Air Data Computer — Page 97, Lesson 114

Let’s start with the big picture, because this chapter is all about how the aircraft’s pressure and temperature data get turned into the numbers you see on your instruments. In a typical aircraft, you get identical sets of air data instruments on the Captain’s and First Officer’s panels. Each set is connected to one of two Air Data Computers — we call them ADCs — and those two ADCs are fed from independent pitot and static sources. The key word there is independent: each ADC has its own pitot and static inputs, so if one source fails, the other side still works. That’s your redundancy. And those two systems can be cross-connected, as shown in Figure 8.3. Now, on top of the two ADCs, there’s a standby barometric altimeter and a standby airspeed indicator. These are fed direct from pitot and static sources that are separate from the ones used by the ADCs. So you have three independent pitot-static systems in total: Captain’s, First Officer’s, and standby. Here’s a clever detail: each of those three systems uses cross-coupled static vents located on each side of the fuselage. Why cross-coupled? Because if you only had a static vent on one side, then during side-slip or yaw, the airflow would distort the static pressure reading. By coupling vents on both sides, the errors tend to cancel out. That arrangement is specifically designed to reduce error due to side-slip or yaw. Now let’s talk about the Air Data Computer itself. In current aircraft, it’s a device that uses analogue or digital computing techniques to convert pressure and temperature data into electrical signals. Those signals are then transmitted to the display instruments and to other systems. So the ADC is the brain that takes raw pitot pressure, static pressure, and temperature, and turns them into usable outputs. There are two types of ADC system: Analogue and Digital. The distinction is based on the method of assessment and transmission of information. The analogue type uses continuous physical variables — like voltage or pressure — to assess and represent the measurements. Let me walk you through the airspeed assessment device from an Analogue ADC, shown in Figure 8.2. It takes inputs of static and pitot pressure. Those pressures are joined together mechanically. Then, using a Pressure Transducer, the result is transmitted forward through the rotation of a shaft. That shaft is driven by a 2-phase servomotor, which in turn is connected to a CX synchro. The CX synchro is where the angular position can be measured and read off as an airspeed. So the mechanical rotation gets converted into an electrical angular signal that represents airspeed. Now, internally, an analogue Air Data Computer can be split into modules for assessment and onward transmission of the data gathered from the temperature, static, and pitot pressure devices. Those modules are: Altitude, Computed Airspeed, Mach Speed, True Airspeed, and — using data from the altitude module via a Rate of Climb module — vertical speed. So let me lay that out clearly. The Altitude module handles altitude. The Computed Airspeed module handles your indicated or computed airspeed. The Mach Speed module handles Mach number. The True Airspeed module handles true airspeed. And the Rate of Climb module takes data from the altitude module to give you vertical speed. That’s the full architecture of the ADC: three independent pitot-static systems, cross-coupled static vents to kill side-slip error, and an ADC that converts pressure and temperature into electrical signals, split into those five modules for altitude, computed airspeed, Mach, true airspeed, and vertical speed.