Standby Altimeter

The Air Data Transducer contains an aneroid capsule, or diaphragms, which expands and contracts with the pressure input from the Pitot tube. The case around the diaphragm is airtight and is vented to the static port. The difference between the pitot pressure and the static pressure is called dynamic pressure. The greater the dynamic pressure, the higher the airspeed.
The units have a circular alloy base 6.7cm diam. with four fixing feet protruding. Unit surmounted by Perspex/plastic cylindrical lid. A solenoid within a circular armature and pitot & static barometric connections are visible. The solenoid coil's motion is sensed by an electromagnetic rotary sensor (around the 6-toothed disc).
The current in the solenoid produces a counter-force against that produced by the pitot-static air pressure difference across the diaphragm attached to the coil. External electronics continuously controls the coil current to keep the rotary sensor signal in the middle of its range (probably its null point). With this balance achieved, the coil current is exactly balancing the air-pressure difference and therefore proportional to it. Suitably manipulating the coil current value yields airspeed.

These little units were used in the Autostabiliser on such aircraft as the Lightning and later the VC10.

The BAC 1-11 AFCS, like that in the VC10, was based on the well proven Bendix PB-20 Autopilot and was designated the Series 2000 AFCS. New features over the PB-20 system include separate pitch and azimuth control computers, a modular Air Data Sensor and a range of units specifically designed for autoflare and autolanding.

Each unit in the BAC 1-11 AFCS is built to a common configuration with circuit modules arranged in stacks either side of the chassis. The stacks are connected by plugs to a mother board and are physically separated into ‘command’ and ‘monitor’ functions to preclude common failures. The computers are entirely solid state and there is a high degree of built-in-test. Self-monitoring techniques and multiple channel redundancy are used to give automatic failure survival in approach and cruise flight.