Digital Altimeter Mechanism

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Smiths Industries were a prolific manufacturer of aircraft instruments for use on military and civil aircraft and this is a servo-driven Altimeter featuring ‘a counter/pointer presentation to provide maximum readability without ambiguity. Smiths claimed that the instrument had a high order of accuracy and sensitivity with virtually no lag up to a maximum rate of height change of 90,000 f .p.m. This servo altimeter can function as a Central Air Data Computer repeater normally, but
upon failure of the CADC, or by choice, it can revert to function as a self-sensing servo altimeter of similar basic accuracy to that of the CADC. In its reversionary mode the altimeter has provision for accepting a pressure-error correction signal' from a small external pressure-error computer.
The principle of the servo altimeter, introduced by Smiths Industries in 1958, was that aneroid capsules were relieved of all but the lightest mechanical work involved in the electrical detection of their position. The operation of the instrument was performed by an electrically powered servo. This resulted in significant gains in accuracy, the possibility of extension of the range to 100,000ft, and the use of five-digit counter presentation making misreading virtually impossible. This conformed to the conclusions reached by the UK Altimeter Committee. In spite of these advantages, and apparent complexity, the servo altimeter achieved a consistent reliability well above that of mechanical altimeters.

This is a Smiths Industries servo altimeter but without a case. The aneroid capsules can be seen and the servo motor. The motor in this case drives a digital readout of altitude in feet and also a secondary digital window displaying possibly pressure in mbars.