Conventional (Analogue) Systems

Each sensor sends the information directly to the corresponding instrument

• Pitot Tube (Pressure Lines)
  • Mach Meter
  • Airspeed Indicator
• Static Port (Pressure Lines)
  • Mach Meter
  • Airspeed Indicator
  • Altimeter
  • Vertical Speed Indicator
• Total Air Temperature (TAT) Probe (Electrical Connection)
  • Total Air Temperature (TAT) Indicator
• Angle of Attack (AoA) Probe (Electrical Connection)
  • Angle of Attack (AoA) Indicator

Each instrument acts independently from one another, meaning that further calculations such as TAS (True Airspeed) or SAT (Static Air Temperature) much be calculated by the crew.

Each indicator is it’s own complex assembly of mechanic parts; which leads to further inaccuracies due to wear, imperfections, backlash (delay) etc. This means that the crew is further required to calculate correction factors to counteract these inaccuracies; such as an altimeter calibration chart published in the aircraft manual. (This is only in certain cases where high accuracy is required, but can’t be achieved due to external conditions)

Air Data Computer (ADC)

The Air Data Computer forms part of the Air Data System, it is the electronic ‘middle-man’ between the sensors on the aircraft and the instruments; allowing it to perform additional calculations with the data provided in order to display even more information about the aircraft. An ADC can either be analogue or digital, depending on the output of the sensors (namely the pressure transducers on the total and static pressure lines)

The fact that the ADC is completely electronic, means that errors introduced due to mechanical wear/inaccuracies in conventional instruments are basically eliminated. Additionally, the ADC can store the the position errors for the sensors under different flight conditions, meaning that it can make these corrections automatically and in real-time.

BITE (Built-In Test Equipment)

A system which regularly monitors the ADC and tests the integrity of the data processing.

• Power Up BITE: When powered up, the unit performs an automatic test of the microprocessor, the memory story and the general functions of the ADC
• Continuous BITE: Regularly monitors the information coming from sensors and data calculated by the ADC to ensure accuracy.

If a malfunction occurs in one or more sensors (for instance a blockage of the pitot tube) the BITE will detect this error and present a flag on all relevant indicators / displays.

Redundancy

Small Aircraft

In the event of a complete failure, smaller aircraft employ a series of back-up analogue indicators that are directly connected to the sensors before they are processed by the ADC

Larger Aircraft

In larger aircraft with multiple sets of instruments (pilot & first officer), two ADCs are typically installed without their own set of dedicated sensors. These ADCs will communicate with another to ensure that they are within tolerance of one another and detect any issues with either set of sensors. There can also be a set of dedicated back-up sensors that are directly connected to analogue indicated.

In the event of an ADC failure, the crew has the functionality to choose one dedicated ADC which will feed both sets of displays.