The Machmeter — Page 97, Lesson 110

I want to walk you through the Machmeter and the Air Data Computer. Let's start with the Machmeter itself. If an aircraft is fitted with an Air Data Computer — we'll call it the ADC — it measures 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 ADC is the central brain that gathers the raw pressures and temperature, and distributes them. Now, there's a combined Mach/Airspeed instrument, and it has two key advantages. First, it can correct for instrument errors and position errors, so it gives you Calibrated Airspeed — CAS — instead of Indicated Airspeed, IAS. Second, it uses digital displays for both Mach number and CAS. So instead of just a needle, you get a precise digital readout. Let me walk you through the face of this instrument, because there are a lot of labelled parts. We have a Mach number scale, and a limit speed pointer — that's the VMO pointer, the maximum operating speed. There's an airspeed pointer, and an external index pointer. There's a command bug, which is a marker you can set, and a setting knob to move it. There's a VMO pointer setting knob, and a command bug setting knob. So you have two setting knobs — one for the command bug, one for the VMO pointer — and two pointers, the airspeed pointer and the VMO pointer. Now, when the instrument is fed from the Air Data Computer, we get a slightly different arrangement. We have a driven cursor, an airspeed pointer, an airspeed manual cursor, and a setting control. The driven cursor is moved by the computer; the manual cursor you set yourself with the setting control. Now let's move to the Air Data Computer itself. In many large aircraft in service today, the conventional pressure instruments — the ones showing altitude, airspeed, and Mach number — are replaced by indicators that display information generated by this central computer, the ADC. So the computer unit, the displays, the sensors that gather pitot pressure, static pressure, and air temperature, plus a power-pack — together these form the aircraft's Air Data System, the ADS. Here's an important point: the ADS is self-contained, but its outputs are essential to the Automatic Flight Control System, the AFCS. So the autopilot depends 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 it's not just for the cockpit displays — it feeds many systems. Now, a number of different aircraft types may use the same basic Air Data Computer. But each aircraft type has different probe positions, so the computer needs to be integrated into each aircraft's systems. This is done with a "Configuration Module." The module can be calibrated to account for differences in pressure and temperature gathering efficiencies caused by the positioning of the gathering probes. That calibration information is entered into the computer to get the most accurate indications possible. So the same ADC hardware can be adapted to different airframes just by configuring the module. The standard ADS instruments show altitude, vertical speed, airspeed, and Mach number — MNo. Additional instruments can display Total Air Temperature, TAT; Static Air Temperature, SAT; and True Airspeed, TAS. So the basic set is four, and you can add three more. Two notes here. First, the weight-on-wheels switch decouples the stall warning system when the aircraft is on the ground — so on the ground, the stall warning is disabled. Second, 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 pressures and temperature; it can also take AOA. Let me show you the schematic of the conventional system. That's the Machmeter and the Air Data Computer. The key idea is that the ADC gathers pitot pressure, static pressure, and TAT, and distributes them to the instruments and systems that need them — giving you CAS instead of IAS, digital displays, and feeding the AFCS, transponder, flight data recorder, and navigation computer.