The Machmeter — Page 92, Lesson 102

Right, let's get into the Machmeter. We've covered the airspeed indicator, and now we're looking at its close cousin, the Machmeter. This instrument tells you your Mach number — the ratio of your true airspeed to the local speed of sound. That's the core definition: Mach number equals TAS divided by LSS, where TAS is true airspeed and LSS is the local speed of sound. Now, before we get into the instrument's internals, let's talk about what happens when things go wrong — specifically, blockages. These are the classic failure modes you need to know cold. First, the static source blocked. If the static source is blocked, the pressure in the Machmeter case will not alter when the aircraft climbs or descends. Think of the case as the reference pressure chamber. If it's sealed off, it's stuck at whatever pressure it had when the blockage occurred. Let's say the blockage happens during a climb. The altitude capsule won't move, because it's sensing the difference between pitot and static, and the static side is frozen. Now, assuming a constant IAS — and therefore a constant dynamic pressure — the airspeed capsule will contract as the static component of pitot pressure reduces. Here's the logic: in a climb, the ambient static pressure is falling. The pitot pressure is made up of static plus dynamic. If dynamic pressure is constant, then pitot pressure falls too, because its static component falls. So the airspeed capsule, which senses pitot pressure, sees a lower pressure and contracts. The result: the Machmeter under-reads. Now reverse it — blockage in a descent, again at constant IAS. The airspeed capsule will expand, because the static component of pitot pressure is increasing as you descend into denser air. The capsule expands, and the Machmeter over-reads. So for a static blockage: under-read in the climb, over-read in the descent. Now the pitot source blocked. Same assumption — climb or descent at constant IAS, constant dynamic pressure. But here the result flips: the Machmeter will over-read in the climb and under-read in the descent. Why? In the climb, the airspeed capsule will expand in error, because the static component of pitot inside the capsule will be greater than the static in the case. The pitot line is frozen at a higher pressure from lower altitude, while the case static keeps falling. So the capsule sees a bigger pressure difference and expands — over-reading. In the descent, the static component of pitot is too small — frozen at a lower pressure from higher altitude — so the airspeed capsule contracts, and the Machmeter under-reads. Here's a handy note to remember: the Machmeter blockage errors are the same as the ASI blockage errors. If you've got the airspeed indicator failure modes memorised, you've got these too. Now, let's talk about the abbreviations you'll see in flight manuals and on performance charts. There are three key ones. MMR — Machmeter reading. That's the uncorrected reading, straight off the dial. 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 important operational point: with TMN available, there is much less risk of an over-speed condition arising. Position error is the error caused by the way the airflow distorts around the aircraft's body at the static ports, and correcting for it gives you a truer picture of your actual Mach number. Now let's pull together the Machmeter summary — the relationships you need to reason through climb and descent performance. First, Mach number equals TAS divided by LSS. We've got that. Second, the speed of sound is proportional to the square root of the absolute temperature. That's a physics fact you must hold onto. And because of that, the speed of sound decreases with the decrease in temperature normally encountered with increase in altitude. Warmer air, faster sound; colder air, slower sound. Climb into the cold, and the local speed of sound drops. Now the climb scenarios. While climbing at a constant Mach number, TAS decreases — because the speed of sound is decreasing, and you're holding a fixed ratio to it. And CAS decreases more rapidly, while the LSS also decreases. So holding Mach constant in a climb, your indicated airspeed falls off faster than your true airspeed does. While climbing at a constant CAS — the opposite case — TAS and Mach number increase, but the LSS decreases. You're holding indicated airspeed fixed, but as the air thins and the speed of sound drops, your true airspeed and your Mach number both rise. That's the classic high-altitude over-speed risk: constant CAS climb, and Mach creeps up toward MMO. Finally, a reminder for calculations involving the Jet Standard Atmosphere. The temperature is assumed to be +15°C at MSL — mean sea level — with a lapse rate of 2° per 1000 feet, with no upper limit, meaning no tropopause. So in the Jet Standard Atmosphere, temperature keeps falling at that steady 2 degrees per thousand feet all the way up, unlike the standard atmosphere where the tropopause caps the lapse rate. Keep that in mind when you're doing Mach-related performance calculations. And if you want to see the rapid rise of Mach number visually — the reason high-altitude over-speed is such a concern — look at Figure 7.2, which shows Mach number rising sharply, in that case far exceeding MMO. That's the Machmeter — the blockages, the error directions, the abbreviations, and the climb/descent relationships. The key takeaway: always reason through which capsule is moving and which pressure is frozen, and you'll never mix up the error directions.