The Machmeter — Page 83, Lesson 100

Let’s start with the Machmeter itself. I want you to think of it as the instrument that tells you the ratio of your true airspeed to the speed of sound at your current altitude — that’s the Mach number. The whole point of this instrument is to show you that ratio, and it does it with a clever mechanical construction. First, the construction. The Machmeter is built from two capsules. One is a simple aneroid altitude capsule, and the other is an airspeed capsule. These two are connected at something called the ratio arm. Now, here’s the key: static pressure enters the case of the instrument, while pitot pressure is fed directly into the airspeed capsule. So the airspeed capsule expands or contracts based on pitot pressure — that’s your indicated airspeed. The altitude capsule, being aneroid, expands or contracts based on static pressure — that’s your altitude. Now, the expansion or contraction of the airspeed capsule is transmitted through the airspeed link and the main shaft to the ratio arm. But the position of the ratio arm is also governed by the expansion or contraction of the altitude capsule. So the ratio arm is the meeting point of both inputs — airspeed and altitude. Then a spring-loaded ranging arm takes the movement of the ratio arm and transmits it to the pointer mechanism. That’s how the needle moves across the scale. Let me make the behaviour clear. If either or both capsules expand — which happens when indicated airspeed increases, or altitude increases, or both — then the ranging arm rotates out of the diagram, and the indicated Mach number increases. Conversely, if airspeed or altitude reduce, the ratio arm and ranging arm move back into the paper, and a lower Mach number is displayed. So the instrument is telling you: more speed or more altitude means a higher Mach number. There’s also an adjustable index on the instrument scale. You can position it with a small knob. This index is set to the limiting Mach number for your aircraft type, in straight and level flight. It gives you a visual warning — a marker on the dial so you can see at a glance when you’re approaching that limit. Now, the errors. This is important for your professional understanding. The Machmeter suffers from only three types of error: instrument error, position error, and manoeuvre induced error. It does not suffer from temperature or density errors, because those errors cancel out. And here’s the neat part about compressibility error: 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 you don’t have to worry about that one. Let’s look at position error. The Machmeter uses the same pitot and static pressure sources as the Airspeed Indicator, so it suffers from position error caused by disturbed airflow at the pitot head and/or the 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. Now, if the sign of the position error is such that the Machmeter under-reads, that could become dangerous at high Mach numbers — because you’d think you’re slower than you actually are. The normal arrangement in modern jet transport aircraft is to allow for instrument and position error such that the Machmeter always over-reads. So it errs on the safe side. Finally, manoeuvre induced error. Whenever the aeroplane manoeuvres, the Machmeter suffers an additional, unpredictable error. This is due to unpredictable changes in the airflow over the static source. So during turns, climbs, or any manoeuvre, you can expect that extra error — and it’s not something you can predict or calibrate out. So, to sum up the whole picture: the Machmeter combines an aneroid altitude capsule and an airspeed capsule at a ratio arm, uses a spring-loaded ranging arm to drive the pointer, and gives you a visual warning via an adjustable index set to the limiting Mach number. Its errors are limited to instrument, position, and manoeuvre induced — with temperature, density, and compressibility errors all taken care of. And in modern jets, the design ensures it always over-reads to keep you safe.