IAS is determined by the dynamic pressure (read off from pitot tube), most pilots do like IAS better because the airplane is gonna behave similarly at the same IAS at the same aoa.

Spacecraft - no
Aircraft - yes

basically IAS is air runnin over yer plane and TAS is true air speed
at altitudes, air get's thinner meaning less IAS.
at altitudes, air gets thinner meaning less drag
.
less drag = more speed

IRL, an aircraft’s “top speed” is its fastest TAS, or how fast it moves through still air. IAS, which is measured by the force of air molecules entering a pitot tube and impacting a diaphragm, varies greatly by air density and is much closer to TAS at sea level, where the air is denser, but much less than TAS at high altitudes, where the air is much “thinner” or less dense. However, IAS is very important to a pilot because an aircraft always stalls at a given IAS...though an aircraft’s TAS may be very high at high altitude, it’s IAS is much lower at that high altitude. So, regardless of altitude, an aircraft will stall at the same IAS (disregarding compressibility) because IAS is the best indication of how much air is flowing over the wings, tail and fuselage as it automatically considers the density of the airflow. Prior to the advent of GPS (or inertial systems or Doppler Radar), pilots and navigators had to compute their TAS based on their IAS, air density (temperature and altitude), and forecast headwind/tailwind, or looking at known landmarks, in order to determine the distance moved across the ground as there was not a “TAS indicator” until the advent of inertial reference systems. However, IAS has always been just as important because IAS, more so than other speed indications, is the easiest and best predictor of aircraft’s ability to remain airborne and handle properly.

1.666 points lol

Alrighty, thanks! @edensk

@Aldriech Always use IAS, mach number is directly proportional to TAS though

Do I still use IAS for achieving max speeds in high altitudes or do I switch to TAS?@edensk

For important speeds/limits use IAS
The higher the altitude the higher the stall speed in TAS is, while in IAS it doesn't change.
I would explain what both exactly are if I wasn't so sleepy, maybe someone else can do it instead

Thank you!

@Aldriech I guess use of TAS is a better option, as of IAS is dependant of air density (kinda altitude)

@ChiChiwerx