"IAS: Indicated Airspeed. This is what you see on the airspeed indicator when flying, obtained by measuring pressure acting on the aircraft by having a little tube pointing into the wind. But here is the trick: What if you are flying in thin air because it is a hot day, or you are high in the air? The air is thinner, so there is less pressure. Well, there will clearly be less pressure on the little tube (the pitot tube) so there will be a lower indication on the airspeed indicator. Aeii! ERROR! But, really, we LIKE this error. Why? Because if there is less pressure pushing on the airspeed indicator, then there is less pressure PUSHING ON THE WINGS AS WELL! So, if we really want to measure the air pressure available for the props and wings (which is what we really care about since more pressure gives more lift and drag) then what we really want is a measure of the pressure acting on the airplane: which is what the airspeed indicator really is, BECAUSE of this error that varies with the pressure! So think of it this way: If you are going 120 mph in thin air, but the pressure is only strong enough to measure 100 mph on the airspeed indicator, then that means the aerodynamic pressure on the WINGS is only 100 mph worth of pressure! And it is this pressure that determines how much lift and drag the wings can put out. So we pilots LIKE this error in the system because it means that if the airplane likes to approach the runway at 100 mph, then we should fly the plane at 100 mph on the airspeed indicator. Maybe one day we are at high altitude or high temperature, and that thin air means we actually need to go 120 mph to get the same pressure on the pitot tube to indicate 100 mph on the airspeed indicator, but that also means we must go 120 mph to get 100 mph worth of pressure on the wings! So the error in the airspeed indicator exactly counter-acts the speed change we need to make the plane fly properly. This is not a coincidence, because the pitot tube and the wings both need the same thing: dynamic pressure, or inertia of air smacking into them to function."

 

Eric D

N58JJ