The Atmosphere — Page 32, Lesson 41

I want to walk you through the relationships between the airspeeds, because this is where a lot of students get tangled up. The key idea I want you to hold onto from the very start is this: of all the speeds we talk about, only one is a true speed through the air. The rest are pressures that we read as speeds. Let's start with Indicated Airspeed, or IAS. This is simply the speed registered on the Airspeed Indicator. It's what the needle shows you in the cockpit. That's the raw reading. Now, that raw reading isn't accurate, because of where the sensors are and how the aircraft is configured. That brings us to Calibrated Airspeed, or CAS. CAS is an accurate measure of dynamic pressure when the aircraft is flying slowly. Here's the problem it corrects: the position of the pitot tube(s) and the static vent(s), together with the aircraft's configuration — that means flaps, landing gear, and so on — and the aircraft's attitude to the airflow, which is angle of attack and sideslip, will all affect the pressures sensed. This is especially true at the static vents. So under the influence of those conditions, a false dynamic pressure is displayed — that false reading is your IAS. When you correct IAS for this 'position' or 'pressure' error, the result is Calibrated Airspeed. Those corrections might be shown on a placard on the flight deck, or in the Flight Manual, and they will include any instrument error as well. Next is Equivalent Airspeed, or EAS. This is an accurate measure of dynamic pressure when the aircraft is flying fast. The problem here is different: air entering the pitot tube(s) gets compressed, which gives a false dynamic pressure reading — again, that's your IAS — but this compression only becomes significant at higher speeds. At a given air density, the amount of compression depends on the speed of the aircraft through the air. So when you correct IAS for both 'position' error AND 'compressibility' error, the result is Equivalent Airspeed. Then we get to True Airspeed, or TAS, sometimes written as V. This is the speed of the aircraft through the air. And I want you to remember this phrase from the text: it is THE ONLY SPEED THERE IS. All the other, so-called, speeds are pressures. There's a formula for it: TAS equals EAS divided by the square root of relative density, where the symbol б, that's the Greek letter rho, is Relative Density. Here's why that matters. The Airspeed Indicator is calibrated for 'standard' sea level density, so it will only read TAS if the density of the air through which the aircraft is flying is 1.225 kg/m³. That's the standard sea level density value. So at 40,000 feet, where the 'standard' density is one quarter of the sea-level value, to maintain the same EAS the aircraft will have to move through the air twice as fast. That's the square root relationship working out — one quarter density means the square root is one half, so TAS doubles. Now let's talk about The Speed of Sound, denoted by the symbol (a). Sound is 'weak' pressure waves which propagate spherically through the atmosphere from their source. The speed at which these pressure waves propagate is proportional to the square root of the absolute temperature of the air. The lower the temperature, the lower the speed of propagation. On a 'standard' day at sea level, the speed of sound is approximately 340 m/s, which is 660 knots TAS. At higher aircraft True Airspeeds and/or higher altitudes, it becomes essential to know the speed of the aircraft in relation to the local speed of sound. This speed relationship is known as the Mach Number, or M. The formula is M equals TAS divided by (a), where (a) is the Local Speed of Sound. So if the True Airspeed of the aircraft is four tenths the speed at which pressure waves propagate through the air mass surrounding the aircraft, the Mach meter will register M 0.4. Finally, we have the Critical Mach Number, or MCRIT. This is the Mach number of the aircraft when the speed of the airflow over some part of the aircraft first reaches the speed of sound. And that point is usually the point of maximum thickness on the aerofoil. So even though the aircraft itself is flying below Mach 1, the airflow accelerating over the wing can locally reach the speed of sound — that's your critical Mach number. So to tie it all together: you start with the raw IAS, correct for position error to get CAS, correct for compressibility to get EAS, and then correct for density to get TAS — the only true speed. And then you relate TAS to the local speed of sound to get Mach number, and you watch for that critical Mach number where local airflow hits the speed of sound.