ben123
I’ll give a go, I’m sure others will do it much more eloquently!
Q1)...A...Decrease.
Put simply, the TAS of the speed of sound (Mach 1.0) decreases with temperature. Temperature decreases with height, so maintaining a constant Mach number whilst climbing means that you are maintaining a constant Mach number whilst the temperature decreases, therefore the TAS will decrease.
An aircraft flying at Mach 1.0, at Sea Level on an ISA day, will have a TAS of 661 kts.
If it maintains Mach 1.0 whilst climbing to 30,000 ft, in ISA conditions, the TAS will decrease to 589 kts.
If it maintains Mach 1.0 up to 40,000 ft, the TAS will decrease further to 573 kts.
Q2)...A...Increase.
Again, as simply as I can, a basic airspeed indicator is just a pressure sensing instrument. The pressure in the pitot tube we assume is ½pV2, and the instrument is calibrated to read correctly in the dense air found at sea level.
In order to maintain a constant airspeed indication (IAS) as we climb we must maintain a constant ½pV2 in the pitot tube. To do this when the outside air density p is decreasing, then V (TAS) must increase to compensate, and keep the pressure in the pitot the same.
Using the ASI in less dense air than it was calibrated for, by climbing significantly above sea level, means the airspeed indications it gives (IAS) will under-indicate the actual airspeed (TAS) of the aircraft because of this reduction in the air density.
An aircraft flying at 160 kts IAS, at Sea Level on an ISA day, will have a TAS of 160 kts.
If it maintains 160 kts IAS, whilst climbing to 20,000 ft, in ISA conditions, the TAS will increase to 220 kts.
If it continues at 160 kts IAS up to 40,000 ft, the TAS will increase further to 325 kts.
Hope this helps!
Regards
Bellerophon