The Machmeter — Page 83, Lesson 100

Let's get into the Machmeter. I want to walk you through how it's built, how it works, and the errors it carries. First, the construction. The Machmeter is essentially two instruments working together inside one case. It has a simple aneroid altitude capsule and an airspeed capsule, and these two are connected at what we call the ratio arm. That ratio arm is the heart of the instrument — it's where the two pressures get compared. Now, let's trace the pressure paths. Static pressure enters the case of the instrument itself. Pitot pressure is fed directly into the airspeed capsule. So the airspeed capsule expands or contracts based on pitot pressure, and that movement is transmitted via the airspeed link and the main shaft up to the ratio arm. But here's the key: the position of the ratio arm is also governed by the expansion or contraction of the altitude capsule, which is sensing static pressure. So the ratio arm's position is a function of both airspeed and altitude. From the ratio arm, a spring-loaded ranging arm transmits the movement to the pointer mechanism. That's the mechanical chain: capsules to ratio arm, ratio arm to ranging arm, ranging arm to pointer. Let's think about what happens physically. If either or both capsules expand — which happens when IAS increases and/or altitude increases — then the ranging arm rotates out of the diagram, and the indicated Mach number increases. Conversely, if airspeed or altitude reduce, the ratio and ranging arms move back into the paper, and a lower Mach number is displayed. So the instrument is telling you the ratio of your true airspeed to the speed of sound, and it does that by comparing pitot and static pressures. There's also an adjustable index on the instrument scale. You can position it with a small knob. This index can be set to the limiting Mach number for your aircraft type, in straight and level flight, to give you a visual warning. So you set that bug at your Mmo, and you can see at a glance when you're approaching it. Now, the errors. This is important. The Machmeter suffers from instrument, position, and manoeuvre induced errors only. It does not suffer from temperature or density errors, because those errors cancel out. And here's the elegant part: since compressibility error depends on the ratio of dynamic pressure to static pressure, and the instrument is calibrated to that very ratio, compressibility error is calibrated out. So the Machmeter is immune to three of the five classic error sources. Let's look at position error. The Machmeter uses the same pitot and static sources as the ASI, so it suffers from position error caused by disturbed airflow at the pitot head and/or static vent. At low Mach numbers, careful design and positioning of the pressure sources ensure that position error on modern jet aircraft is small. But at higher Mach numbers, changes in airflow may cause position error to become bigger and possibly change its sign. And here's the danger: if the sign of the position error is such that the Machmeter under-reads, the error could become dangerous at high Mach numbers. That's why the normal arrangement in modern jet transport aircraft is to allow for instrument and position error such that the Machmeter always over-reads. You'd rather see a slightly high Mach number than a dangerously low one. Finally, manoeuvre induced error. The Machmeter will suffer an additional, unpredictable error whenever the aeroplane manoeuvres. This is due to the unpredictable changes in the airflow over the static source. So when you're pulling G, the static pressure at the vent can be disturbed, and that shows up as an error you can't predict or correct for. So to sum up: the Machmeter compares pitot and static pressures through two capsules and a ratio arm to display Mach number, it's free of temperature, density, and compressibility errors, and its remaining errors — position and manoeuvre induced — are managed by design so it tends to over-read rather than under-read.