Air Data Computer — Page 97, Lesson 114

I want to walk you through the Air Data Computer now — this is the brain of the whole pitot-static system. Let's start with how the plumbing is arranged on a typical aircraft, because that sets up everything else. On a typical aircraft, you get identical sets of air data instruments on the Captain's panel and on the First Officer's panel. Each set is connected to one of two Air Data Computers — we call them ADCs — and the reason there are two is redundancy. If one computer fails, the other side still works. Each ADC is fed from independent pitot and static sources, and those sources can be cross-connected, as shown in Figure 8.3. So the two systems aren't completely isolated — they can back each other up. Now, in addition to the indicators powered by those 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 independent systems makes use of cross-coupled static vents located on each side of the fuselage. Cross-coupled means the static pressure is taken from both sides of the aircraft and averaged or combined. Why? Because this arrangement is designed to reduce error due to side-slip or yaw. If the aircraft is slipping sideways, the static pressure on one side can be distorted — by taking vents from both sides, you cancel out that error. Now let's get to 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 electrical signals are then transmitted to the display instruments and to other systems. So the ADC takes raw physical measurements — pressure and temperature — and turns them into something the rest of the aircraft can use. There are two types of ADC system, and they're described as either Analogue or Digital, based on the method of assessment and transmission of information used. That's the key distinction — how the information is assessed and how it's transmitted. Let me explain the analogue type first. The analogue type uses continuous physical variables — such as voltage or pressure — to assess and represent the measurements obtained. So instead of converting to discrete digital numbers, it works with continuously varying signals. Figure 8.2 shows an airspeed assessment device from an Analogue ADC, and it indicates the inputs of static and pitot pressure. Here's the mechanism: the two pressures are joined together mechanically. Then, using a Pressure Transducer, the result is transmitted forward for use through the rotation of a shaft. That shaft is driven by a 2-phase servomotor. The servomotor, in turn, is connected to a CX synchro — that's a control transformer synchro — where angular position can be measured and read off as an airspeed. So the pressure difference between pitot and static gets turned into a shaft rotation, and that rotation angle is read as airspeed. Now, internally, an analogue Air Data Computer may be split into modules for assessment and onward transmission of the data obtained through the temperature, static, and pitot pressure gathering devices. Let me list those modules: there's an Altitude module, a Computed Airspeed module, a Mach Speed module, a True Airspeed module, and — using data from the altitude module — a Rate of Climb module, which gives you vertical speed. So the analogue ADC is modular: each module handles one computed parameter, and they feed each other. The Rate of Climb module, for instance, takes its input from the altitude module to derive vertical speed. Let me just make sure you've got the full picture of the pitot-static redundancy. Three independent systems — Captain's ADC, First Officer's ADC, and standby instruments — each with cross-coupled static vents on both sides of the fuselage to kill side-slip error. And the ADC itself, whether analogue or digital, is the converter that turns pressure and temperature into electrical signals for the displays and other systems. The analogue version does it with continuous variables, mechanical linkage, a pressure transducer, a 2-phase servomotor, and a CX synchro to read angular position as airspeed — and it's split into modules for altitude, computed airspeed, Mach, true airspeed, and rate of climb.