Abstract: A servo motor is coupled through an electric clutch to the needle on a mechanical airspeed indicator. The servo motor is controlled by a signal processor to position the needle to indicate airspeed based on a value calculated from static and pitot pressure indicated by output signals from a solid state transducer. The needle is also connected to an aneroid mechanism that positions the needle based on static and pitot pressure but which is overcome by the servo motor when the clutch is activated. The clutch is deactivated, allowing the aneroid mechanism to position the needle, if a fault is detected by the signal processor.

Abstract: An altimeter system for aircraft employs an aneroid, dash mounted altimeter having a needle coupled to an encoder. The encoder provides a signal indicating that the needle points to a certain altitude. A solid state transducer senses outside pressure and supplies a signal to a signal processor, which instructs a transponder to transmit an ICAA coded altitude signal. When the signal from the encoder is provided to the signal processor, it compares the altitude from the needle position with the altitude that is squawked. If the error exceeds a value stored in the signal processor, a fault is indicated to the pilot and squawked. If the error is within the stored value, an offset is computed from that value and is summed with the needle altitude and the sum is squawked. The squawked altitude is the value between the altimeter reading and the altitude indicated by the transducer.