IAS or actual air speed

TAS (True Airspeed) Pros- You know for sure how fast you're going

Cons: All the aircraft checklists are in KIAS (Knots Indicated Airspeed), which make it confusing to read.
You can get a close approximation from the GPS, bottom left corner, G/S is the abbreviation, then below that, is the speed. While it's ground speed, you won't really need it all that much anyway.

The reason indicated airspeed is most important and useful is that it's a direct measurement of the aerodynamic stress on the plane. For instance, you can't lower the landing gear above a certain indicated airspeed, because the force of the air at that speed would rip it off, or at least damaged it. It doesn't matter what our true speed is in this case, what matters is how much aerodynamic stress is on the plane.

So you should keep your airspeed indicator displaying indicated airspeed, and if you want to know your true speed over the ground, refer to the GPS.

Ed

Thanks chaps....I knew it was important to use KIAS, I tried TAS once and err....well I shan't be doing it again. My 747 is buried somewhere near Manchester.

The indicated air speed is the important one because it is irrelevant to your ground speed. It took me a while to get my head round that (there are documents on the Internet explaining it all) but imagine a 30kt wind speed - flying into it your ground speed will be less, fly with the wind and your ground speed will be higher. However your indicated airspeed won't alter.

IAS is also critical to keep you from stalling the plane or rotating to early on takeoff. Remmember, the airspeed indicator reads aerodynamic pressure. By doing so, its information remains the same regardless of atmospheric conditions. ALWAYS use the IAS setting on the airspeed indicator.