as it would lose lift? Or it could extend flaps maybe.
There's no reason at all for it to slow down. If you've ever been on one of those moving walkways think about it like that, yes by walking on it you're moving faster than the people around you, but to you you're not walking any faster than normal even though you'll arrive at your destination earlier.
That's actually a really good analogy. Mind if I throw some numbers on it to flesh things out?
Let's set that moving walkway going at 5mph, and we'll put ourselves on that walkway, on a turned-off rascal scooter. The scooter is stationary on the belt, but it's still moving at 5mph - that's your tailwind pushing the air around the plane forward.
Now, let's turn that scooter on and throttle it up to 5mph. The scooter is plugging along comfortably at 5mph, but it's actually moving at 10mph. This is your plane flying with a tailwind, performing normally for its indicated air speed, while having a much higher ground speed.
Curiously, this does make the phrase "supersonic speeds" somewhat debatable. While they were traveling over the ground faster than sound would, they weren't moving faster than sound would in the air around them.
Ooh, there's another fun one...
"Indicated" airspeed isn't actually how fast you are going relative to the air. "Indicated" airspeed is how hard the air is pushing into the front of the pitot tube. But, at high altitude, the air is thinner, and doesn't push as hard. To get the same indicated airspeed at altitude, you have to be flying much, much faster.
Indicated airspeed is how fast the wings "think" they are moving. If the stall speed is 80kts, and the true airspeed is 200kts, but you're so high that the wings "think" they are only moving at 75kts, the aircraft is in a stall.
Next up, "critical mach number" and "coffin corner"