The Machmeter — Page 97, Lesson 110

I want to walk you through the Machmeter and the Air Data Computer that feeds it. Let's start with the Machmeter itself. If the aircraft is fitted with an Air Data Computer — and I'll explain what that is in a moment — it will measure three things: pitot pressure, static pressure, and Total Air Temperature. It then sends that information electronically to any instruments and other computers that need it. So the Machmeter isn't just a standalone gauge; it's part of a bigger system. Now, the combined Mach/Airspeed instrument has two key advantages. First, it can correct for instrument and position errors to give you Calibrated Airspeed, or CAS, instead of Indicated Airspeed, IAS. That's a real improvement, because IAS is what the raw instrument reads, but CAS is the corrected value that accounts for those errors. Second, it uses digital displays for both Mach number and CAS. So you get precise digital readouts rather than just a needle. Let me describe the face of this combined instrument, because there's a lot on it. You have a Mach number scale, and you have a limit speed pointer, which is the VMO pointer — VMO is the maximum operating speed. There's also an airspeed pointer, and a command bug with its setting knob. The command bug is a marker you can set to a target speed. And there's a VMO pointer setting knob, so you can set that limit speed pointer to the appropriate value. There's also an external index pointer. So the pilot can set both a command speed and a limit speed on the same instrument. Now, when the instrument is fed from the Air Data Computer, the design changes a bit. You still have the airspeed pointer and the Mach number scale, but now you also have a VMO pointer that is a driven cursor — meaning the computer drives it automatically rather than you setting it manually. And you have an airspeed manual cursor with its setting control. So the computer-driven version automates some of what you'd otherwise set by hand. Let me now introduce the Air Data Computer itself. In many large aircraft currently in service, the conventional pressure instruments that show altitude, airspeed, and Mach number — that's MNo — are replaced by indicators displaying information generated by a central computer, the Air Data Computer, or ADC. So instead of each instrument doing its own pressure sensing, one central computer does the work and feeds the displays. The computer unit and the displays, together with the sensors for the basic data — pitot pressure, static pressure, and air temperature — plus a power-pack, together form the aircraft's Air Data System, or ADS. So the ADS is the whole package: sensors, computer, displays, and power supply. Now, an important point: while the ADS is self-contained, its outputs are essential to the operation of the aircraft's Automatic Flight Control System, the AFCS. So the autopilot and flight director depend on the air data. The ADS outputs may also be used in the altitude transponder, the flight data recorder, the navigation computer, and more. So this one system feeds many other systems. Here's a practical detail. A number of different aircraft types may use the same basic Air Data Computer. So the same computer hardware can be fitted to different aircraft. To make that work, the device needs to be integrated into each aircraft's systems, and that's achieved by a "Configuration Module." The module can be calibrated to take into account differences in pressure and temperature gathering efficiencies due to the positioning of the gathering probes. So if the pitot probe is in a slightly different place on one aircraft versus another, the configuration module compensates for that. That information is then entered into the computer to obtain the most accurate indications possible. The standard ADS instruments show altitude, vertical speed, airspeed, and Mach number, MNo. Additional instruments can display Total Air Temperature, or TAT, Static Air Temperature, or SAT, and True Airspeed, TAS. So the basic set is four, and you can add three more. Two final notes. The weight on wheels switch decouples the stall warning system when the aircraft is on the ground. So when the aircraft is on the ground, the stall warning is disabled. And Angle of Attack, AOA, may also be an input to the ADC for use in some aircraft systems. So the ADC doesn't just take pressure and temperature; it can also take angle of attack. That's the Machmeter and the Air Data Computer system that drives it.