Chapter 8
Air Data Computer

1. Introduction – The Air Data System (ADS)

In many modern large aircraft, conventional pressure instruments (altitude, airspeed, Mach number) are replaced by indicators driven by a central computer — the Air Data Computer (ADC). Together with sensors and displays, it forms the Air Data System (ADS).

ADS Components

• Pitot pressure sensor
• Static pressure sensor
• Total Air Temperature (TAT) probe
• Power supply
• The ADC unit itself
• Display instruments

Standard ADS Instrument Outputs

Configuration Module

Because many aircraft types may use the same basic ADC, a Configuration Module is used to integrate it into each specific aircraft. This module is calibrated for differences in probe positioning efficiency to obtain the most accurate indications possible.

The ADS outputs are essential to the AFCS (Automatic Flight Control System) and also feed: altitude transponder, flight data recorder, navigation computer, and other systems.

2. Pitot-Static System and Redundancy

A typical aircraft has identical instrument sets on the Captain's and First Officer's panels. Each set connects to one of two independent ADCs fed from independent pitot and static sources that can be cross-connected.

Standby Instruments

In addition to ADC-fed instruments, there is always:

• A standby barometric altimeter — fed directly from an independent pitot-static source
• A standby airspeed indicator — fed directly from the same independent source

Each of the three independent pitot-static systems uses cross-coupled static vents on each side of the fuselage to reduce error from sideslip or yaw.

3. Air Data Computer – Analogue and Digital Types

3.1 Analogue ADC

Uses continuous physical variables (voltage, pressure, shaft rotation) to represent and transmit measurements. A Pressure Transducer converts the pressure inputs to shaft rotation via a 2-phase servomotor connected to a CX synchro. The analogue ADC is internally divided into modules:

• Altitude module
• Computed Airspeed (CAS) module
• Mach Speed module
• True Airspeed (TAS) module
• Rate of Climb module (using data from altitude module)

3.2 Digital ADC

Uses digital (binary) data for assessment and transmission. Analogue-to-Digital Converters (ADCs) at the input side convert pressure, temperature, and AOA measurements from analogue to digital form for internal processing and onward transmission to flight deck displays.

flowchart LR
    A["Pitot Pressure"] --> C["ADC"]
    B["Static Pressure"] --> C
    T["TAT Probe"] --> C
    AOA["AOA (some types)"] --> C
    C --> D["Altitude"]
    C --> E["Airspeed (CAS)"]
    C --> F["Mach Number"]
    C --> G["TAS"]
    C --> H["Vertical Speed"]
    C --> I["TAT / SAT"]
    C --> J["AFCS / FDR / XPDR / Nav Computer"]

4. System Redundancy

Redundancy is provided through:

• Change-over cocks — permit an alternative static source to connect to the computer.
• Electrical switching — Captain's instruments can be fed from the First Officer's ADC and vice versa.
• Mixed sourcing — in some aircraft, ADC outputs are mixed to both sides of the panel, reducing the risk of an undetected malfunction.
• Standby instruments — available for total ADC failure due to power loss.

5. Failure Warning

A comparison monitor can be incorporated to automatically compare the outputs of both ADCs and warn the pilot of malfunction. With purely mechanical instruments, comparison was done visually only.

6. Built-In Test Equipment (BIT / BITE)

7. Advantages of an Air Data System

7.1 Improved Displays

Electrically-servoed instrumentation gives complete freedom to design unambiguous, easier-to-read displays including digital, moving tape, and combined formats.

7.2 Reduced Instrument and Lag Errors

Friction loss in mechanical linkages is the major source of instrument error in conventional instruments, also causing lag. Servomotors in ADS largely eliminate both problems.

7.3 Error Correction

Computing height, airspeed, etc. within one computer allows error corrections to be applied through specially shaped cams. For example, Position Error Correction (PEC) can be computed in the Mach channel and then applied also in the height and airspeed channels.

7.4 Central Source for Other Systems

The ADC provides air data in many forms required by other aircraft systems — AFCS, FDR, transponder, navigation computer, and more.

7.5 Clean Design

Electrically-driven instruments reduce the amount of pneumatic plumbing behind the panel to only the standby instruments' pitot-static lines. Benefits: space saving, easier maintenance, shorter pitot-static lines reduce acoustic errors.

Master Reference Tables

Answer Key (Instructor-Generated Questions)