Ground speed or Air speed?

I know this sounds a bit stupid, but do I use ground speed or air speed to see how fast I’m going while flying. I pretty sure it’s air speed but at one point my auto pilot was set to Mach 0.8, but I consis stayed at 250 kts, yet my ground speed was more like Mach 0.8. Anyone know what’s up?

The higher up you go, the less dense the air is. This makes indicated air speed less and less when ground speed increases/stays the same.

Airspeed is always used in air. Ground speed is used on the ground. Airspeed decreases the higher you go because the air is thinner

And that ground speed is used to calculate the rate of descent.

It’s not like I was at 50,000 feet though I was barely at 30,000 which at that height I’ve gone much faster

Being @Aernout is a real world A380 pilot, he should be able to explain this best.

at 30,000 ft air is much less thinner so the drag from the air is greatly reduced so this allows you to travel faster (in terms of ground speed)

but this doesn’t always mean that the higher you go the faster you’ll always be because airplane engine get less productive at higher altitudes.
there is this cruise altitude where the reduced drag and the decreased engine thrust are balanced so that you are travelling at the most efficient speed.

To answer your question as the higher altitude you fly, outside A/C temperature(OAT) decreases. and speed of sound is directly proportional to the temperature. So as OAT decreases so does the true airspeed associated with a given Mach.

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For ATC instructions you use airspeed, there is a point which is normally around 30.000 - 33.000 feet where you go from Airspeed to Mach to fly. Also your indicated airspeeds decreases while you climb but your mach number and groundspeed increase.

Groundspeed is just used a reference for your speed compared to the ground. It is not used for any instructions by atc or so.

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