The Machmeter — Page 92, Lesson 102

Let’s start with the Machmeter’s failure modes, because that’s where this excerpt opens — and it’s a classic exam trap. Static Source Blocked. If the static source is blocked, the pressure inside the Machmeter case stays frozen at whatever it was when the blockage happened. It won’t change when the aircraft climbs or descends. Now, picture a climb with a blocked static source, at a constant IAS. Constant IAS means constant dynamic pressure — that’s the difference between pitot pressure and static pressure. The altitude capsule won’t move, because it senses static pressure, which is stuck. But the airspeed capsule senses pitot pressure, which is the sum of static plus dynamic. As you climb, the static component of pitot pressure reduces, so the airspeed capsule contracts. A contracted airspeed capsule means the Machmeter under-reads in a climb. Now reverse it — a descent with a blocked static source, again at constant IAS. The static component of pitot pressure increases as you descend, so the airspeed capsule expands. An expanded capsule means the Machmeter over-reads in a descent. Pitot Source Blocked. Now the pitot source is blocked instead. Again, assume a climb or descent at constant IAS, hence constant dynamic pressure. Here the Machmeter over-reads in the climb and under-reads in the descent — exactly opposite to the static blockage case. Why? In a climb, the static component of pitot pressure inside the airspeed capsule is greater than the static pressure in the case, so the capsule expands in error — over-reading. In a descent, the static component of pitot is too small, so the capsule contracts — under-reading. And here’s the note that ties it together: the Machmeter blockage errors are the same as the ASI blockage errors. If you already know how an airspeed indicator behaves with a blocked static or pitot source, you already know the Machmeter’s behaviour — they share the same pressure-sensing principle. Now let’s move to the abbreviations, because these are the exact terms you’ll see in Flight Manuals. MMR — Machmeter reading. That’s the uncorrected reading straight off the instrument. IMN — Indicated Mach number. That’s the MMR corrected for instrument error. The values quoted in Flight Manuals are normally IMN. TMN — True Mach number. That’s the IMN corrected for position error. And here’s the safety point: MMO — that’s the maximum operating Mach number. There is much less risk of an over-speed condition arising when TMN is available, because you’re seeing the true Mach number, not one distorted by position error. Now the Machmeter summary — the core relationships. Mach number = TAS / LSS. That’s true airspeed divided by local speed of sound. Speed of sound is proportional to the square root of the absolute temperature. So as temperature decreases — which normally happens with increasing altitude — the speed of sound decreases too. Now the three climb scenarios, and you need to keep these straight. Climbing at a constant Mach number: TAS decreases, CAS decreases more rapidly, and the LSS also decreases. So you’re holding Mach steady, but the actual airspeed is falling off, and the indicated airspeed falls even faster. Climbing at a constant CAS: TAS and Mach number both increase, but the LSS decreases. So holding indicated airspeed constant, your true airspeed climbs and your Mach number climbs — and that’s exactly why Mach becomes the limiting factor at high altitude. And finally, the Jet Standard Atmosphere assumption, because you’ll use it in calculations: temperature is assumed to be +15°C at MSL — mean sea level — with a lapse rate of 2° per 1000 feet, and with no upper limit, meaning no tropopause. So the temperature keeps decreasing at that constant rate all the way up. Let me show you the climb-and-descent behaviour visually, because the inversion and constant-CAS figures make it concrete. So the takeaway: blockage errors mirror the ASI exactly — static blocked gives under-read in climb, over-read in descent; pitot blocked gives the reverse. And the Mach number itself is simply TAS divided by local speed of sound, with the speed of sound shrinking as temperature drops with altitude.